How could c++ std::bind return value assigned to std::function?

user_4964392 0
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Consider some easy code below:

int f1(int a) {
  std::cout << a << std::endl;
}

int main (int agrc, char* argv[]) {
  std::function<int(int)> f = std::bind(&f1, std::placeholders::_1);
  f(123);

  return 0;
}

I've read some documents about std::function and std::bind, but still don't understand how does it work.
The compiler shows that call of std::bind returns an object of type _Bind_helper, however, I don't see the std::function class has a construction function with input type _Bind_helper, so how does the std::function xxx = std::bind xxx work?

bind c++11 function stl c++
2 Answers

I don't see the std::function class has a construction function with input type _Bind_helper, so how does the std::function xxx = std::bind xxx work?

It does have such constructor. It's this one:

template< class F >
function( F f );

Note that this converting constructor participates in overload resolution only if the (deduced) template argument is a Callable - which the return type of std::bind is.

user_20793030

According to ccpreference.com:

Class template std::function is a general-purpose polymorphic function wrapper. Instances of std::function can store, copy, and invoke any Callable target -- functions, lambda expressions, bind expressions, or other function objects, as well as pointers to member functions and pointers to data members.

The stored callable object is called the target of std::function. If a std::function contains no target, it is called empty. Invoking the target of an empty std::function results in std::bad_function_call exception being thrown.

std::function satisfies the requirements of CopyConstructible and CopyAssignable.

std::function is basically a wrapper around a callable object. It's known as a type erasure object - it erases the details of operations to provide a common interface.

Your std::bind expression

std::bind(&f1, std::placeholders::_1)

produces a function object that calls the function f1 with a parameter that you provide in a placeholder and the return type is int. Hence you can assign that to a function object with the same signature:

std::function<int(int)>

which uses a constructor that take a callable object.

user_99675710
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