Initialize object pointer in another class with function pointer - c++

Question

Consider the following. Class A has a function pointer as a member and accepts a function in its constructor to pass to this member. In a separate file, I have a class B that contains a pointer to class A as a member, and class B also has as a member the function I want to pass to class A.
Below is an example and the errors I receive. What's the standard method of doing something like this?
A.h:
class A {
private:
int (*func)(int);
public:
A(int (*func_)(int));
};
A::A(int (*func_)(int)) : func(func_) {}
B.h:
#include "A.h" // Why can't I forward declare A instead?
class B {
private:
A *A_ptr;
int function(int); // some function
public:
B();
~B();
};
int B::function(int n) {
return n+2; // some return value
}
B::B() {
A_ptr = new A(function);
}
B::~B() {
delete A_ptr;
}
main.cpp:
#include "B.h"
int main() {
B b;
}
Errors I get:
B.h: In constructor ‘B::B()’:
B.h:18:25: error: no matching function for call to ‘A::A(<unresolved overloaded function type>)’
B.h:18:25: note: candidates are:
A.h:9:1: note: A::A(int (*)(int))
A.h:9:1: note: no known conversion for argument 1 from ‘<unresolved overloaded function type>’ to ‘int (*)(int)’
A.h:1:7: note: A::A(const A&)
A.h:1:7: note: no known conversion for argument 1 from ‘<unresolved overloaded function type>’ to ‘const A&’

Answer 1

To answer your question regarding "What's the standard method of doing something like this" I'll assume you mean passing member functions and/or general function pointers around and executing them with some data. Some popular implementations which provide this ability are:
FastDelegate
std::function
boost::function
It really comes down to preference and library choice. Personally, I've used FastDelegate most of the time and then std::function after that.
All the links I posted should have tutorial information to get you up and running and show you how to properly pass and store member functions and/or general function pointers with ease.
Here's an example of using a FastDelegate with your example:
class A
{
public:
// [1] This creates a delegate type. Can used for any function,
// class function, static function, that takes one int
// and has a return type of an int.
typedef FastDelegate1< int, int > Delegate;
// [2] Pass the delegate into 'A' and save a copy of it.
A( const Delegate& delegate ) : _delegate( delegate ) { };
void execute()
{
// [7]
// Result should be 10!
int result = _delegate( 8 );
}
private:
// [3] Storage to save the Delegate in A.
Delegate _delegate;
};
class B
{
public:
B()
{
// [4] Create the delegate
A::Delegate bDelegate;
bDelegate.bind( this, &B::function );
// [5] Create 'A' passing in the delegate.
_aPtr = new A( bDelegate );
// [6] Test it out!! :)
// This causes `A` to execute the Delegate which calls B::function.
_aPtr->execute();
}
~B()
{
delete _aPtr;
}
int function( int n )
{
return n+2;
}
private:
A* _aPtr;
};