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90 changes: 44 additions & 46 deletions 09_iterators.html
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Original file line number Diff line number Diff line change
Expand Up @@ -140,7 +140,7 @@ <h2>History of iterators</h2>
and for all.</p>

<p>Let me tell you a little about them.
Anybody who programs in C++ is forced to use vector
Anybody who programs in C++ is forced to use <code>std::vector</code>
and those have iterators.
But, there are lots of people who do not quite understand what they are,
partially because iterators are called iterators.
Expand All @@ -157,12 +157,8 @@ <h2>History of iterators</h2>
But, many people at <a href="https://en.wikipedia.org/wiki/Carnegie_Mellon_University">CMU</a> got tenure because of it.
It has some interesting ideas, including the idea of a generator.
For those of you who know <a href="https://en.wikipedia.org/wiki/Python_(programming_language)">Python</a>, it is like an iterator in Python<sup id="fnref:1"><a href="#fn:1" rel="footnote">1</a></sup>.
Barbara Liskov said, wouldn&rsquo;t it be nice to write:</p>

<pre><code>for x in thing
</code></pre>

<p>and iterators allow you to do that<sup id="fnref:2"><a href="#fn:2" rel="footnote">2</a></sup>.
Barbara Liskov said, &ldquo;wouldn&rsquo;t it be nice to write something like: <code>for x in thing</code>&rdquo;.
Iterators allow you to do that<sup id="fnref:2"><a href="#fn:2" rel="footnote">2</a></sup>.
It is like a generator in Alphard.
It is a procedure which returns multiple values, one by one.
It&rsquo;s a procedural abstraction.
Expand All @@ -173,12 +169,12 @@ <h2>History of iterators</h2>
<p>At the same time, I was working on how to do algorithms and I
introduced the notion of position.
A better name is coordinate, the name which Paul and I use
in our book &ldquo;Elements of Programming&rdquo;<sup id="fnref:3"><a href="#fn:3" rel="footnote">3</a></sup>.
in our book &ldquo;Elements of Programming&rdquo;.
A coordinate is some way of indicating where in the data structure you are.
It is not a control structure, it&rsquo;s just the pointer into a data structure,
or generalized notion of a coordinate.
It is something which allows me to navigate through the data structure in a
natural way.</p>
natural way<sup id="fnref:3"><a href="#fn:3" rel="footnote">3</a></sup>.</p>

<p>Eventually I started talking to C++ guys and showed them coordinates
and they said, &ldquo;we call it iterator&rdquo;.
Expand Down Expand Up @@ -209,8 +205,8 @@ <h2>History of iterators</h2>
Maybe I was stupid,
but I wasn&rsquo;t ignorant.</p>

<a name="List-pool-iterators"></a>
<h2>List pool iterators</h2>
<a name="Affiliated-types-for-iterators"></a>
<h2>Affiliated types for iterators</h2>

<p>We always need to distinguish between how we type
code in C++ and what the notion is behind it.
Expand All @@ -224,18 +220,18 @@ <h2>List pool iterators</h2>
gives me a pointer type, I want a way to find out what type
it points to.</p>

<p>To do this we need this notion of <em>type functions</em> which accept one type
<p>To do this we need this notion of <strong>type functions</strong> which accept one type
and return a different type.
This problem of needing to obtain affiliated types is not specific to C and C++.
It appears in Java and Python as well.
In spite of Python&rsquo;s <a href="https://en.wikipedia.org/wiki/Duck_typing">duck typing</a> there&rsquo;s <em>still</em> a connection
between types, even if they are duck types.</p>

<p>So we need this notion of <em>type functions</em>,
<p>So we need this notion of type functions,
but C doesn&rsquo;t let us do this,
and neither does C++.
Instead of type functions we are going to solve
this problem for iterators by using typedefs.</p>
this problem for iterators by using <code>typedef</code>s.</p>

<p>For an iterator, there are <a href="https://en.cppreference.com/w/cpp/iterator/iterator_traits">5 types</a> that we always need to define.
Three of them are primary, two are secondary.
Expand All @@ -257,32 +253,12 @@ <h2>List pool iterators</h2>

<p> In C++ (without concepts) we use tag types to designate the iterator categories.
Every iterator uses a tag type to denote which theory it supports.
The tag lets you do compile time dispatch<sup id="fnref:5"><a href="#fn:5" rel="footnote">5</a></sup>.</p>

<p> What category is the iterator for <code>list_pool</code> from last chapter?
We need <code>ForwardIterator</code>, because there is no way in a singly linked list
to go backwards.</p></li>
The tag lets you do compile time dispatch<sup id="fnref:5"><a href="#fn:5" rel="footnote">5</a></sup>.</p></li>
</ol>


<p>Let&rsquo;s define these types in the <code>list_pool</code> iterator.</p>

<pre><code>#include &lt;iterator&gt;

// class list_pool {
// ...

struct iterator {
typedef list_pool::value_type value_type;
typedef list_pool::list_type difference_type;
typedef std::forward_iterator_tag iterator_category;
};

// };
</code></pre>

<a name="Iterator-reference-types"></a>
<h3>Iterator reference types</h3>
<a name="Historical-artifacts"></a>
<h3>Historical artifacts</h3>

<p>The fourth and fifth types to be defined are required only for historical reasons.
Microsoft said they were going to vote against STL unless it accommodated
Expand Down Expand Up @@ -317,15 +293,33 @@ <h3>Iterator reference types</h3>
<p>&emsp;4. <code>reference</code>: the type of a reference to the value. <br/>
&emsp;5. <code>pointer</code>: the type of a pointer to the value.</p>

<p>Here is our implementation:</p>

<pre><code>typedef value_type&amp; reference;
typedef value_type* pointer;
</code></pre>

<p>It&rsquo;s not particularly harmful, but it obfuscates things
and it provides &ldquo;language experts&rdquo; with steady employment.</p>

<a name="List-pool-iterators"></a>
<h2>List pool iterators</h2>

<p>Let&rsquo;s define these types in the <code>list_pool</code> iterator.
What category is the iterator for <code>list_pool</code> from last chapter?
We need <code>ForwardIterator</code>, because there is no way in a singly linked list
to go backwards.</p>

<pre><code>#include &lt;iterator&gt;

// class list_pool {
// ...

struct iterator {
typedef list_pool::value_type value_type;
typedef list_pool::list_type difference_type;
typedef std::forward_iterator_tag iterator_category;
typedef value_type&amp; reference;
typedef value_type* pointer;
};

// };
</code></pre>

<a name="Constructors"></a>
<h3>Constructors</h3>

Expand Down Expand Up @@ -528,7 +522,7 @@ <h3>Everything on a computer is totally ordered</h3>
but it establishes a total ordering.
A total ordering does not have to be topologically induced by the traversal.</p>

<p><strong>Exercise:</strong> extend the iterator to linked iterator
<p><strong>Exercise:</strong> Extend the iterator to linked iterator
so we can assign the next on a node.
Specifically we want to be able to modify the successor of
an iterator.</p>
Expand Down Expand Up @@ -576,9 +570,13 @@ <h2>Code</h2>
<li id="fn:2">
See this <a href="http://web.mit.edu/ghudson/info/iterators">brief description of CLU iterators</a>.<a href="#fnref:2" rev="footnote">&#8617;</a></li>
<li id="fn:3">
See chapter 7 of &ldquo;Elements of Programming&rdquo; on coordinate structures.<a href="#fnref:3" rev="footnote">&#8617;</a></li>
See chapter 7 of &ldquo;Elements of Programming&rdquo; on coordinate structures.
An interesting discussion on the general idea of &ldquo;coordinatisation&rdquo;
is found in chapter 1 of &ldquo;Basic Notions of Algebra&rdquo; by Shafarevich.<a href="#fnref:3" rev="footnote">&#8617;</a></li>
<li id="fn:4">
<a href="https://www.boost.org/">Boost</a> is a popular collection of C++ libraries generally accepted as the next tool to reach for beyond the standard library.
<a href="https://www.boost.org/">Boost</a> is a popular collection of C++ libraries, covering a wide range of uses,
generally accepted as the next tool to reach for beyond the standard library.
Many standard library features, such as smart pointers, were initially developed in Boost.
Alex speaks positively of some parts (see <a href="http://stepanovpapers.com/siekforeword.pdf">his foreword</a> for &ldquo;The Boost Graph Library&rdquo;), but others he is more critical of.<a href="#fnref:4" rev="footnote">&#8617;</a></li>
<li id="fn:5">
Some algorithms can be implemented more efficiently for certain
Expand Down
106 changes: 52 additions & 54 deletions 09_iterators.md
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Expand Up @@ -10,7 +10,7 @@ other algorithms and learn to write iterators right once
and for all.

Let me tell you a little about them.
Anybody who programs in C++ is forced to use vector
Anybody who programs in C++ is forced to use `std::vector`
and those have iterators.
But, there are lots of people who do not quite understand what they are,
partially because iterators are called iterators.
Expand All @@ -27,11 +27,8 @@ It's a mythical language. It was never implemented.
But, many people at [CMU][cmu] got tenure because of it.
It has some interesting ideas, including the idea of a generator.
For those of you who know [Python][python], it is like an iterator in Python[^python-iterator].
Barbara Liskov said, wouldn't it be nice to write:

for x in thing

and iterators allow you to do that[^clu-iterators].
Barbara Liskov said, "wouldn't it be nice to write something like: `for x in thing`".
Iterators allow you to do that[^clu-iterators].
It is like a generator in Alphard.
It is a procedure which returns multiple values, one by one.
It's a procedural abstraction.
Expand All @@ -42,12 +39,12 @@ It was a generalization of a control structure.
At the same time, I was working on how to do algorithms and I
introduced the notion of position.
A better name is coordinate, the name which Paul and I use
in our book "Elements of Programming"[^eop].
in our book "Elements of Programming".
A coordinate is some way of indicating where in the data structure you are.
It is not a control structure, it's just the pointer into a data structure,
or generalized notion of a coordinate.
It is something which allows me to navigate through the data structure in a
natural way.
natural way[^coordinate-references].

Eventually I started talking to C++ guys and showed them coordinates
and they said, "we call it iterator".
Expand Down Expand Up @@ -78,9 +75,13 @@ It wasn't out of ignorance.
Maybe I was stupid,
but I wasn't ignorant.

[^eop]: See chapter 7 of "Elements of Programming" on coordinate structures.
[^coordinate-references]: See chapter 7 of "Elements of Programming" on coordinate structures.
An interesting discussion on the general idea of "coordinatisation"
is found in chapter 1 of "Basic Notions of Algebra" by Shafarevich.

[^boost]: [Boost][boost] is a popular collection of C++ libraries generally accepted as the next tool to reach for beyond the standard library.
[^boost]: [Boost][boost] is a popular collection of C++ libraries, covering a wide range of uses,
generally accepted as the next tool to reach for beyond the standard library.
Many standard library features, such as smart pointers, were initially developed in Boost.
Alex speaks positively of some parts (see [his foreword][alex-graph-foreword] for "The Boost Graph Library"), but others he is more critical of.


Expand Down Expand Up @@ -125,7 +126,8 @@ but I wasn't ignorant.
[snyder]: https://dblp.org/pid/04/4444.html
[alex-graph-foreword]: http://stepanovpapers.com/siekforeword.pdf

## List pool iterators

## Affiliated types for iterators

We always need to distinguish between how we type
code in C++ and what the notion is behind it.
Expand All @@ -139,18 +141,18 @@ way to obtain `int` from `int*`. That is, if somebody
gives me a pointer type, I want a way to find out what type
it points to.

To do this we need this notion of *type functions* which accept one type
To do this we need this notion of **type functions** which accept one type
and return a different type.
This problem of needing to obtain affiliated types is not specific to C and C++.
It appears in Java and Python as well.
In spite of Python's [duck typing][duck] there's *still* a connection
between types, even if they are duck types.

So we need this notion of *type functions*,
So we need this notion of type functions,
but C doesn't let us do this,
and neither does C++.
Instead of type functions we are going to solve
this problem for iterators by using typedefs.
this problem for iterators by using `typedef`s.

For an iterator, there are [5 types][cpp-iterator-traits] that we always need to define.
Three of them are primary, two are secondary.
Expand All @@ -175,40 +177,7 @@ Let's start with the primary:
Every iterator uses a tag type to denote which theory it supports.
The tag lets you do compile time dispatch[^compile-time-dispatch].

What category is the iterator for `list_pool` from last chapter?
We need `ForwardIterator`, because there is no way in a singly linked list
to go backwards.

Let's define these types in the `list_pool` iterator.

#include <iterator>

// class list_pool {
// ...

struct iterator {
typedef list_pool::value_type value_type;
typedef list_pool::list_type difference_type;
typedef std::forward_iterator_tag iterator_category;
};

// };


[^compile-time-dispatch]: Some algorithms can be implemented more efficiently for certain
iterator categories. For example [`std::distance`][std-distance] can be
implemented as a constant time algorithm for `RandomAccessIterators` but
only a linear time algorithm for other iterator categories. The
`iterator_category` tag allows the appropriate algorithm to be selected at
compile time. This technique is known as [tag disptach][tag-dispatch].

[duck]: https://en.wikipedia.org/wiki/Duck_typing
[ptrdiff]: https://en.cppreference.com/w/c/types/ptrdiff_t
[cpp-iterator-traits]: https://en.cppreference.com/w/cpp/iterator/iterator_traits
[std-distance]: https://en.cppreference.com/w/cpp/iterator/distance
[tag-dispatch]: https://quuxplusone.github.io/blog/2021/06/07/tag-dispatch-and-concept-overloading

### Iterator reference types
### Historical artifacts

The fourth and fifth types to be defined are required only for historical reasons.
Microsoft said they were going to vote against STL unless it accommodated
Expand Down Expand Up @@ -243,22 +212,51 @@ So now we need to provide it.
&emsp;4. `reference`: the type of a reference to the value. <br/>
&emsp;5. `pointer`: the type of a pointer to the value.

Here is our implementation:

typedef value_type& reference;
typedef value_type* pointer;

It's not particularly harmful, but it obfuscates things
and it provides "language experts" with steady employment.

[^ms-dos-pointers]: See ["A look back at memory models in 16-bit MS-DOS"][far-ptr-article]
for a brief overview of these various pointer types.
The more general concept behind them is [memory segmentation][memory-segmentation].

[^compile-time-dispatch]: Some algorithms can be implemented more efficiently for certain
iterator categories. For example [`std::distance`][std-distance] can be
implemented as a constant time algorithm for `RandomAccessIterators` but
only a linear time algorithm for other iterator categories. The
`iterator_category` tag allows the appropriate algorithm to be selected at
compile time. This technique is known as [tag disptach][tag-dispatch].

[duck]: https://en.wikipedia.org/wiki/Duck_typing
[ptrdiff]: https://en.cppreference.com/w/c/types/ptrdiff_t
[cpp-iterator-traits]: https://en.cppreference.com/w/cpp/iterator/iterator_traits
[std-distance]: https://en.cppreference.com/w/cpp/iterator/distance
[tag-dispatch]: https://quuxplusone.github.io/blog/2021/06/07/tag-dispatch-and-concept-overloading
[lvalue]: https://en.wikipedia.org/wiki/Value_(computer_science)#lrvalue
[far-ptr-article]: https://devblogs.microsoft.com/oldnewthing/20200728-00/?p=104012
[memory-segmentation]: https://en.wikipedia.org/wiki/Memory_segmentation

## List pool iterators

Let's define these types in the `list_pool` iterator.
What category is the iterator for `list_pool` from last chapter?
We need `ForwardIterator`, because there is no way in a singly linked list
to go backwards.

#include <iterator>

// class list_pool {
// ...

struct iterator {
typedef list_pool::value_type value_type;
typedef list_pool::list_type difference_type;
typedef std::forward_iterator_tag iterator_category;
typedef value_type& reference;
typedef value_type* pointer;
};

// };

### Constructors

Let's write constructors for our iterator:
Expand Down Expand Up @@ -472,7 +470,7 @@ A total ordering does not have to be topologically induced by the traversal.

[lex]: https://en.wikipedia.org/wiki/Lexicographic_order

**Exercise:** extend the iterator to linked iterator
**Exercise:** Extend the iterator to linked iterator
so we can assign the next on a node.
Specifically we want to be able to modify the successor of
an iterator.
Expand Down
6 changes: 5 additions & 1 deletion index.html
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Expand Up @@ -330,9 +330,13 @@
<li><a href="09_iterators.html#L9.-Iterators">9. Iterators</a>
<ul>
<li><a href="09_iterators.html#History-of-iterators">History of iterators</a></li>
<li><a href="09_iterators.html#Affiliated-types-for-iterators">Affiliated types for iterators</a>
<ul>
<li><a href="09_iterators.html#Historical-artifacts">Historical artifacts</a></li>
</ul>
</li>
<li><a href="09_iterators.html#List-pool-iterators">List pool iterators</a>
<ul>
<li><a href="09_iterators.html#Iterator-reference-types">Iterator reference types</a></li>
<li><a href="09_iterators.html#Constructors">Constructors</a></li>
<li><a href="09_iterators.html#Dereference">Dereference</a></li>
<li><a href="09_iterators.html#Pre-2d-increment-2c--post-2d-increment">Pre-increment, post-increment</a></li>
Expand Down