C++1y 没有从 std::bind 到 std::function 的可行转换

我正在尝试将转发函数存储到std::function。如果我使用std::bind，我收到错误消息，例如no viable conversion from ...。如果我使用 lambda，它编译没问题。

这是示例代码

#include <functional>

template<typename Handler>void func1(int a, Handler&& handler) {}
template<typename Handler>void func2(Handler&& handler)
{
    // this line compile fine
    std::function<void ()> funcA = [handler = std::move(handler)]() { func1(1, std::move(handler)); };
    // this line got compile error
    std::function<void ()> funcB = std::bind(func1<Handler>, 1, std::move(handler));
}

int main()
{
    func2(&main); // this just a sample, I am using functor as argument in real code
}

两者都尝试g++ --std=c++1y(v4.9.0) 和clang++ --std=c++1y(v3.4.1) 产生相同的结果

编辑：clang++ 错误消息

main.cpp:8:28: error: no viable conversion from 'typename _Bind_helper<__is_socketlike<void (*)(int, int (*&&)())>::value, void (*)(int, int
      (*&&)()), int, int (*)()>::type' (aka '_Bind<__func_type (typename decay<int>::type, typename decay<int (*)()>::type)>') to
      'std::function<void ()>'
    std::function<void ()> funcB = std::bind(&func1<Handler>, 1, std::move(handler));
                           ^       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
main.cpp:14:5: note: in instantiation of function template specialization 'func2<int (*)()>' requested here
    func2(&main);
    ^
/usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/4.9.0/../../../../include/c++/4.9.0/functional:2181:7: note: candidate constructor not viable: no
      known conversion from 'typename _Bind_helper<__is_socketlike<void (*)(int, int (*&&)())>::value, void (*)(int, int (*&&)()), int, int
      (*)()>::type' (aka '_Bind<__func_type (typename decay<int>::type, typename decay<int (*)()>::type)>') to 'nullptr_t' for 1st argument
      function(nullptr_t) noexcept
      ^
/usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/4.9.0/../../../../include/c++/4.9.0/functional:2192:7: note: candidate constructor not viable: no
      known conversion from 'typename _Bind_helper<__is_socketlike<void (*)(int, int (*&&)())>::value, void (*)(int, int (*&&)()), int, int
      (*)()>::type' (aka '_Bind<__func_type (typename decay<int>::type, typename decay<int (*)()>::type)>') to 'const std::function<void ()> &'
      for 1st argument
      function(const function& __x);
      ^
/usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/4.9.0/../../../../include/c++/4.9.0/functional:2201:7: note: candidate constructor not viable: no
      known conversion from 'typename _Bind_helper<__is_socketlike<void (*)(int, int (*&&)())>::value, void (*)(int, int (*&&)()), int, int
      (*)()>::type' (aka '_Bind<__func_type (typename decay<int>::type, typename decay<int (*)()>::type)>') to 'std::function<void ()> &&' for
      1st argument
      function(function&& __x) : _Function_base()
      ^
/usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/4.9.0/../../../../include/c++/4.9.0/functional:2226:2: note: candidate template ignored:
      substitution failure [with _Functor = std::_Bind<void (*(int, int (*)()))(int, int (*&&)())>]: no matching function for call to object of
      type 'std::_Bind<void (*(int, int (*)()))(int, int (*&&)())>'
        function(_Functor);
        ^
1 error generated.

介绍

std::绑定会尝试打电话func1<Handler>与左值引用，但是你的实例化func1会让它只接受rvalues.

解释

在这里，我们将您的测试用例减少到最低限度以显示正在发生的情况，下面的代码片段格式不正确，随后将解释原因。

#include <functional>

template<class T>
void foobar (T&& val);

int main() {
  std::function<void()> f = std::bind (&foobar<int>, std::move (123));
}

在上面我们将实例化foobar with T = int，这使得参数类型val成为一个右值引用 to int (int&&).

std::move(123) will 移动构造我们的值存储在创建的对象中std::绑定，但标准说，当std::绑定稍后调用存储的函数，所有参数都传递为TiD cv &; IE。作为lvalues.

此行为是由标准强制执行的（n3797 http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3797.pdf），如章节所述[func.bind.bind]p10.

通过将之前的格式错误的代码片段更改为以下内容，不会引发任何错误，因为foobar<int>现在接受一个左值引用;适合绑定到lvalue传递给我们的函数函数对象由返回std::绑定.

  std::function<void()> f = std::bind (&foobar<int&>, std::move (123));

???

#include <functional>
#include <type_traits>
#include <iostream>

int main() {
  auto is_lvalue = [](auto&& x) {
    return std::is_lvalue_reference<decltype(x)> { };
  };

  auto check = std::bind (is_lvalue, std::move (123));
  bool res   = check (); // res = true
}