unable to pass Template function as a callback parameter

Question

Please refer the code below:

typedef void (*TimerCallback)(int RequestID_in, void* AdditionalParameter_in);
class  MyTimer
{
    public:
        MyTimer(){}
        bool schedule( int Interval_in, TimerCallback TimerCallback_in, void* AdditionalParameter_in)
        {
            //some logic
            return true;
        }
};

namespace
{
    template <class T>
    void myTimerFunc(int RequestID_in, void* AdditionalParameter_in)
    {
        MyLogic<T>* pLogic = static_cast<MyLogic<T>*>(AdditionalParameter_in);
        if(pLogic)
        {
            //do something
        }
    }
}

template <class T>
class MyLogic
{
public:
    MyLogic(){}

    void testMe()
    {
        MyTimer aTimer;
        aTimer.schedule(10, myTimerFunc<T>, this);
    }
};

int main()
{
    MyLogic<int> myLogic;
    myLogic.testMe();
}

I am using VC6 compiler and the compiler throws following error:

error C2664: 'schedule' : cannot convert parameter 2 from 'void (int,void *)' to 'void (__cdecl *)(int,void *)' None of the functions with this name in scope match the target type E:\test\BTest\BTest.cpp(46) : while compiling class-template member function 'void __thiscall MyLogic::testMe(void)'

I tested this code in Visual Studio 2008 and it works fine without any issues.

I know VC6 is an outdated compiler but my project source code(legacy) is still compiled with VC6.

Hence, any work around possible to make this code compile?

Answer 1

I believe you're missing an __stdcall in there:

typedef void (__stdcall *TimerCallback)(int RequestID_in, void* AdditionalParameter_in);

I'm not sure why, but that's what the error message is suggesting. Perhaps you've got a compiler parameter that's telling it to assume __stdcall instead of __cdecl, or vice versa.

Answer 2

All version of Visual Studio I've encountered get seizures when they have to resolve/convert template functions into function pointers. The workaround I use, is to use a id() function which returns its argument unchanged:

template<class T>
T id(T t) 
{
    return t;
}

template <class T>
class MyLogic
{
public:
    MyLogic(){}

    void testMe()
    {
        MyTimer aTimer;
        aTimer.schedule(10, id(myTimerFunc<T>), this);
        //------------------^^(              )
    }
};

This compiles under Visual Studio 6. Please note, that the type of myTimerFunc does not change here, it merely helps the compiler to resolve things.