c++11 标准模板（STL）（std::pair）（七）访问 pair 的一个元素

定义于头文件 <utility>

std::pair 是一个结构体模板，其可于一个单元存储两个相异对象。 pair 是 std::tuple 的拥有两个元素的特殊情况。

访问 pair 的一个元素

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std::get(std::pair)

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| template< size_t I, class T1, class T2 > typename std::tuple_element<I, std::pair<T1,T2> >::type& get( pair<T1, T2>& p ) noexcept; | (1) | (C++11 起) (C++14 前) |
| template< size_t I, class T1, class T2 > constexpr std::tuple_element_t<I, std::pair<T1,T2> >& get( pair<T1, T2>& p ) noexcept; | (1) | (C++14 起) |
| template< size_t I, class T1, class T2 > const typename std::tuple_element<I, std::pair<T1,T2> >::type& get( const pair<T1,T2>& p ) noexcept; | (2) | (C++11 起) (C++14 前) |
| template< size_t I, class T1, class T2 > constexpr const std::tuple_element_t<I, std::pair<T1,T2> >& get( const pair<T1,T2>& p ) noexcept; | (2) | (C++14 起) |
| template< size_t I, class T1, class T2 > typename std::tuple_element<I, std::pair<T1,T2> >::type&& get( std::pair<T1,T2>&& p ) noexcept; | (2) | (3) | (C++11 起) (C++14 前) |
| template< size_t I, class T1, class T2 > constexpr std::tuple_element_t<I, std::pair<T1,T2> >&& get( std::pair<T1,T2>&& p ) noexcept; | (2) | (3) | (C++14 起) |
| template< size_t I, class T1, class T2 > constexpr const std::tuple_element_t<I, std::pair<T1,T2> >&& get( const std::pair<T1,T2>&& p ) noexcept; | (2) | (4) | (C++17 起) |
| template <class T, class U> constexpr T& get(std::pair<T, U>& p) noexcept; | (2) | (5) | (C++14 起) |
| template <class T, class U> constexpr const T& get(const std::pair<T, U>& p) noexcept; | (2) | (6) | (C++14 起) |
| template <class T, class U> constexpr T&& get(std::pair<T, U>&& p) noexcept; | (2) | (7) | (C++14 起) |
| template <class T, class U> constexpr const T&& get(const std::pair<T, U>&& p) noexcept; | (2) | (8) | (C++17 起) |
| template <class T, class U> constexpr T& get(std::pair<U, T>& p) noexcept; | (2) | (9) | (C++14 起) |
| template <class T, class U> constexpr const T& get(const std::pair<U, T>& p) noexcept; | (2) | (10) | (C++14 起) |
| template <class T, class U> constexpr T&& get(std::pair<U, T>&& p) noexcept; | (2) | (11) | (C++14 起) |
| template <class T, class U> constexpr const T&& get(const std::pair<U, T>&& p) noexcept; | (2) | (12) | (C++17 起) |
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用类 tuple 的接口从 pair 提取元素。

若序号 I 不是 0 或 1 则基于范围的重载 (1-4) 无法编译。

若类型 T 与 U 相同则基于类型的重载 (5-12) 无法编译。

参数

|---|---|-------------|
| p | - | 要提取内容的 pair |

返回值

1-4) 若 I==0 则返回到 p.first 的引用，若 I==1 则返回到 p.second 的引用。

5-8) 返回到 p.first 的引用。

9-12) 返回到 p.second 的引用。

调用示例

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#include <iostream>
#include <string>
#include <iomanip>
#include <complex>
#include <tuple>
#include <typeinfo>

struct Cell
{
    int x;
    int y;

    Cell() = default;
    Cell(int a, int b): x(a), y(b) {}
    bool operator ==(const Cell &cell) const
    {
        return x == cell.x && y == cell.y;
    }

    bool operator <(const Cell &cell) const
    {
        if (x < cell.x)
        {
            return true;
        }
        return y < cell.y;
    }
};

std::ostream &operator<<(std::ostream &os, const Cell &cell)
{
    os << "{" << cell.x << "," << cell.y << "}";
    return os;
}

std::ostream &operator<<(std::ostream &os, const std::pair<int, Cell> &pair)
{
    os << "pair{" << pair.first << " {" << pair.second.x << "," << pair.second.y << "}}";
    return os;
}

int main()
{
    std::pair<int, Cell> pair1(101, Cell(102, 103));
    std::cout << "pair1:" << std::setw(8) << std::get<0>(pair1) << "    "
              << std::get<1>(pair1) << std::endl;
    std::pair<int, Cell> pair2(101, Cell(102, 103));
    std::cout << "pair2:" << std::setw(8) << std::get<0>(std::move(pair2)) << "    "
              << std::get<1>(std::move(pair2)) << std::endl;

    return 0;
}

输出

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pair1:     101    {102,103}
pair2:     101    {102,103}