Can I define a function pointer pointing to an object of std::function type?

Question

Something like the following:

#include <functional>

int main()
{
    std::function<int(int)> func = [](int x){return x;};
    int* Fptr(int) = &func; //error
}

The errors I get are

temp.cpp: In function ‘int main()’:
temp.cpp:6:15: warning: declaration of ‘int* Fptr(int)’ has ‘extern’ and is initialized
  int* Fptr(int) = &func; //error
               ^
temp.cpp:6:20: error: invalid pure specifier (only ‘= 0’ is allowed) before ‘func’
  int* Fptr(int) = &func; //error
                    ^
temp.cpp:6:20: error: function ‘int* Fptr(int)’ is initialized like a variable

A more direct way to go from lambda function to function pointer would be useful to know as well.

Answer 1

int* Fptr(int)

declares a function "Fptr" that takes an int and returns int*.

A function pointer declaration looks like

int (*Fptr)(int)

Further,std::function<int(int)> is not the type of your lambda function, but your lambda function can be implicitly converted to that type.

Fortunately, a (non-capturing) lambda function can also be implicitly converted to a function pointer, so the most direct way from lambda function to function pointer is

int (*Fptr)(int) = [](int x){return x;};