C++11新特性std::make_tuple的使用

std::tuple是C++ 11中引入的一个非常有用的结构，以前我们要返回一个包含不同数据类型的返回值，一般都需要自定义一个结构体或者通过函数的参数来返回，现在std::tuple就可以帮我们搞定。

1.引用头文件

#include <tuple>

2. Tuple初始化

std::tuple的初始化可以通过构造函数实现。

// Creating and Initializing a tuple
std::tuple<int, double, std::string> result1 { 22, 19.28, "text" };

这种初始化方式要定义各个元素的数据类型，比较繁琐，C++11也提供了另外一种方式std::make_tuple。

3. std::make_tuple

// Creating a tuple using std::make_tuple
auto result2 = std::make_tuple( 7, 9.8, "text" );

这种初始化方式避免需要逐个指定元素类型的问题，自动化实现各个元素类型的推导。

完整例子一：

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

int main()
{
  // Creating and Initializing a tuple
  std::tuple<int, double, std::string> result1 { 22, 19.28, "text" };
  // Compile error, as no way to deduce the types of elements in tuple
  //auto result { 22, 19.28, "text" }; // Compile error
  // Creating a tuple using std::make_tuple
  auto result2 = std::make_tuple( 7, 9.8, "text" );
  // std::make_tuple automatically deduced the type and created tuple
  // Print values
  std::cout << "int value = " << std::get < 0 > (result2) << std::endl;
  std::cout << "double value = " << std::get < 1 > (result2) << std::endl;
  std::cout << "string value = " << std::get < 2 > (result2) << std::endl;
  return 0;
}

输出：

<strong>int</strong> value = 7

<strong>double</strong> value = 9.8

string value = text

完整例子二：

#include <iostream>
#include <tuple>
#include <functional>
 
std::tuple<int, int> f() // this function returns multiple values
{
  int x = 5;
  return std::make_tuple(x, 7); // return {x,7}; in C++17
}
 
int main()
{
  // heterogeneous tuple construction
  int n = 1;
  auto t = std::make_tuple(10, "Test", 3.14, std::ref(n), n);
  n = 7;
  std::cout << "The value of t is " << "("
       << std::get<0>(t) << ", " << std::get<1>(t) << ", "
       << std::get<2>(t) << ", " << std::get<3>(t) << ", "
       << std::get<4>(t) << ")\n";
 
  // function returning multiple values
  int a, b;
  std::tie(a, b) = f();
  std::cout << a << " " << b << "\n";
}

输出：

The value of t is (10, Test, 3.14, 7, 1)

5 7

参考材料

https://thispointer.com/c11-make_tuple-tutorial-example/

https://en.cppreference.com/w/cpp/utility/tuple/make_tuple

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