Pointers to Class Functions

I'm trying to send a pointer to a class function to another class for execution. Here's an example:

Code:

class B
{
public:
	void DoThis(int num, bool (*p2Func)(std::ofstream&));
private:
	std::ofstream m_fileOut;
};

void B::DoThis(int num, bool (*p2Func)(std::ofstream&))
{
	(*p2Func)(m_fileOut);
}

class A
{
public:
	A() {m_pB = new B();}
	~A() {delete m_pB;}
	bool Serialize(std::ifstream& ar) {return true;}
	bool Serialize(std::ofstream& ar) {return true;}
	void DoSomethingElse();
private:
	B* m_pB;
};

void A::DoSomethingElse()
{
	//do some stuff
	m_pB->DoThis(0,Serialize);
}

I get the following compiler error:
error C2664: 'DoThis' : cannot convert parameter 2 from 'bool (std::ofstream &)' to 'bool (*)(std::ofstream &)'
None of the functions with this name in scope match the target type

When I try changing the call to DoThis to:

Code:

	m_pB->DoThis(0,&Serialize);

I get the following compiler error:
error C2276: '&' : illegal operation on bound member function expression

I'm not exactly sure what I'm doing wrong. Can someone point me in a better direction?

Making Serialize() a static function would work, I think. Otherwise, if you're trying to make m_pB's DoThis() execute (calling 'A' object)->Serialize(), you'll probably have to pass m_pB either a functor or else this and a pointer-to-member-function (syntax is slightly different).

**EDIT**
From what your functions do, it looks like the static solution would work. If it isn't static, then the function looks for an additional (hidden) parameter, the this pointer of the object. Since Serialize() seems to act very much like a global function (but stuck within a class for organization's sake?), you can get away with removing the this pointer by declaring it static, in effect making it an ordinary global function.

I think my example was a little too simple. I can't make Serialize a static function because Serialize makes use of other class members that are not static.

Originally Posted by Hunter2

you'll probably have to pass m_pB either a functor or else this and a pointer-to-member-function (syntax is slightly different).

I've been using http://www.newty.de/fpt/index.html as a reference for function pointers. I'll take a look at functors and see if I can wrap my head around it. What is this other syntax for passing this and a pointer-to-member-function?

Thanks for the help, Hunter2. A functor looks like what I needed. I ended up with the following test code:

Code:

#include <iostream>
#include <string>

using std::cout;
using std::endl;
using std::string;

// abstract base class
class ISPrintFunctor
{
public:
	virtual void operator()(const char * ar) = 0;
};

// derived template class
template <class T>
class ISPrintSpecificFunctor : public ISPrintFunctor
{
private:
	void (T::*m_p2Func)(const char *);
	T* m_pObj;
public:
	ISPrintSpecificFunctor(T* pObj, void(T::*p2Func)(const char *))
		: m_pObj(pObj), m_p2Func(p2Func) {};
	
	virtual void operator()(const char * ar)
		{
			(*m_pObj.*m_p2Func)(ar);
		};
};

class B
{
public:
	B(const string& a) : m_a(a) {}
	void DoThis(int num, ISPrintFunctor* functor)
		{
			(*functor)(m_a.c_str());
		};
private:
	string m_a;
};

class A
{
public:
	A(string a)
		{
			m_pB = new B(a);
		}
	~A()
		{
			delete m_pB;
		}
	void DoSomethingElse()
		{
			//do some stuff
			ISPrintSpecificFunctor<A> functor(this,&A::Print);
			m_pB->DoThis(0,&functor);
		};
	void Print(const char * ar)
		{
			cout << ar << endl;
		};
private:
	B* m_pB;
};

// main program
int main()
{
	A a("Hello World!");
	a.DoSomethingElse();
	
	return 0;
}

Certainly a warped way to Hello World, but it works. As a side note, you can make Print a private member of class A and it will still work. Doesn't that violate encapsulation? Once again, thanks for the help.

The functor is an extra level of indirection that you may not need...
Here's what you wanted to do originally, but didn't have the syntax quite right:

Code:

#include <iostream>
#include <string>

using std::cout;
using std::endl;
using std::string;

class B
{
public:
    B(const string& a) : m_a(a) {}
    
    template <class T>
    void DoThis(T* pObj, void(T::*p2Func)(const char *))
    {
        (pObj->*p2Func)(m_a.c_str());
    }

private:
    string m_a;
};

class A
{
public:
    A(const string &a) {m_pB = new B(a);}
    ~A() {delete m_pB;}

    void DoSomethingElse()
    {
        m_pB->DoThis(this, &A::Print);
    }

    void Print(const char * ar)
    {
        cout << ar << endl;
    }
    
private:
    B *m_pB;
};

int main()
{
    A a("Hello World!");
    a.DoSomethingElse();
    
    return 0;
}

gg

And thus, both methods have been demonstrated pretty much exactly as I had envisioned them I ordinarily would use the 'this and pointer-to-member-function' approach, but syntactically the functor approach looks nicer once all the appropriate templates and classes have been created (all necessary information contained in a single object rather than being passed in pieces - nice if the functor will be shuffled around a bit before use), and has the benefit of (perhaps) better swappability with ordinary function pointers for later code reuse.

Originally Posted by 7stud??

In three sentences or less, can someone explain what is meant by a call back function?

It's a user-specified function, usually set via a function pointer, that gets called by the inner workings of an independent module when a given event occurs.

Originally Posted by Hunter2

It's a user-specified function, usually set via a function pointer, that gets called by the inner workings of an independent module when a given event occurs.

lol, how in the world did you manage to post in the wrong thread?

I don't know. I got an email from CBoard saying 7stud replied to THIS thread, with THAT question. Hence the "??" in the quote name

Originally Posted by CProg Notifier Email

Hello Hunter2,

7stud has just replied to a thread you have subscribed to entitled - Pointers to Class Functions - in the C++ Programming forum of C Board.

This thread is located at:
Pointers to Class Functions

Here is the message that has just been posted:
***************
Hi,

Every time I see the term call back function, my eyes glaze over, and I know I no longer really understand what the author is talking about.

In three sentences or less, can someone explain what is meant by a call back function?

Thanks.