Template Function error, "no matching function for call to..."

Question

See,I have this code:

template<typename T=int>struct color
{
    T red, green, blue;
    color(T r, T g, T b)
    :red(r), green(g), blue(b)
    {
    }

    #ifdef SDL_BACKEND
    template<typename R,typename S> R map(S surf)
    {
        return SDL_MapRGB(surf->format,red,green,blue);
    }
    #endif /* SDL_BACKEND */
};

and I use it here:

pro::color<int> black(0,0,0);
SDL_FillRect(screen, 0, black.map(screen));

Now here's the error I'm getting:

error: no matching function for call to 'pro::color::map(SDL_Surface*&)'|

I'm not that experienced in templates, so I haven't seen this error before. What exactly is the problem?

NOTE: I didn't tag this with the "SDL" tag, because IMHO this question is more related to templates , the fact that I am using SDL is irrelevant. Also I'm using gcc-4.4x with -std=c++0x.

Kerrek SB · Accepted Answer · 2012-01-24 05:28:39Z

4

This has nothing to do with meta programming. It's just a matter of using templates correctly. The return type cannot be deduced, so you have to specify it; either in the function or in the template instantiation. I.e., pick one of these two:

// Version #1: Change function definition
template<typename S> Uint32 map(S surf) { return SDL_MapRGB(surf->format,red,green,blue); }


// Version #2: Change invocation
black.map<Uint32>(screen);

(In fact, I don't really understand why you need a template here at all. Why not just make the function into Uint32 map(SDL_Surface *)?)

answered Jan 24, 2012 at 5:28

Kerrek SB

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8 Comments

ApprenticeHacker Over a year ago

In fact, I don't really understand why you need a template here at all . I'm not doing something important, I just wrote this function to learn more about templates. +1 , by the way.

Kerrek SB Over a year ago

@IntermediateHacker: Rule 1 of Template Club: Don't use templates :-) (That is, "use them if you need them, not because you're there". Think of templates as you think of fire extinguishers.)

ApprenticeHacker Over a year ago

lol, that saves me a lot of extra trouble learning about them. :D

Sion Sheevok Over a year ago

Agreed regarding the lack of necessity for templating. Documentation does indeed suggest that, since you are returning the result of SDL_MapRGB and SDL_MapRGB returns a Uint32, there's no point in making the return type be a template function. I'm not familiar with SDL, but it seems since your method only uses part of the SDL_Surface interface, you don't need the interface of any of its derived types and could merely take an SDL_Surface*&.

Kerrek SB Over a year ago

@IntermediateHacker: Well, you need them when you need them... as a very rough rule of thumb, you need class templates for generic data structures, and function templates when you need to deduce arguments. Those two often go hand in hand. A template in the right place is pure magic, but it comes at a considerable ontological cost.

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