C ++如何制作ofstreams的元组？

Question

我有三个 std::ofstream 对象，我将它们放在如下函数中并在 main() 中使用它们。编译时收到很多错误消息，有人可以帮我解决这个问题吗？

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

std::tuple<std::ofstream, std::ofstream, std::ofstream>
set_ofstream_tuple() {
   std::ofstream out1("out1.txt", std::ofstream::out);
   out1 << "this is out1.txt" << std::endl;
   std::ofstream out2("out2.txt", std::ofstream::out);
   out2 << "this is out2.txt" << std::endl;
   std::ofstream out3("out3.txt", std::ofstream::out);
   out3 << "this is out3.txt" << std::endl;

   return std::make_tuple(out1, out2, out3);
}

int main() {
  std::ofstream out1, out2, out3;
  std::tie(out1, out2, out3) = set_ofstream_tuple();
  /// make use of there ofstreams more here
}

Answer 1

这一行：

return std::make_tuple<out1, out2, out3>;

很奇怪。你想做什么？ :)

std::make_tuple 是一个函数，它从它的参数中生成一个元组：

return std::make_tuple(std::move(out1), std::move(out2), std::move(out3));

注意std::move，这些是必需的，因为您不能复制std::ofstream。

Answer 2

问题是您在返回之前将不可复制的ofstream 复制到元组中。

一种解决方案是在返回之前将 ofstreams 移动到元组中：

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

std::tuple<std::ofstream, std::ofstream, std::ofstream>
set_ofstream_tuple() 
{
   std::ofstream out1("out1.txt", std::ofstream::out);
   out1 << "this is out1.txt" << std::endl;
   std::ofstream out2("out2.txt", std::ofstream::out);
   out2 << "this is out2.txt" << std::endl;
   std::ofstream out3("out3.txt", std::ofstream::out);
   out3 << "this is out3.txt" << std::endl;

   return std::make_tuple(std::move(out1), std::move(out2), std::move(out3));
}

int main() {
  std::ofstream out1, out2, out3;
  std::tie(out1, out2, out3) = set_ofstream_tuple();
  /// make use of there ofstreams more here

}

证明：on godbolt

另一种解决方案是在创建过程中使用元组作为主存储：

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

std::tuple<std::ofstream, std::ofstream, std::ofstream>
set_ofstream_tuple() 
{
  auto streams = std::make_tuple(std::ofstream("out1.txt", std::ofstream::out),
                                 std::ofstream("out2.txt", std::ofstream::out),
                                 std::ofstream("out3.txt", std::ofstream::out));

  auto& out1 = std::get<0>(streams);
  auto& out2 = std::get<1>(streams);
  auto& out3 = std::get<2>(streams);

  out1 << "this is out1.txt" << std::endl;
  out2 << "this is out2.txt" << std::endl;
  out3 << "this is out3.txt" << std::endl;

  return streams;
}

int main() {
  std::ofstream out1, out2, out3;
  std::tie(out1, out2, out3) = set_ofstream_tuple();
  /// make use of there ofstreams more here

}

从 c++17 开始，我们将能够使用即将推出的 std::optional 避免空的 ofstreams 的冗余构造（此处使用 std::experimental 进行演示）：

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

#include <experimental/optional>

std::tuple<std::ofstream, std::ofstream, std::ofstream>
set_ofstream_tuple() 
{
  auto streams = std::make_tuple(std::ofstream("out1.txt", std::ofstream::out),
                                 std::ofstream("out2.txt", std::ofstream::out),
                                 std::ofstream("out3.txt", std::ofstream::out));

  auto& out1 = std::get<0>(streams);
  auto& out2 = std::get<1>(streams);
  auto& out3 = std::get<2>(streams);

  out1 << "this is out1.txt" << std::endl;
  out2 << "this is out2.txt" << std::endl;
  out3 << "this is out3.txt" << std::endl;

  return streams;
}

int main() {
  std::experimental::optional<std::ofstream> out1, out2, out3;
  std::tie(out1, out2, out3) = set_ofstream_tuple();
  /// make use of there ofstreams more here

}