Cannot get operator() pointer of std::bind() returned object

I need to extract the type of a function object parameter.

Lambdas get translated into a closure object with the operator(). std::function has got the operator(), too.

So, I can get a pointer to the operator() to pass to another function, in this way:

template <typename F, typename T, typename R, typename ... Args>
void helper(F&, R (T::*)(Args...) const)
{
    // do something with Args types
}

template <typename F>
void bar(F f)
{
    helper(f, &F::operator());
}

void freefunc(int) {}

void foo()
{
    // lambda: ok
    bar([](int){});
    // std::function: ok
    const std::function<void(double)> f = [](double){};
    bar(f);
    // std::bind: does not compile
    auto g = std::bind(freefunc, std::placeholders::_1);
    bar(g);
}

std::bind should create an object with the operator(), too. However, my code does not work with std::bind(), and I cannot understand why.

gcc produces this error:

In instantiation of 'void bar(F) [with F = std::_Bind<void (*(std::_Placeholder<1>))(int)>]':
<source>:58:8:   required from here
<source>:47:11: error: no matching function for call to 'helper(std::_Bind<void (*(std::_Placeholder<1>))(int)>&, <unresolved overloaded function type>)'
   47 |     helper(f, &F::operator());
      |     ~~~~~~^~~~~~~~~~~~~~~~~~~
<source>:39:6: note: candidate: 'template<class F, class T, class R, class ... Args> void helper(F&, R (T::*)(Args ...) const)'
   39 | void helper(F&, R (T::*)(Args...) const)
      |      ^~~~~~
<source>:39:6: note:   template argument deduction/substitution failed:
<source>:47:11: note:   couldn't deduce template parameter 'T'
   47 |     helper(f, &F::operator());
      |     ~~~~~~^~~~~~~~~~~~~~~~~~~
ASM generation compiler returned: 1
<source>: In instantiation of 'void bar(F) [with F = std::_Bind<void (*(std::_Placeholder<1>))(int)>]':
<source>:58:8:   required from here
<source>:47:11: error: no matching function for call to 'helper(std::_Bind<void (*(std::_Placeholder<1>))(int)>&, <unresolved overloaded function type>)'
   47 |     helper(f, &F::operator());
      |     ~~~~~~^~~~~~~~~~~~~~~~~~~
<source>:39:6: note: candidate: 'template<class F, class T, class R, class ... Args> void helper(F&, R (T::*)(Args ...) const)'
   39 | void helper(F&, R (T::*)(Args...) const)
      |      ^~~~~~
<source>:39:6: note:   template argument deduction/substitution failed:
<source>:47:11: note:   couldn't deduce template parameter 'T'
   47 |     helper(f, &F::operator());

What's the correct way to do the same with std::bind?

Solution

What you want to do is unfortunately impossible because the return type of std::bind is too loosely specified by the standard.

std::function::operator() is clearly defined by the standard, so you can match it against R (T::*)(Args... ), see [func.wrap.func.general],
for lambda functions, it's not that clear from [expr.prim.lambda.closure#3], but I'd say that it should work,
for std::bind, the specification [func.bind.bind#4] is much broader because it only says that you can call g(u1, u2, …, uM) where g is the returned value from std::bind, so there is no guarantee that the return type of std::bind even has an operator() member-function.

The actual implementation problem here, which is the same for gcc, clang and msvc, is that the operator() member-function of the return value is actually a template, so you cannot use &F::operator() directly — you cannot take the address of a templated (member-)function.