可变参数模板，类型推导和std :: function [英] Variadic templates, type deduction and std::function

问题描述

我正在尝试制作一个模板函数，可以将任何类型和数量的参数传递给其他函数并将其绑定到 std :: function 。我设法做到了：

I'm trying to make a template function to which is possible to pass some other function with any type and number of parameters and bind it to a std::function. I managed to do this:

#include <iostream>
#include <functional>

int foo(int bar)
{
  std::cout << bar << std::endl;
  return bar;
}

template <typename Ret, typename... Args>
std::function<Ret (Args...)> func(std::function<Ret (Args...)> f)
{
  return f;
}

int main()
{
  //auto barp = func(foo); // compilation error
  auto bar  = func(std::function<void (int)>(foo));

  bar (0); // prints 0
}

我只想打电话给 auto barp = func（foo）; 并推断出类型，但是此行给出以下编译错误：

I would like to just call auto barp = func(foo); and have the types deduced, but this line gives the following compilation errors:

error: no matching function for call to ‘func(void (&)(int))’
    auto barp = func(foo);
                        ^
note: candidate is:
note: template<class Ret, class ... Args> std::function<_Res(_ArgTypes ...)> func(std::function<_Res(_ArgTypes ...)>)
 std::function<Ret (Args...)> func(std::function<Ret (Args...)> f)
                              ^
note:   template argument deduction/substitution failed:
note:   mismatched types ‘std::function<_Res(_ArgTypes ...)>’ and ‘int (*)(int)’
   auto barp = func(foo);
                       ^

为什么要匹配 std :: function< ; _Res（_ArgTypes ...）> 与 int（*）（int）吗？我觉得我应该让编译器以某种方式将 _Res（_ArgTypes ...）扩展为 int（int），但是

Why is it trying to match std::function<_Res(_ArgTypes ...)> with int (*)(int)? I feel I should get the compiler somehow to expand _Res(_ArgTypes ...) to int(int), but how?

推荐答案

函数不是 std :: function ，它可以转换为一个。
但是，您可以推断出函数的参数，除非对重载有歧义。

A function is not an std::function, it is convertible to one. You can deduce the arguments of a function, however, barring ambiguity about overloads.

#include <iostream>
#include <functional>

int foo(int bar)
{
  std::cout << bar << std::endl;
  return 0;
}

// Will cause error.
//int foo(double); 

template <typename Ret, typename... Args>
std::function<Ret (Args...)> func(Ret f(Args...))
{
  return f;
}

int main()
{
  auto bar = func(foo);
  bar (0); // prints 0
}

您要对原始<$ c $进行的操作c> std :: function 与此类似，但显然不起作用：

What you want to do with the original std::function is similar to this, which more obviously does not work:

template<typename T>
struct A
{
  A(T);  
};

template<typename T>
void func(A<T> a);

int main()
{
    func(42);
}

42 不是 A ，但可以将其转换为一个。但是，将其转换为一个将需要知道 T 。

42 is not a A, it can be converted to one, though. However, converting it to one would require T to already be known.

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