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# iter_const_reference_t | ||
* iterator[meta header] | ||
* std[meta namespace] | ||
* type-alias[meta id-type] | ||
* cpp20[meta cpp] | ||
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```cpp | ||
namespace std { | ||
template<indirectly_readable It> | ||
using iter_const_reference_t = common_reference_t<const iter_value_t<It>&&, iter_reference_t<It>>; | ||
} | ||
``` | ||
* indirectly_readable[link ./indirectly_readable.md] | ||
* common_reference_t[link /reference/type_traits/common_reference.md] | ||
* iter_value_t[link ./iter_value_t.md] | ||
* iter_reference_t[link ./iter_reference_t.md] | ||
## 概要 | ||
任意のイテレータ型`It`から、そのイテレータの要素への`const`参照型を取得する。 | ||
## 型の決定 | ||
`iter_value_t<It>`が非参照型であり`iter_reference_t<It>`から修飾を除いた型であるとすると、`iter_reference_t<It>`に対して`iter_const_reference_t<It>`は基本的には次のようになる | ||
|`iter_reference_t`|`iter_const_reference_t<It>`| | ||
|---|---| | ||
|`T&`|`const T&`| | ||
|`T&&`|`const T&&`| | ||
|`const T&`|`const T&`| | ||
|`const T&&`|`const T&&`| | ||
|`T`|`T`| | ||
|`const T`|`T`| | ||
`It`に対して`iterator_traits`の特殊化が存在する場合や、`const iter_value_t<It>&&`と`iter_reference_t<It>`について`common_reference`が特殊化されている場合はこれと異なる結果となりうる。標準ライブラリにあるそのようなイテレータ型を持つものについて一部例を示すと、次のようになる | ||
|`It`の取得元の範囲|`iter_value_t<It>`|`iter_reference_t<It>`|`iter_const_reference_t<It>`| | ||
|---|---|---|---| | ||
|[`std::vector<bool>`](/reference/vector/vector.md)|`bool`|`std::vector<bool>::reference`|`bool`| | ||
|`const std::vector<bool>`|`bool`|`bool`|`bool`| | ||
|[`views::zip`](/reference/ranges/zip_view.md.nolink)|`std::tuple<T, U>`|`std::tuple<T&, U&>`|`std::tuple<const T&, const D&>`| | ||
|[`views::enumrate`](/reference/ranges/enumrate_view.md.nolink)|`std::tuple<D, T>`|`std::tuple<D, T&>`|`std::tuple<D, const T&>`| | ||
表中の`T, U`はそれそれの`view`に入力された範囲の値型、`D`は`views::enumrate`の入力範囲の`iter_difference_t`とする。 | ||
## 例 | ||
```cpp example | ||
#include <iterator> | ||
#include <tuple> | ||
// indirectly_readableとなる最低限の型 | ||
// Rが参照型、Vが値型となる | ||
template<typename R, typename V> | ||
struct test_indirectly_readable { | ||
using value_type = V; | ||
using difference = std::ptrdiff_t; | ||
auto operator*() const -> R; | ||
}; | ||
int main() { | ||
static_assert(std::same_as<std::iter_const_reference_t<test_indirectly_readable<int& , int>>, const int&>); | ||
static_assert(std::same_as<std::iter_const_reference_t<test_indirectly_readable<int&& , int>>, const int&&>); | ||
static_assert(std::same_as<std::iter_const_reference_t<test_indirectly_readable<const int& , int>>, const int&>); | ||
static_assert(std::same_as<std::iter_const_reference_t<test_indirectly_readable<const int&&, int>>, const int&&>); | ||
static_assert(std::same_as<std::iter_const_reference_t<test_indirectly_readable<int , int>>, int>); | ||
static_assert(std::same_as<std::iter_const_reference_t<test_indirectly_readable<const int , int>>, int>); | ||
static_assert(std::same_as<std::iter_const_reference_t<test_indirectly_readable<std::tuple<int, double> , std::tuple<int, double>>>, std::tuple<int, double>>); | ||
static_assert(std::same_as<std::iter_const_reference_t<test_indirectly_readable<std::tuple<int&, double&>, std::tuple<int, double>>>, std::tuple<const int&, const double&>>); | ||
static_assert(std::same_as<std::iter_const_reference_t<test_indirectly_readable<std::tuple<int, double>& , std::tuple<int, double>>>, const std::tuple<int, double>&>); | ||
} | ||
``` | ||
* std::iter_const_reference_t[color ff0000] | ||
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### 出力 | ||
``` | ||
``` | ||
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## バージョン | ||
### 言語 | ||
- C++23 | ||
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### 処理系 | ||
- [Clang](/implementation.md#clang): ?? | ||
- [GCC](/implementation.md#gcc): 13.1 | ||
- [Visual C++](/implementation.md#visual_cpp): 2022 Update 6 | ||
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## 参照 | ||
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- [P2278R4 `cbegin` should always return a constant iterator](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2278r4.html) |