C++ 那些被遗漏的细节3 std::get

简述

• 在之前的文章C++11 std::tuple 中有使用std::get获取std::tuple元素的例子。
• std::get除了通过索引(C++11)获取std::tuple的元素，还能通过元素类型获取元素(C++14)

语法

// 头文件 <tuple>
template< std::size_t I, class... Types >
typename std::tuple_element<I, tuple<Types...> >::type&
    get( tuple<Types...>& t ) noexcept; (1)	(since C++11)
(constexpr since C++14)
template< std::size_t I, class... Types >
typename std::tuple_element<I, tuple<Types...> >::type&&
    get( tuple<Types...>&& t ) noexcept; (2)	(since C++11)
(constexpr since C++14)
template< std::size_t I, class... Types >
typename std::tuple_element<I, tuple<Types...> >::type const&
    get( const tuple<Types...>& t ) noexcept; (3)	(since C++11)
(constexpr since C++14)
template< std::size_t I, class... Types >
typename std::tuple_element<I, tuple<Types...> >::type const&&
    get( const tuple<Types...>&& t ) noexcept; (4)	(since C++11)
(constexpr since C++14)
template< class T, class... Types >
constexpr T& get(tuple<Types...>& t) noexcept; (5)	(since C++14)
template< class T, class... Types >
constexpr T&& get(tuple<Types...>&& t) noexcept; (6)	(since C++14)
template< class T, class... Types >
constexpr const T& get(const tuple<Types...>& t) noexcept; (7)	(since C++14)
template< class T, class... Types >
constexpr const T&& get(const tuple<Types...>&& t) noexcept; (8)	(since C++14)

• 1-4通过索引获取元素，5-8通过元素类型获取元素，特别注意，如果该类型的元素不唯一，将获取失败。

例子

#include <iostream>
#include <tuple>
#include <string>

int main()
{
  std::tuple<int, int> t1{1, 1};
  std::cout << "t1 is {" << std::get<0>(t1) << ", " << std::get<1>(t1) << "}" << std::endl;

  std::tuple<int, std::string> t2{1, "hello world"};
  std::cout << "t2 is {" << std::get<int>(t2) << ", " << std::get<std::string>(t2) << "}" << std::endl;

  std::tuple<int, int> t3{1, 1};
  // error: static_assert failed due to requirement 'value != __ambiguous' 
  // "type occurs more than once in type list"
  // std::cout << "t3 is {" << std::get<int>(t3);
  return 0;
}

参考

• std::get