f.cpp

#include "Header.h"
const type_info &g () { return C ().func.target_type (); }

Header.h

#include <functional>
#include <typeinfo>

struct C { std::function<void ()> func = [] {}; C () { }  };
const type_info &g ();

main.cpp

#include "Header.h"
#include <stdio.h>

int main ()
{
  printf ("%s\n%s\n", C ().func.target_type ().name (), g ().name ());
}