Templates and Function Pointers (C++)

dob99 · 23-05-2006 03:23PM #1

Hi,
I am writing a template class to call a specific function on an object. Generally, the function is virtual and may be overridden, but I want to specify an override to call. (Ultimately, it'll run in a separate thread, with a message loop to handle UI updates, but I omitted that for clarity.) The code is below:

template <class OP>
class Operation
{
typedef bool (OP::*OP_FUNC)();

public:
	Operation(OP* pOp, OP_FUNC pFunc = 0) : m_pOp(pOp), m_pFunc(pFunc) { }
	
	bool Do()
	{
		if (m_pFunc)
		{
			// This doesn't compile
			//return m_pOp->(OP::*(m_pFunc)());
			
			// This calls the "last" override
			return (m_pOp->*(m_pFunc))();
		}
		else
		{
			// This calls the correct override
			return m_pOp->OP::Do();
		}
	}

protected:
	OP* m_pOp;
	OP_FUNC m_pFunc;
};



#include <stdio.h>

class A
{
public:
	A() { }
	
public:
	virtual bool Do() { puts("A::Do() called."); return true; }
	virtual bool fn() { puts("A::fn() called."); return true; }
};

class B : public A
{
public:
	B() { }
	
public:
	virtual bool Do() { puts("B::Do() called."); return false; }
	virtual bool fn() { puts("B::fn() called."); return false; }
};



int main(int argc, char** argv)
{
	B b;
	
	Operation<A> do_operation(&b);
	Operation<A> fn_operation(&b, A::fn);
	
	do_operation.Do();
	fn_operation.Do();

	return 0;
}

The output from this is:

A::Do() called.
B::fn() called.

whereas I want it to be

A::Do() called.
A::fn() called.

I don't want to have to modify A or B - in reality there are many different A's and B's in the project.

Anyone any ideas?

dob99 · 23-05-2006 04:18PM

Consider this more serious example:

class C : public A
{
public:
	C() { }

public:
	virtual bool Do() { puts("C::Do() called."); return A::Do(); }
	virtual bool fn()
	{ 
		Operation<A> fn_op(this, A::fn);
		puts("C::fn() called.");
		return fn_op.Do();
	}
};


int main(int argc, char** argv)
{
	B b;
	
	Operation<A> do_operation(&b);
	Operation<A> fn_operation(&b, A::fn);
	
	do_operation.Do();
	fn_operation.Do();

	C c;
	c.fn();

	return 0;
}

This results in an infinite loop (and actually more acurately represents what I'm trying to do).

Templates and Function Pointers (C++)

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