std :: make_pair中的c ++ 11 rvalue引用 [英] c++11 rvalue references in `std::make_pair`

问题描述

对于C ++ 98

template <class T1, class T2>
  pair<T1,T2> make_pair (T1 x, T2 y);

对于C ++ 11

template <class T1, class T2>
  pair<V1,V2> make_pair (T1&& x, T2&& y);  // see below for definition of V1 and V2

如果T1和/或T2是右值引用，则将移动对象，并且x和/或y处于未定义但有效的状态.

If T1 and/or T2 are rvalue references, the objects are moved and x and/or y are left in an undefined but valid state.

// make_pair example
#include <utility>      // std::pair
#include <iostream>     // std::cout
#include <functional>
#include <unordered_set>
using namespace std;

int main () {
  int i = 2;
  int j = 3;

  pair<int, int> p0 = make_pair<int, int>(2, 3); // fine
  pair<int, int> p1 = make_pair<int, int>(move(i), move(j)); // fine
  pair<int, int> p2 = make_pair<int, int>(i+0, j+0); // fine
  pair<int, int> p3 = pair<int, int>(i, j); // fine

  const int i2 = 2;
  const int j2 = 3;
  // error: no matching function for call to ‘make_pair(const int&, const int&)’
  pair<int, int> p4 = make_pair<int, int>(i2, j2); // wrong

  // error: no matching function for call to ‘make_pair(int&, int&)’
  pair<int, int> p5 = make_pair<int, int>(i, j); // wrong

  return 0;
}

问题>我发现用 make_pair 创建带有非右值引用变量的 pair 是不方便的.如您在上面的代码中看到的那样，以下行不会编译.

Question> I found that it is inconvenient to create a pair with non-rvalue reference variables by make_pair. As you can see above code, the following line doesn't compile.

  // error: no matching function for call to ‘make_pair(int&, int&)’
  pair<int, int> p4 = make_pair<int, int>(i, j); // wrong

在实践中，如何将 make_pair 与非右值引用变量一起使用?

In practice, how do you make use of make_pair with non-rvalue reference variables?

谢谢

推荐答案

std :: make_pair 的意义是为您推断参数的类型.否则，您可以这样写:

The point of std::make_pair is to deduce type of your arguments for you. Otherwise you could just write:

auto p = std::pair<int, double>(42, -1.5);

相反，您写

auto p = std::make_pair(42, -1.5);

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