C++11新特性-std::tuple

1. 引入头文件

#include <tuple>

2. std::tuple初始化

std::tuple<int, std::string, float> t1(10, "Test", 3.14);

这里要注意，不是所有的C++ 11编译器都支持copy-list-initialization的方式。如下代码所示。

std::tuple<int, int> foo_tuple() 
{
  return {1, -1};  // Error until N4387
  return std::tuple<int, int>{1, -1}; // Always works
  return std::make_tuple(1, -1); // Always works
}

3. 打印std::tuple

打印std::tuple可以将它的元素逐个打印出来，不过非常繁琐，我们可以通过如下通用的打印函数，帮助我们一次性的将tuple的所有要素打印出来。

#include <iostream>
#include <tuple>
#include <string>
 
// helper function to print a tuple of any size
template<class Tuple, std::size_t N>
struct TuplePrinter {
    static void print(const Tuple& t) 
    {
        TuplePrinter<Tuple, N-1>::print(t);
        std::cout << ", " << std::get<N-1>(t);
    }
};
 
template<class Tuple>
struct TuplePrinter<Tuple, 1> {
    static void print(const Tuple& t) 
    {
        std::cout << std::get<0>(t);
    }
};
 
template<typename... Args, std::enable_if_t<sizeof...(Args) == 0, int> = 0>
void print(const std::tuple<Args...>& t)
{
    std::cout << "()\n";
}
 
template<typename... Args, std::enable_if_t<sizeof...(Args) != 0, int> = 0>
void print(const std::tuple<Args...>& t)
{
    std::cout << "(";
    TuplePrinter<decltype(t), sizeof...(Args)>::print(t);
    std::cout << ")\n";
}
// end helper function
 
int main()
{
    std::tuple<int, std::string, float> t1(10, "Test", 3.14);
    print(t1);
}

输出：

(10, Test, 3.14)

4、合并多个std::tuple

std::tuple_cat函数可以将多个std::tuple合并为一个tuple。

int main()
{
    std::tuple<int, std::string, float> t1(10, "Test", 3.14);
    int n = 7;
    auto t2 = std::tuple_cat(t1, std::make_tuple("Foo", "bar"), t1, std::tie(n));
    n = 42;
    print(t2);
}

输出：

(10, Test, 3.14, Foo, bar, 10, Test, 3.14, 42)

5. std::tuple的解包(unpack)

std::tie能够将std::tuple包含的要素解包(unpack)成单个的对象。

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

int main() {
  auto info = std::make_tuple(3.8, 'A', "Lisa Simpson");
  
  double score = 0.0;
  char grade;
  std::string name;
  std::tie(score, grade, name) = info;

  std::cout << "score:" << score << ", grade:" << grade << ", name:" << name << std::endl;

return 0;
}

输出:

score:3.8, grade:A, name:Lisa Simpson

std::tie还支持std::pair对象的解包(unpack)。

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

int main() {
  auto info = std::make_pair(3.8, "Lisa Simpson");
  
  double score = 0.0;
  std::string name;
  std::tie(score, name) = info;

  std::cout << "score:" << score << ", name:" << name << std::endl;

return 0;
}

输出:

score:3.8, name:Lisa Simpson

当我们不关注tuple中的某个元素时，可以使用std::ignore忽略该元素。

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

int main() {
  auto info = std::make_pair(3.8, "Lisa Simpson");
  
  double score = 0.0;
  std::string name;
  std::tie(score, std::ignore) = info;

  std::cout << "score:" << score << ", name:" << name << std::endl;

return 0;
}

输出:

score:3.8, name:

参考材料

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