Question

I have this template :

        template <class SourceFormat, class DestFormat, void (*convert)(DestFormat, SourceFormat)>
    static void _draw(...);

And these functions :

    template <class Class1, class Class2>
    inline static void convertNone(Class1& dest, Class2& source) {
        dest = source;
    };
    inline static void convertARGB_GREY(unsigned __int32& dest, unsigned __int8& source) {
        dest = source + (source << 8);
        dest += (dest << 16);
    };

I use the template in another function :

    void Blitter::draw(...) {

    if (...) {
        _draw<unsigned __int32, unsigned __int32, &convertNone>(...);
    } else {
        _draw<unsigned __int32, unsigned __int8, &convertARGB_GREY>(...); // ERRORS go here!
    }
}

I get these errors :

Error   1   error C2440: 'specialization' : cannot convert from 'void (__cdecl *)(unsigned int &,unsigned char &)' to 'void (__cdecl *const )(unsigned char,unsigned int)'  d:\projects\fanlib\source\blitter.cpp   102
Error   2   error C2973: 'FANLib::Blitter::_draw' : invalid template argument 'void (__cdecl *)(unsigned int &,unsigned char &)'    d:\projects\fanlib\source\blitter.cpp   102

I suppose it’s rather obvious that I don’t fully comprehend functions-as-parameters… 🙁

Many thanks in advance

Answer 1

I don’t know if you’ve done it intentionally, but your template parameters go Source/Destintaion and then Destination/Source.

Notice that when you do _draw<unsigned __int32, unsigned __int8, &convertARGB_GREY>(...); your template definition fills them in as:

SourceFormat = unsigned __int32
DestFormat = unsigned __int8
void (*convert)(unsigned __int8, unsigned __int32)

You don’t have a function of that definition though.

You do have one that matches
void (*convert)(unsigned __int32&, unsigned __int8&)
Do you see how the parameters don’t match?

Declare your template like this:

template <
    class SourceFormat,
    class DestFormat,
    void (*convert)(SourceFormat, DestFormat)> 
static void _draw(...);

and your function declaration like this:

void convertARGB_GREY(
    unsigned __int32 source,        // notice how I removed the '&'
    unsigned __int8 destination)    // character on these two lines

and it will compile. (Since you lose the references, I’d recommend returning the result in this case: unsigned __int8 destination convertARGB_GREY(...) though.)