Question

I need to solve such a problem.
There is a base class and two inherited classes. The base class contains method which needs a function-pointer as a parameter. But such functions are defined in inherited classes.

class CBase;

typedef bool (CBase::*FPredicate)();

class CBase
{
public:
    CBase() {}
    ~CBase() {}
protected:
    //this method waits until 'predicate' is true or until 'timeout' ms. passed
    //and returns true if 'predicate' is true eventually
    bool WaitEvent(FPredicate predicate, int timeout)
    {
        bool result = false;
        int time1 = GetTickCount();
        int time2;

        bool isEnd = false;
        while(!isEnd)
        {
            result = isEnd = (this->*predicate)();              

            time2 = GetTickCount();
            if(time2 - time1 > timeout && !isEnd)
                isEnd = true;
        }
        return result;
    }
};

class CChildA : public CBase
{
protected:
    bool a1() {/*some work*/}
    bool a2() {/*some work*/}
    void a_main()
    {
        ...
        WaitEvent(&CChildA::a1, 100);
        ...
        WaitEvent(&CChildA::a2, 100);
        ...
    }
};

class CChildB : public CBase
{
protected:
    bool b1() {/*some work*/}
    bool b2() {/*some work*/}
    void b_main()
    {
        ...
        WaitEvent(&CChildB::b1, 100);
        ...
        WaitEvent(&CChildB::b2, 100);
        ...
    }
};

MSVC 2005 compiler gives an error on WaitEvent calls:

error C2664: ‘CBase::WaitEvent’ : cannot convert parameter 1 from ‘bool (__thiscall CChildA::* )(void)’ to ‘FPredicate’

A question is: how shall I change the code to make it work? will it be safe to rewrite WaitEvent call as
WaitEvent((FPredicate)(&CChildA::a1), 100)?

In this case compiler tells of no error but is it safe? Or is there a better way of solving a problem?

Thank you in advance.

Answer 1

The problem is that the implicitly passed this differs in type. Either you cast it, but that will probably fail in the presence of multiple inheritance. A better & more robust solution would be to change the signature to:

template< typename T >
bool WaitEvent( bool ( T::*predicate )(), int timeout ) { ... }