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<h2 id="C-17-多态分配器，调试资源和自定义类型"><a href="#C-17-多态分配器，调试资源和自定义类型" class="headerlink" title="C++17: 多态分配器，调试资源和自定义类型"></a>C++17: 多态分配器，调试资源和自定义类型</h2><p>从C++17开始支持的多态分配器是标准库对标准分配器的增强。</p>
<p>它比常规分配器容易使用得多，并且允许相同的类型容器具有不同的分配器，甚至有可能在运行时改变分配器。</p>
<p>多态分配器符合标准库分配器的规则。尽管如此，它的核心还是使用内存资源对象来进行内存管理。</p>
<p>多态分配器包含指向内存资源类的指针，这就是为什么它可以使用虚函数调度。您可以在运行时更改内存资源，同时保留分配器的类型。这与常规分配器相反，后者使用不同分配器时是不同类型。</p>
<p>多态分配器的所有类型都位于单独的命名空间<code>std::pmr</code>在头文件<code><memory_resource></code>（PMR, Polymorphic Memory Resource）中。</p>
<h2 id="pmr核心要素"><a href="#pmr核心要素" class="headerlink" title="pmr核心要素"></a><code>pmr</code>核心要素</h2><p><code>pmr</code>的主要部分：</p>
<ul>
<li><code>std::pmr::memory_resource</code> - 是所有其他实现的抽象基类。它定义了以下纯函数： <pre class="line-numbers language-cpp" data-language="cpp"><code class="language-cpp"><span class="token keyword">virtual</span> <span class="token keyword">void</span><span class="token operator">*</span> <span class="token function">do_allocate</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span>size_t bytes<span class="token punctuation">,</span> std<span class="token double-colon punctuation">::</span>size_t alignment<span class="token punctuation">)</span>
<span class="token keyword">virtual</span> <span class="token keyword">void</span> <span class="token function">do_deallocate</span><span class="token punctuation">(</span><span class="token keyword">void</span><span class="token operator">*</span> p<span class="token punctuation">,</span> std<span class="token double-colon punctuation">::</span>size_t bytes<span class="token punctuation">,</span> std<span class="token double-colon punctuation">::</span>size_t alignment<span class="token punctuation">)</span>
<span class="token keyword">virtual</span> <span class="token keyword">bool</span> <span class="token function">do_is_equal</span><span class="token punctuation">(</span><span class="token keyword">const</span> std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>memory_resource<span class="token operator">&</span> other<span class="token punctuation">)</span> <span class="token keyword">const</span> <span class="token keyword">noexcept</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span></span></code></pre>
</li>
<li><code>std::pmr::polymorphic_allocator</code> - 是标准分配器的实现，它使用 <code>memory_resource</code> 对象来管理内存分配和释放。</li>
<li><code>new_delete_resource()</code>和<code>null_memory_resource()</code>访问的是全局内存资源。</li>
<li>一组预先定义的内存池资源类：<ul>
<li><code>synchronized_pool_resource</code></li>
<li><code>unsynchronized_pool_resource</code></li>
<li><code>monotonic_buffer_resource</code></li>
</ul>
</li>
<li>模板特化的标准容器与多态分配器，例如<code>std::pmr::vector</code>，<code>std::pmr::string</code>，<code>std::pmr::map</code>等。每个特化容器都在与相应容器相同的头文件中定义。</li>
<li>同样值得一提的是，池资源可以链式包含。如果资源池中没有可用内存，分配器将从”上游”资源中分配。</li>
</ul>
<h2 id="预先定义的内存资源"><a href="#预先定义的内存资源" class="headerlink" title="预先定义的内存资源"></a>预先定义的内存资源</h2><h3 id="1-new-delete-resource"><a href="#1-new-delete-resource" class="headerlink" title="1. new_delete_resource()"></a>1. <code>new_delete_resource()</code></h3><p>它是一个函数，返回指向全局“默认”内存资源的指针。它使用全局 new 和 delete 管理内存。</p>
<h3 id="2-null-memory-resource"><a href="#2-null-memory-resource" class="headerlink" title="2. null_memory_resource()"></a>2. <code>null_memory_resource()</code></h3><p>它返回一个指向全局“空”内存资源的指针，它在每次分配时抛出<code>std::bad_alloc</code>。虽然听起来没什么用，但当你想保证你的对象不会在堆上分配任何内存时，它可能会很方便。或用于测试。</p>
<h3 id="3-synchronized-pool-resource"><a href="#3-synchronized-pool-resource" class="headerlink" title="3. synchronized_pool_resource"></a>3. synchronized_pool_resource</h3><p>这是一个线程安全的分配器，用于管理不同大小的池。每个池都是一组块，这些块被分成大小均匀的块。</p>
<h3 id="4-unsynchronized-pool-resource"><a href="#4-unsynchronized-pool-resource" class="headerlink" title="4. unsynchronized_pool_resource"></a>4. unsynchronized_pool_resource</h3><p>非线程安全的资源池。</p>
<h3 id="5-monotonic-buffer-resource"><a href="#5-monotonic-buffer-resource" class="headerlink" title="5. monotonic_buffer_resource"></a>5. monotonic_buffer_resource</h3><p>这是一个非线程安全的、快速的、特殊用途的资源，它从预先分配的缓冲区中获取内存，但不会通过release来释放它。它只能单向递增。</p>
<h2 id="多态分配器示例"><a href="#多态分配器示例" class="headerlink" title="多态分配器示例"></a>多态分配器示例</h2><p>使用<code>monotonic_buffer_resource</code>和<code>pmr::vector</code>的简单例子.</p>
<pre class="line-numbers language-cpp" data-language="cpp"><code class="language-cpp"><span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><iostream></span></span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><memory_resource></span></span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><vector></span></span>
<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token keyword">int</span> argc<span class="token punctuation">,</span> <span class="token keyword">char</span><span class="token operator">*</span> argv<span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">char</span> buffer<span class="token punctuation">[</span><span class="token number">64</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token number">0</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span><span class="token function">fill_n</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span><span class="token function">begin</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span><span class="token punctuation">,</span> std<span class="token double-colon punctuation">::</span><span class="token function">size</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span> <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token char">'-'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> buffer <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>monotonic_buffer_resource pool<span class="token punctuation">{</span>std<span class="token double-colon punctuation">::</span><span class="token function">data</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span><span class="token punctuation">,</span>
std<span class="token double-colon punctuation">::</span><span class="token function">size</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>vector<span class="token operator"><</span><span class="token keyword">char</span><span class="token operator">></span> vec<span class="token punctuation">{</span><span class="token operator">&</span>pool<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">char</span> i <span class="token operator">=</span> <span class="token char">'a'</span><span class="token punctuation">;</span> i <span class="token operator"><=</span> <span class="token char">'z'</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
vec<span class="token punctuation">.</span><span class="token function">push_back</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> buffer <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>输出：</p>
<pre class="line-numbers language-none"><code class="language-none">---------------------------------------------------------------
aababcdabcdefghabcdefghijklmnopabcdefghijklmnopqrstuvwxyz------<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span></span></code></pre>
<p>在上面的例子中，我们使用了一个<code>monotonic_buffer_resource</code>，这个资源是用堆栈中的一个内存块(<code>buffer[]</code>)初始化的。通过使用一个简单的 <code>char buffer[]</code> 数组，我们可以轻松打印“内存”的内容。<code>vector</code>从池中获取内存（并且由于它在堆栈上，所以速度非常快），如果没有更多可用空间，它将从“上游”资源中请求内存。该示例显示了需要插入更多元素时的<code>vector</code>重新分配。向量每次获取更多空间来存放所有字母。正如您所见，<code>monotonic_buffer_resource</code>不会删除任何内存，它只会向前增长。</p>
<p>我们也可以在向量上使用<code>reserve()</code>，这会限制内存分配的数量，但这个例子的重点是说明容器的“扩展”。</p>
<p>在容器中存储比<code>char</code>大的类型会怎么样？</p>
<h3 id="存储pmr-string"><a href="#存储pmr-string" class="headerlink" title="存储pmr::string"></a>存储<code>pmr::string</code></h3><p>将字符串插入<code>pmr::vector</code>会怎么样？</p>
<p>多态分配器的好处在于，如果容器中的对象也使用多态分配器，那么它们将要求父容器的分配器来管理内存。</p>
<p>如果你想使用这个特性，你必须使用<code>std::pmr::string</code>而不是<code>std::string</code>.</p>
<p>请查看下面的示例，其中我们预先在堆栈上分配缓冲区，然后将其传递到字符串<code>vector</code>：</p>
<pre class="line-numbers language-cpp" data-language="cpp"><code class="language-cpp"><span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><iostream></span></span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><memory_resource></span></span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><vector></span></span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><string></span></span>
<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token keyword">int</span> argc<span class="token punctuation">,</span> <span class="token keyword">char</span><span class="token operator">*</span> argv<span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token string">"sizeof(std::string): "</span> <span class="token operator"><<</span> <span class="token keyword">sizeof</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span>string<span class="token punctuation">)</span> <span class="token operator"><<</span> <span class="token char">'\n'</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token string">"sizeof(std::pmr::string): "</span> <span class="token operator"><<</span> <span class="token keyword">sizeof</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>string<span class="token punctuation">)</span>
<span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token keyword">char</span> buffer<span class="token punctuation">[</span><span class="token number">256</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token number">0</span><span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">// a small buffer on the stack</span>
std<span class="token double-colon punctuation">::</span><span class="token function">fill_n</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span><span class="token function">begin</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span><span class="token punctuation">,</span> std<span class="token double-colon punctuation">::</span><span class="token function">size</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span> <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token char">'-'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> <span class="token keyword">auto</span> BufferPrinter <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span>string_view buf<span class="token punctuation">,</span>
std<span class="token double-colon punctuation">::</span>string_view title<span class="token punctuation">)</span> <span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> title <span class="token operator"><<</span> <span class="token string">":\n"</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">auto</span><span class="token operator">&</span> ch <span class="token operator">:</span> buf<span class="token punctuation">)</span> <span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token punctuation">(</span>ch <span class="token operator">>=</span> <span class="token char">' '</span> <span class="token operator">?</span> ch <span class="token operator">:</span> <span class="token char">'#'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token function">BufferPrinter</span><span class="token punctuation">(</span>buffer<span class="token punctuation">,</span> <span class="token string">"zeroed buffer"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>monotonic_buffer_resource pool<span class="token punctuation">{</span>std<span class="token double-colon punctuation">::</span><span class="token function">data</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span><span class="token punctuation">,</span>
std<span class="token double-colon punctuation">::</span><span class="token function">size</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>vector<span class="token operator"><</span>std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>string<span class="token operator">></span> vec<span class="token punctuation">{</span><span class="token operator">&</span>pool<span class="token punctuation">}</span><span class="token punctuation">;</span>
vec<span class="token punctuation">.</span><span class="token function">reserve</span><span class="token punctuation">(</span><span class="token number">5</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
vec<span class="token punctuation">.</span><span class="token function">push_back</span><span class="token punctuation">(</span><span class="token string">"Hello World"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
vec<span class="token punctuation">.</span><span class="token function">push_back</span><span class="token punctuation">(</span><span class="token string">"One Two Three"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">BufferPrinter</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span><span class="token function">string_view</span><span class="token punctuation">(</span>buffer<span class="token punctuation">,</span> std<span class="token double-colon punctuation">::</span><span class="token function">size</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
<span class="token string">"after two short strings"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
vec<span class="token punctuation">.</span><span class="token function">emplace_back</span><span class="token punctuation">(</span><span class="token string">"This is a longer string"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">BufferPrinter</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span><span class="token function">string_view</span><span class="token punctuation">(</span>buffer<span class="token punctuation">,</span> std<span class="token double-colon punctuation">::</span><span class="token function">size</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
<span class="token string">"after longer string strings"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
vec<span class="token punctuation">.</span><span class="token function">push_back</span><span class="token punctuation">(</span><span class="token string">"Four Five Six"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">BufferPrinter</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span><span class="token function">string_view</span><span class="token punctuation">(</span>buffer<span class="token punctuation">,</span> std<span class="token double-colon punctuation">::</span><span class="token function">size</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
<span class="token string">"after the last string"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>以下是我在gcc 11.1 上收到的输出</p>
<pre class="line-numbers language-shell" data-language="shell"><code class="language-shell">sizeof<span class="token punctuation">(</span>std::string<span class="token punctuation">)</span>: <span class="token number">32</span>
sizeof<span class="token punctuation">(</span>std::pmr::string<span class="token punctuation">)</span>: <span class="token number">40</span>
zeroed buffer:
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
after two short strings:
<span class="token variable"><span class="token variable">`</span><span class="token comment">#!6#####!6###########Hello World####</span><span class="token variable">`</span></span><span class="token comment">#!6#####!6###########One Two Three###-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------#</span>
after longer string strings:
<span class="token variable"><span class="token variable">`</span><span class="token comment">#!6#####!6###########Hello World####</span><span class="token variable">`</span></span><span class="token comment">#!6#####!6###########One Two Three###`#!6#####!6###################----------------------------------------------------------------------------------------This is a longer string#-------------------------------#</span>
after the last string:
<span class="token variable"><span class="token variable">`</span><span class="token comment">#!6#####!6###########Hello World####</span><span class="token variable">`</span></span><span class="token comment">#!6#####!6###########One Two Three###`#!6#####!6###################--------`#!6###`#!6###########Four Five Six###----------------------------------------This is a longer string#-------------------------------#</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>以下是这个例子观察到的主要事情：</p>
<ul>
<li><code>pmr::string</code>大小大于常规<code>std::string</code>。这是因为分配器不是无状态的，它必须存储指向内存资源的指针。</li>
<li>例子中的<code>vector</code>保留了五个元素，因此当插入四个元素时<code>vector</code>不会变长。</li>
<li>前两个字符串很短，因此它们可以放入<code>vector</code>的内存块中，此处没有动态内存分配。</li>
<li>但是对于第三个字符串，要求它是一个单独的内存块，并且<code>vector</code>只存储一个指向它的指针。正如您在输出中所见，`”This is a longer string”几乎位于缓冲区的末尾。</li>
<li>当我们插入另一个短字符串，然后它再次进入<code>vector</code>内存块。</li>
</ul>
<p>比较一下，下面使用常规<code>std::string</code>时的输出：</p>
<pre class="line-numbers language-shell" data-language="shell"><code class="language-shell">sizeof<span class="token punctuation">(</span>std::string<span class="token punctuation">)</span>: <span class="token number">32</span>
sizeof<span class="token punctuation">(</span>std::pmr::string<span class="token punctuation">)</span>: <span class="token number">40</span>
zeroed buffer:
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
after two short strings:
@l<span class="token comment">#############Hello World####`l#############One Two Three###-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------#</span>
after longer string strings:
@l<span class="token comment">#############Hello World####`l#############One Two Three#####n3#V##################-----------------------------------------------------------------------------------------------------------------------------------------------------------------------#</span>
after the last string:
@l<span class="token comment">#############Hello World####`l#############One Two Three#####n3#V##################--------#l#############Four Five Six###-------------------------------------------------------------------------------------------------------------------------------#</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>这次容器中的元素使用更少的内存，因为不需要存储指向内存资源的指针。短字符串存储在<code>vector</code>的内存块中，但请注意较长的字符串……它不在缓冲区中！正确地说，向量存储一个指向分配长字符串的内存块的指针，但默认分配器分配了它，因此它不会出现在我们的输出中。</p>
<p>我提到，如果内存资源不够，则分配器将从上游资源获取内存。我们如何观察它？</p>
<h3 id="一些手段"><a href="#一些手段" class="headerlink" title="一些手段"></a>一些手段</h3><p>在前面的例子中，上游内存资源是默认的，因为我们没有改变它。这意味着使用的是new() 和 delete()。但是，我们必须记住， do_allocate() 和 do_deallocate() 成员函数也采用对齐参数。</p>
<p>这就是为什么如果我们想要查看内存是否由 <code>new()</code> 分配，我们必须使用 C++17 带对齐支持的<code>new()</code> ：</p>
<pre class="line-numbers language-cpp" data-language="cpp"><code class="language-cpp"><span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><iostream></span></span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><memory_resource></span></span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><vector></span></span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><string></span></span>
<span class="token keyword">void</span><span class="token operator">*</span> lastAllocatedPtr <span class="token operator">=</span> <span class="token keyword">nullptr</span><span class="token punctuation">;</span>
size_t lastSize <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token keyword">void</span><span class="token operator">*</span> <span class="token keyword">operator</span> <span class="token keyword">new</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span>size_t size<span class="token punctuation">,</span> std<span class="token double-colon punctuation">::</span>align_val_t align<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">auto</span> ptr <span class="token operator">=</span> <span class="token function">aligned_alloc</span><span class="token punctuation">(</span><span class="token generic-function"><span class="token function">static_cast</span><span class="token generic class-name"><span class="token operator"><</span>std<span class="token double-colon punctuation">::</span>size_t<span class="token operator">></span></span></span><span class="token punctuation">(</span>align<span class="token punctuation">)</span><span class="token punctuation">,</span> size<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>ptr<span class="token punctuation">)</span> <span class="token keyword">throw</span> std<span class="token double-colon punctuation">::</span>bad_alloc<span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token string">"new: "</span> <span class="token operator"><<</span> size
<span class="token operator"><<</span> <span class="token string">", align: "</span> <span class="token operator"><<</span> <span class="token generic-function"><span class="token function">static_cast</span><span class="token generic class-name"><span class="token operator"><</span>std<span class="token double-colon punctuation">::</span>size_t<span class="token operator">></span></span></span><span class="token punctuation">(</span>align<span class="token punctuation">)</span>
<span class="token operator"><<</span> <span class="token string">", ptr: "</span> <span class="token operator"><<</span> ptr <span class="token operator"><<</span> <span class="token char">'\n'</span><span class="token punctuation">;</span>
lastAllocatedPtr <span class="token operator">=</span> ptr<span class="token punctuation">;</span>
lastSize <span class="token operator">=</span> size<span class="token punctuation">;</span>
<span class="token keyword">return</span> ptr<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">void</span> <span class="token keyword">operator</span> <span class="token keyword">delete</span><span class="token punctuation">(</span><span class="token keyword">void</span><span class="token operator">*</span> ptr<span class="token punctuation">,</span> std<span class="token double-colon punctuation">::</span>size_t size<span class="token punctuation">,</span>
std<span class="token double-colon punctuation">::</span>align_val_t align<span class="token punctuation">)</span> <span class="token keyword">noexcept</span>
<span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token string">"delete: "</span> <span class="token operator"><<</span> size
<span class="token operator"><<</span> <span class="token string">", align: "</span> <span class="token operator"><<</span> <span class="token generic-function"><span class="token function">static_cast</span><span class="token generic class-name"><span class="token operator"><</span>std<span class="token double-colon punctuation">::</span>size_t<span class="token operator">></span></span></span><span class="token punctuation">(</span>align<span class="token punctuation">)</span>
<span class="token operator"><<</span> <span class="token string">", ptr : "</span> <span class="token operator"><<</span> ptr <span class="token operator"><<</span> <span class="token char">'\n'</span><span class="token punctuation">;</span>
<span class="token function">free</span><span class="token punctuation">(</span>ptr<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token keyword">int</span> argc<span class="token punctuation">,</span> <span class="token keyword">char</span><span class="token operator">*</span> argv<span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">constexpr</span> size_t buf_size <span class="token operator">=</span> <span class="token number">32</span><span class="token punctuation">;</span>
<span class="token keyword">uint16_t</span> buffer<span class="token punctuation">[</span>buf_size<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token number">0</span><span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">// a small buffer on the stack</span>
std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>monotonic_buffer_resource pool<span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span><span class="token function">data</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span><span class="token punctuation">,</span> std<span class="token double-colon punctuation">::</span><span class="token function">size</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span> <span class="token operator">*</span> <span class="token keyword">sizeof</span><span class="token punctuation">(</span><span class="token keyword">uint16_t</span><span class="token punctuation">)</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>vector<span class="token operator"><</span><span class="token keyword">uint16_t</span><span class="token operator">></span> vec<span class="token punctuation">{</span><span class="token operator">&</span>pool<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span>size_t i <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span> i <span class="token operator"><=</span> <span class="token number">20</span><span class="token punctuation">;</span> <span class="token operator">++</span>i<span class="token punctuation">)</span> vec<span class="token punctuation">.</span><span class="token function">push_back</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span>size_t i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator"><</span> buf_size<span class="token punctuation">;</span> <span class="token operator">++</span>i<span class="token punctuation">)</span> std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> buffer<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator"><<</span> <span class="token string">" "</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token keyword">auto</span><span class="token operator">*</span> bufTemp <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token keyword">uint16_t</span><span class="token operator">*</span><span class="token punctuation">)</span>lastAllocatedPtr<span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span>size_t i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator"><</span> lastSize<span class="token punctuation">;</span> <span class="token operator">++</span>i<span class="token punctuation">)</span> std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> bufTemp<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator"><<</span> <span class="token string">" "</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>在上面的代码中实现了对齐的<code>new()</code></p>
<p>还有两个丑陋的全局变量 :) 然而，多亏了它们，我们可以看到我们的内存何时消失.</p>
<p>这一次，我们存储<code>uint16_t</code>而不是<code>char</code>.</p>
<p>该程序试图在一个向量中存储 20 个数字，但由于向量不断增长，我们需要的不仅仅是预定义的缓冲区（只有 32 ）。这就是为什么在某些时候分配器会转向全局<code>new</code>和<code>delete</code>。</p>
<p>以下可能获得的输出：</p>
<pre class="line-numbers language-shell" data-language="shell"><code class="language-shell">new: <span class="token number">128</span>, align: <span class="token number">16</span>, ptr: 0x55e22faebeb0
<span class="token number">1</span> <span class="token number">1</span> <span class="token number">2</span> <span class="token number">1</span> <span class="token number">2</span> <span class="token number">3</span> <span class="token number">4</span> <span class="token number">1</span> <span class="token number">2</span> <span class="token number">3</span> <span class="token number">4</span> <span class="token number">5</span> <span class="token number">6</span> <span class="token number">7</span> <span class="token number">8</span> <span class="token number">1</span> <span class="token number">2</span> <span class="token number">3</span> <span class="token number">4</span> <span class="token number">5</span> <span class="token number">6</span> <span class="token number">7</span> <span class="token number">8</span> <span class="token number">9</span> <span class="token number">10</span> <span class="token number">11</span> <span class="token number">12</span> <span class="token number">13</span> <span class="token number">14</span> <span class="token number">15</span> <span class="token number">16</span> <span class="token number">0</span>
<span class="token number">1</span> <span class="token number">2</span> <span class="token number">3</span> <span class="token number">4</span> <span class="token number">5</span> <span class="token number">6</span> <span class="token number">7</span> <span class="token number">8</span> <span class="token number">9</span> <span class="token number">10</span> <span class="token number">11</span> <span class="token number">12</span> <span class="token number">13</span> <span class="token number">14</span> <span class="token number">15</span> <span class="token number">16</span> <span class="token number">17</span> <span class="token number">18</span> <span class="token number">19</span> <span class="token number">20</span> <span class="token number">0</span> <span class="token number">0</span> <span class="token number">0</span> <span class="token number">0</span> <span class="token number">0</span> <span class="token number">0</span> <span class="token punctuation">..</span><span class="token punctuation">..</span><span class="token punctuation">..</span>
delete: <span class="token number">128</span>, align: <span class="token number">16</span>, ptr <span class="token builtin class-name">:</span> 0x55e22faebeb0<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>
<p>看起来预定义的缓冲区最多只能存储第 16 个元素，当我们插入数字 17 时，向量必须增长，这就是我们看到新分配128 字节的原因。第二行显示自定义缓冲区的内容，而第三行显示通过<code>new() </code>分配的内存。</p>
<h3 id="更好的解决方案"><a href="#更好的解决方案" class="headerlink" title="更好的解决方案"></a>更好的解决方案</h3><p>前面的示例有效并向我们展示了一些东西，但是在生产代码中使用<code>new()</code> 和 <code>delete()</code> 进行特殊操作并不是您应该做的。事实上，内存资源是可扩展的，如果你想要最好的解决方案，你可以重用你的资源！</p>
<p>您所要做的就是实现以下内容：</p>
<ul>
<li>继承<code>std::pmr::memory_resource</code></li>
<li>实现相关接口：<ul>
<li><code>do_allocate()</code></li>
<li><code>do_deallocate()</code></li>
<li><code>do_is_equal()</code></li>
</ul>
</li>
<li>将自定义内存资源设置为对象和容器的资源池。</li>
</ul>
<p>下面是一些如何实现的参考：</p>
<ul>
<li><a target="_blank" rel="noopener" href="https://www.youtube.com/watch?v=v3dz-AKOVL8">CppCon 2017: Pablo Halpern “Allocators: The Good Parts” - YouTube</a></li>
<li><a target="_blank" rel="noopener" href="https://www.youtube.com/watch?v=FcpmMmyNNv8">Taming dynamic memory - An introduction to custom allocators in C++ - Andreas Weis - code::dive 2018 - YouTube</a></li>
<li>A whole extensive chapter in Nicolai’s book on C++17: <a target="_blank" rel="noopener" href="https://leanpub.com/cpp17">C++17 - The Complete Guide</a>.</li>
<li><a target="_blank" rel="noopener" href="https://www.youtube.com/watch?v=q6A7cKFXjY0&">C++ Weekly - Ep 222 - 3.5x Faster Standard Containers With PMR! - YouTube</a></li>
</ul>
<h3 id="调试内存资源"><a href="#调试内存资源" class="headerlink" title="调试内存资源"></a>调试内存资源</h3><p>为了有效地使用分配器，如果有一个工具可以让我们跟踪容器中的内存分配，这会很方便。</p>
<pre class="line-numbers language-cpp" data-language="cpp"><code class="language-cpp"><span class="token keyword">class</span> <span class="token class-name">debug_resource</span> <span class="token operator">:</span> <span class="token base-clause"><span class="token keyword">public</span> std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span><span class="token class-name">memory_resource</span></span>
<span class="token punctuation">{</span>
<span class="token keyword">public</span><span class="token operator">:</span>
<span class="token keyword">explicit</span> <span class="token function">debug_resource</span><span class="token punctuation">(</span>
std<span class="token double-colon punctuation">::</span>string name<span class="token punctuation">,</span>
std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>memory_resource<span class="token operator">*</span> up <span class="token operator">=</span> std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span><span class="token function">get_default_resource</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token operator">:</span> _name<span class="token punctuation">{</span>std<span class="token double-colon punctuation">::</span><span class="token function">move</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span><span class="token punctuation">}</span><span class="token punctuation">,</span> _upstream<span class="token punctuation">{</span>up<span class="token punctuation">}</span>
<span class="token punctuation">{</span><span class="token punctuation">}</span>
<span class="token keyword">void</span><span class="token operator">*</span> <span class="token function">do_allocate</span><span class="token punctuation">(</span>size_t bytes<span class="token punctuation">,</span> size_t alignment<span class="token punctuation">)</span> <span class="token keyword">override</span>
<span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> _name <span class="token operator"><<</span> <span class="token string">" do_allocate(): "</span> <span class="token operator"><<</span> bytes <span class="token operator"><<</span> <span class="token char">'\n'</span><span class="token punctuation">;</span>
<span class="token keyword">void</span><span class="token operator">*</span> ret <span class="token operator">=</span> _upstream<span class="token operator">-></span><span class="token function">allocate</span><span class="token punctuation">(</span>bytes<span class="token punctuation">,</span> alignment<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> ret<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">void</span> <span class="token function">do_deallocate</span><span class="token punctuation">(</span><span class="token keyword">void</span><span class="token operator">*</span> ptr<span class="token punctuation">,</span> size_t bytes<span class="token punctuation">,</span> size_t alignment<span class="token punctuation">)</span> <span class="token keyword">override</span>
<span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> _name <span class="token operator"><<</span> <span class="token string">" do_deallocate(): "</span> <span class="token operator"><<</span> bytes <span class="token operator"><<</span> <span class="token char">'\n'</span><span class="token punctuation">;</span>
_upstream<span class="token operator">-></span><span class="token function">deallocate</span><span class="token punctuation">(</span>ptr<span class="token punctuation">,</span> bytes<span class="token punctuation">,</span> alignment<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">bool</span> <span class="token function">do_is_equal</span><span class="token punctuation">(</span>
<span class="token keyword">const</span> std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>memory_resource<span class="token operator">&</span> other<span class="token punctuation">)</span> <span class="token keyword">const</span> <span class="token keyword">noexcept</span> <span class="token keyword">override</span>
<span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token keyword">this</span> <span class="token operator">==</span> <span class="token operator">&</span>other<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">private</span><span class="token operator">:</span>
std<span class="token double-colon punctuation">::</span>string _name<span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>memory_resource<span class="token operator">*</span> _upstream<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p><code>debug_resource</code>只是实际内存资源的包装器。正如在<code>do_allocate</code>/<code>do_deallocate</code>函数中看到的那样，我们只记录数字，然后使用上游资源进行实际内存分配。</p>
<p>示例用例：</p>
<pre class="line-numbers language-cpp" data-language="cpp"><code class="language-cpp"><span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token keyword">int</span> argc<span class="token punctuation">,</span> <span class="token keyword">char</span><span class="token operator">*</span> argv<span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">constexpr</span> size_t BUF_SIZE <span class="token operator">=</span> <span class="token number">128</span><span class="token punctuation">;</span>
<span class="token keyword">char</span> buffer<span class="token punctuation">[</span>BUF_SIZE<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token number">0</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span><span class="token function">fill_n</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span><span class="token function">begin</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span><span class="token punctuation">,</span> std<span class="token double-colon punctuation">::</span><span class="token function">size</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span> <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token char">'-'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
debug_resource default_dbg<span class="token punctuation">{</span><span class="token string">"default"</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>monotonic_buffer_resource pool<span class="token punctuation">{</span>std<span class="token double-colon punctuation">::</span><span class="token function">data</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span><span class="token punctuation">,</span>
std<span class="token double-colon punctuation">::</span><span class="token function">size</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token operator">&</span>default_dbg<span class="token punctuation">}</span><span class="token punctuation">;</span>
debug_resource dbg<span class="token punctuation">{</span><span class="token string">"pool"</span><span class="token punctuation">,</span> <span class="token operator">&</span>pool<span class="token punctuation">}</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>vector<span class="token operator"><</span>std<span class="token double-colon punctuation">::</span>string<span class="token operator">></span> strings<span class="token punctuation">{</span><span class="token operator">&</span>dbg<span class="token punctuation">}</span><span class="token punctuation">;</span>
strings<span class="token punctuation">.</span><span class="token function">emplace_back</span><span class="token punctuation">(</span><span class="token string">"Hello Short String"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
strings<span class="token punctuation">.</span><span class="token function">emplace_back</span><span class="token punctuation">(</span><span class="token string">"Hello Short String 2"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>输出：</p>
<pre class="line-numbers language-shell" data-language="shell"><code class="language-shell">pool do_allocate<span class="token punctuation">(</span><span class="token punctuation">)</span>: <span class="token number">32</span>
pool do_allocate<span class="token punctuation">(</span><span class="token punctuation">)</span>: <span class="token number">64</span>
pool do_deallocate<span class="token punctuation">(</span><span class="token punctuation">)</span>: <span class="token number">32</span>
pool do_deallocate<span class="token punctuation">(</span><span class="token punctuation">)</span>: <span class="token number">64</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>
<p>上面我们使用了两次<code>debug_resource</code>，第一个“池”用于记录请求到<code>monotonic_buffer_resource</code>的每个分配。在输出中，您可以看到我们有两次分配和两次解除分配。</p>
<p>还有另一个<code>debug_resource</code> - <code>default</code>。这被配置为monotonic_buffer_resource 的父级。这意味着如果池需要分配。那么它必须通过我们的“default”对象请求内存。</p>
<p>如果您添加三个字符串，如下所示：</p>
<pre class="line-numbers language-cpp" data-language="cpp"><code class="language-cpp">strings<span class="token punctuation">.</span><span class="token function">emplace_back</span><span class="token punctuation">(</span><span class="token string">"Hello Short String"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
strings<span class="token punctuation">.</span><span class="token function">emplace_back</span><span class="token punctuation">(</span><span class="token string">"Hello Short String 2"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
strings<span class="token punctuation">.</span><span class="token function">emplace_back</span><span class="token punctuation">(</span><span class="token string">"Hello A bit longer String"</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span></span></code></pre>
<p>那么输出就不一样了：</p>
<pre class="line-numbers language-shell" data-language="shell"><code class="language-shell">pool do_allocate<span class="token punctuation">(</span><span class="token punctuation">)</span>: <span class="token number">32</span>
pool do_allocate<span class="token punctuation">(</span><span class="token punctuation">)</span>: <span class="token number">64</span>
pool do_deallocate<span class="token punctuation">(</span><span class="token punctuation">)</span>: <span class="token number">32</span>
pool do_allocate<span class="token punctuation">(</span><span class="token punctuation">)</span>: <span class="token number">128</span>
default do_allocate<span class="token punctuation">(</span><span class="token punctuation">)</span>: <span class="token number">256</span>
pool do_deallocate<span class="token punctuation">(</span><span class="token punctuation">)</span>: <span class="token number">64</span>
pool do_deallocate<span class="token punctuation">(</span><span class="token punctuation">)</span>: <span class="token number">128</span>
default do_deallocate<span class="token punctuation">(</span><span class="token punctuation">)</span>: <span class="token number">256</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>这次您可以注意到，对于第三个字符串，我们预定义的小缓冲区内没有空间，这就是为什么<code>monotonic_buffer_resource</code>必须要求另外256个字节的“default”。</p>
<h2 id="自定义类型"><a href="#自定义类型" class="headerlink" title="自定义类型"></a>自定义类型</h2><p>定义一个自定义类型：</p>
<pre class="line-numbers language-cpp" data-language="cpp"><code class="language-cpp"><span class="token keyword">struct</span> <span class="token class-name">Product</span> <span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span>string name<span class="token punctuation">;</span>
<span class="token keyword">char</span> cost<span class="token punctuation">{</span><span class="token number">0</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>
<p>如果将它插入向量：</p>
<pre class="line-numbers language-cpp" data-language="cpp"><code class="language-cpp">std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>vector<span class="token operator"><</span>Product<span class="token operator">></span> vec<span class="token punctuation">{</span><span class="token operator">&</span>pool<span class="token punctuation">}</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>
<p>然后，vector将使用提供的内存资源，但不会应用于自定义类型中。所以，如果 Product 必须为name分配内存，它将使用默认分配器。</p>
<p>我们必须“启用”我们的类型并让它知道分配器，以便它可以利用来自父容器的分配器。</p>
<p>配备了调试资源和一些“缓冲区打印技术”，我们现在可以检查我们的自定义类型是否与分配器一起工作。让我们来看看：</p>
<pre class="line-numbers language-cpp" data-language="cpp"><code class="language-cpp"><span class="token keyword">struct</span> <span class="token class-name">SimpleProduct</span> <span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span>string name<span class="token punctuation">;</span>
<span class="token keyword">char</span> cost<span class="token punctuation">{</span><span class="token number">0</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token keyword">int</span> argc<span class="token punctuation">,</span> <span class="token keyword">char</span><span class="token operator">*</span> argv<span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">constexpr</span> size_t BUF_SIZE <span class="token operator">=</span> <span class="token number">256</span><span class="token punctuation">;</span>
<span class="token keyword">char</span> buffer<span class="token punctuation">[</span>BUF_SIZE<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token number">0</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span><span class="token function">fill_n</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span><span class="token function">begin</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span><span class="token punctuation">,</span> std<span class="token double-colon punctuation">::</span><span class="token function">size</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span> <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token char">'-'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> <span class="token keyword">auto</span> BufferPrinter <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span>string_view buf<span class="token punctuation">,</span>
std<span class="token double-colon punctuation">::</span>string_view title<span class="token punctuation">)</span> <span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> title <span class="token operator"><<</span> <span class="token string">":\n"</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span>size_t i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator"><</span> buf<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token operator">++</span>i<span class="token punctuation">)</span> <span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token punctuation">(</span>buf<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator">>=</span> <span class="token char">' '</span> <span class="token operator">?</span> buf<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator">:</span> <span class="token char">'#'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>i <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">%</span> <span class="token number">64</span> <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token char">'\n'</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token char">'\n'</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token function">BufferPrinter</span><span class="token punctuation">(</span>buffer<span class="token punctuation">,</span> <span class="token string">"initial buffer"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
debug_resource default_dbg<span class="token punctuation">{</span><span class="token string">"default"</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>monotonic_buffer_resource pool<span class="token punctuation">{</span>std<span class="token double-colon punctuation">::</span><span class="token function">data</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span><span class="token punctuation">,</span>
std<span class="token double-colon punctuation">::</span><span class="token function">size</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token operator">&</span>default_dbg<span class="token punctuation">}</span><span class="token punctuation">;</span>
debug_resource dbg<span class="token punctuation">{</span><span class="token string">"buffer"</span><span class="token punctuation">,</span> <span class="token operator">&</span>pool<span class="token punctuation">}</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>vector<span class="token operator"><</span>SimpleProduct<span class="token operator">></span> products<span class="token punctuation">{</span><span class="token operator">&</span>dbg<span class="token punctuation">}</span><span class="token punctuation">;</span>
products<span class="token punctuation">.</span><span class="token function">reserve</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
products<span class="token punctuation">.</span><span class="token function">emplace_back</span><span class="token punctuation">(</span>SimpleProduct<span class="token punctuation">{</span><span class="token string">"car"</span><span class="token punctuation">,</span> <span class="token char">'7'</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
products<span class="token punctuation">.</span><span class="token function">emplace_back</span><span class="token punctuation">(</span>SimpleProduct<span class="token punctuation">{</span><span class="token string">"TV"</span><span class="token punctuation">,</span> <span class="token char">'9'</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
products<span class="token punctuation">.</span><span class="token function">emplace_back</span><span class="token punctuation">(</span>SimpleProduct<span class="token punctuation">{</span><span class="token string">"a bit longer product name"</span><span class="token punctuation">,</span> <span class="token char">'4'</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">BufferPrinter</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span>string_view<span class="token punctuation">{</span>buffer<span class="token punctuation">,</span> BUF_SIZE<span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token string">"after insertion"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>输出：</p>
<pre class="line-numbers language-shell" data-language="shell"><code class="language-shell">initial buffer:
----------------------------------------------------------------
----------------------------------------------------------------
----------------------------------------------------------------
---------------------------------------------------------------
buffer do_allocate<span class="token punctuation">(</span><span class="token punctuation">)</span>: <span class="token number">160</span>
after insertion:
<span class="token comment">###############car#er##@P#####7-------###############TV##er##</span>
@P<span class="token comment">#####9-------##N##U##################--------4---------------</span>
----------------------------------------------------------------
---------------------------------------------------------------<span class="token comment">#</span>
buffer do_deallocate<span class="token punctuation">(</span><span class="token punctuation">)</span>: <span class="token number">160</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>让我们解密代码和输出：</p>
<p>该<code>vector</code>包含<code>SimpleProduct</code>对象，它只是一个字符串和一个数字。我们保留了四个元素，您可以注意到我们的调试资源记录了 160 字节的分配。插入三个元素后，我们可以发现car和数字 7（这就是我使用 char 作为价格类型的原因）。然后是 9 的TV。我们也可以注意到<code>4</code>作为第三个元素的价格，但没有名字<code>a bit longer product name</code></p>
<p>让自定义类型分配器感知并不是很难，但我们必须记住以下几点：</p>
<ul>
<li>尽可能使用 pmr::* 类型，以便您可以将分配器传递给它们。</li>
<li>声明 allocator_type 以便分配器特征可以“识别”您的类型使用分配器。您还可以为分配器特征声明其他属性，但在大多数情况下，默认值就可以了。</li>
<li>声明接受分配器并将其进一步传递给您的成员的构造函数。</li>
<li>也注意声明分配器的复制和移动构造函数。</li>
<li>与分配和移动操作相同。</li>
</ul>
<p>这意味着我们相对简单的自定义类型声明必须变长：</p>
<pre class="line-numbers language-cpp" data-language="cpp"><code class="language-cpp"><span class="token keyword">struct</span> <span class="token class-name">Product</span> <span class="token punctuation">{</span>
<span class="token keyword">using</span> allocator_type <span class="token operator">=</span> std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>polymorphic_allocator<span class="token operator"><</span><span class="token keyword">char</span><span class="token operator">></span><span class="token punctuation">;</span>
<span class="token keyword">explicit</span> <span class="token function">Product</span><span class="token punctuation">(</span>allocator_type alloc <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token operator">:</span> _name<span class="token punctuation">{</span>alloc<span class="token punctuation">}</span>
<span class="token punctuation">{</span><span class="token punctuation">}</span>
<span class="token function">Product</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>string name<span class="token punctuation">,</span> <span class="token keyword">char</span> price<span class="token punctuation">,</span>
<span class="token keyword">const</span> allocator_type<span class="token operator">&</span> alloc <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token operator">:</span> _name<span class="token punctuation">{</span>std<span class="token double-colon punctuation">::</span><span class="token function">move</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span><span class="token punctuation">,</span> alloc<span class="token punctuation">}</span><span class="token punctuation">,</span> _price<span class="token punctuation">{</span>price<span class="token punctuation">}</span>
<span class="token punctuation">{</span><span class="token punctuation">}</span>
<span class="token function">Product</span><span class="token punctuation">(</span><span class="token keyword">const</span> Product<span class="token operator">&</span> other<span class="token punctuation">,</span> <span class="token keyword">const</span> allocator_type<span class="token operator">&</span> alloc<span class="token punctuation">)</span>
<span class="token operator">:</span> _name<span class="token punctuation">{</span>other<span class="token punctuation">.</span>_name<span class="token punctuation">,</span> alloc<span class="token punctuation">}</span><span class="token punctuation">,</span> _price<span class="token punctuation">{</span>other<span class="token punctuation">.</span>_price<span class="token punctuation">}</span>
<span class="token punctuation">{</span><span class="token punctuation">}</span>
<span class="token function">Product</span><span class="token punctuation">(</span>Product<span class="token operator">&&</span> other<span class="token punctuation">,</span> <span class="token keyword">const</span> allocator_type<span class="token operator">&</span> alloc<span class="token punctuation">)</span>
<span class="token operator">:</span> _name<span class="token punctuation">{</span>std<span class="token double-colon punctuation">::</span><span class="token function">move</span><span class="token punctuation">(</span>other<span class="token punctuation">.</span>_name<span class="token punctuation">)</span><span class="token punctuation">,</span> alloc<span class="token punctuation">}</span><span class="token punctuation">,</span> _price<span class="token punctuation">{</span>other<span class="token punctuation">.</span>_price<span class="token punctuation">}</span>
<span class="token punctuation">{</span><span class="token punctuation">}</span>
Product<span class="token operator">&</span> <span class="token keyword">operator</span><span class="token operator">=</span><span class="token punctuation">(</span><span class="token keyword">const</span> Product<span class="token operator">&</span> other<span class="token punctuation">)</span> <span class="token operator">=</span> <span class="token keyword">default</span><span class="token punctuation">;</span>
Product<span class="token operator">&</span> <span class="token keyword">operator</span><span class="token operator">=</span><span class="token punctuation">(</span>Product<span class="token operator">&&</span> other<span class="token punctuation">)</span> <span class="token operator">=</span> <span class="token keyword">default</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>string _name<span class="token punctuation">;</span>
<span class="token keyword">char</span> _price<span class="token punctuation">{</span><span class="token char">'0'</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token keyword">int</span> argc<span class="token punctuation">,</span> <span class="token keyword">char</span><span class="token operator">*</span> argv<span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">constexpr</span> size_t BUF_SIZE <span class="token operator">=</span> <span class="token number">256</span><span class="token punctuation">;</span>
<span class="token keyword">char</span> buffer<span class="token punctuation">[</span>BUF_SIZE<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token number">0</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span><span class="token function">fill_n</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span><span class="token function">begin</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span><span class="token punctuation">,</span> std<span class="token double-colon punctuation">::</span><span class="token function">size</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span> <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token char">'-'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> <span class="token keyword">auto</span> BufferPrinter <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span>string_view buf<span class="token punctuation">,</span>
std<span class="token double-colon punctuation">::</span>string_view title<span class="token punctuation">)</span> <span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> title <span class="token operator"><<</span> <span class="token string">":\n"</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span>size_t i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator"><</span> buf<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token operator">++</span>i<span class="token punctuation">)</span> <span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token punctuation">(</span>buf<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator">>=</span> <span class="token char">' '</span> <span class="token operator">?</span> buf<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator">:</span> <span class="token char">'#'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>i <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">%</span> <span class="token number">64</span> <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token char">'\n'</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> <span class="token char">'\n'</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
debug_resource default_dbg<span class="token punctuation">{</span><span class="token string">"default"</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>monotonic_buffer_resource pool<span class="token punctuation">{</span>std<span class="token double-colon punctuation">::</span><span class="token function">data</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span><span class="token punctuation">,</span>
std<span class="token double-colon punctuation">::</span><span class="token function">size</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token operator">&</span>default_dbg<span class="token punctuation">}</span><span class="token punctuation">;</span>
debug_resource dbg<span class="token punctuation">{</span><span class="token string">"buffer"</span><span class="token punctuation">,</span> <span class="token operator">&</span>pool<span class="token punctuation">}</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span>pmr<span class="token double-colon punctuation">::</span>vector<span class="token operator"><</span>Product<span class="token operator">></span> products<span class="token punctuation">{</span><span class="token operator">&</span>dbg<span class="token punctuation">}</span><span class="token punctuation">;</span>
products<span class="token punctuation">.</span><span class="token function">reserve</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
products<span class="token punctuation">.</span><span class="token function">emplace_back</span><span class="token punctuation">(</span>Product<span class="token punctuation">{</span><span class="token string">"car"</span><span class="token punctuation">,</span> <span class="token char">'7'</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
products<span class="token punctuation">.</span><span class="token function">emplace_back</span><span class="token punctuation">(</span>Product<span class="token punctuation">{</span><span class="token string">"TV"</span><span class="token punctuation">,</span> <span class="token char">'9'</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
products<span class="token punctuation">.</span><span class="token function">emplace_back</span><span class="token punctuation">(</span>Product<span class="token punctuation">{</span><span class="token string">"a bit longer product name"</span><span class="token punctuation">,</span> <span class="token char">'4'</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">BufferPrinter</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span>string_view<span class="token punctuation">{</span>buffer<span class="token punctuation">,</span> BUF_SIZE<span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token string">"after insertion"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>输出：</p>
<pre class="line-numbers language-shell" data-language="shell"><code class="language-shell">buffer do_allocate<span class="token punctuation">(</span><span class="token punctuation">)</span>: <span class="token number">192</span>
buffer do_allocate<span class="token punctuation">(</span><span class="token punctuation">)</span>: <span class="token number">26</span>
after insertion:
<span class="token comment">#######(##############car#+## ###+##7-------#######X######</span>
<span class="token comment">########TV##+## ###+##9-------##############################</span>
--------4-------------------------------------------------------
a bit longer product name<span class="token comment">#-------------------------------------#</span>
buffer do_deallocate<span class="token punctuation">(</span><span class="token punctuation">)</span>: <span class="token number">26</span>
buffer do_deallocate<span class="token punctuation">(</span><span class="token punctuation">)</span>: <span class="token number">192</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>在输出中，第一个内存分配<code>192</code>字节用于<code>vector.reserve(3)</code>，然后有另一个用于更长的字符串（第三个元素）。还打印了完整的缓冲区，显示字符串所在的位置。</p>
<p>我们的自定义对象由其他<code>pmr</code>容器组成，因此更加简单！而且我想在大多数情况下您可以利用现有类型。但是，如果您需要访问分配器并执行自定义内存分配，那么您应该查看 Pablo 的演讲，他在其中介绍了自定义列表容器的示例。</p>
<p><a target="_blank" rel="noopener" href="https://www.youtube.com/watch?v=v3dz-AKOVL8">CppCon 2017: Pablo Halpern “Allocators: The Good Parts” - YouTube</a></p>
<h2 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h2><p>通过本文，我想向您展示一些有关pmr的基本示例和多态分配器的概念。如您所见，为<code>vector</code>设置分配器比使用常规分配器简单得多。您可以使用一组预定义的分配器，并且实现您的自定义版本相对容易。文章中的代码仅展示了一个简单的例子，以说明从何处获取内存。</p>
<p>在这篇博文中，我们在标准库的深层进行了另一次旅程。虽然分配器很可怕，但似乎多态分配器让事情变得更舒服。</p>
<h2 id="参考"><a href="#参考" class="headerlink" title="参考"></a>参考</h2><ul>
<li><a target="_blank" rel="noopener" href="https://www.youtube.com/watch?v=v3dz-AKOVL8">CppCon 2017: Pablo Halpern “Allocators: The Good Parts” - YouTube</a> - in-depth explanations of allocators and the new PMR stuff. Even with a test implementation of some node-based container.</li>
<li><a target="_blank" rel="noopener" href="https://www.youtube.com/watch?v=LIb3L4vKZ7U">CppCon 2015: Andrei Alexandrescu “std::allocator…” - YouTube</a> - from the introduction you can learn than <code>std::allocator</code> was meant to fix far/near issues and make it consistent, but right now we want much more from this system.</li>
<li><a target="_blank" rel="noopener" href="https://stackoverflow.com/questions/4502691/what-is-the-purpose-of-allocator-traitst-in-c0x">c++ - What is the purpose of allocator_traits in C++0x? - Stack Overflow</a></li>
<li><a target="_blank" rel="noopener" href="https://jguegant.github.io/blogs/tech/dense-hash-map3.html#dense-hash-map3">Jean Guegant’s Blog – Making a STL-compatible hash map from scratch - Part 3 - The wonderful world of iterators and allocators</a> - this is a super detailed blog post on how to make more use of allocators, not to mention good anecdotes and jokes :)</li>
<li><a target="_blank" rel="noopener" href="https://blog.feabhas.com/2019/03/thanks-for-the-memory-allocator/">Thanks for the memory (allocator) - Sticky Bits</a> - a valuable introduction to allocators, their story and how the new model of PMR fit in. You can also see how to write your tracking pmr allocator and how <code>*_pool_resource</code> works.</li>
<li><a target="_blank" rel="noopener" href="https://www.youtube.com/watch?v=IejdKidUwIg">CppCon 2018: Arthur O’Dwyer “An Allocator is a Handle to a Heap”</a> - a great talk from Arthur where he shares all the knowledge needed to understand allocators.</li>
<li><a target="_blank" rel="noopener" href="https://amzn.to/3gsR3u9">C++17 - The Complete Guide</a> by Nicolai Josuttis - inside the book, there’s a long chapter about PMR allocators.</li>
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