Question

Related:

• Ambiguous overload accessing argument-less template functions with variadic parameters
• Simple variadic template function can't instantinate
• Why is this variadic function ambiguous?

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Consider this pair of variadic templates:

template<typename Dummy>
bool All(Param& c) {
    return true;
}

template<typename Dummy, Func* f, Func* ...rest>
bool All(Param& c) {
    return f(c) && All<Dummy, rest...>(c);
}

This works and compiles. However, how to write it without the first template parameter?

Sounds trivial? Well, that’s what I thought. 🙂 Let’s consider some ideas.

Idea #1:

template<Func* f, Func* ...rest>
bool All(Param& c) {
    return f(c) && All<rest...>(c);
}
template<>
bool All(Param& c) {
    return true;
}

Won’t work… When I attempted this I had specialization in mind, but on the second thought that’s not how it works.

In the original example I created two different templates of overloads, first taking 1 template parameter and second taking 2 or more. No ambiguities and no specialization involved. am I getting it right?

Idea #2:

bool All(Param& c) {
    return true;
}

template<Func* f, Func* ...rest>
bool All(Param& c) {
    return f(c) && All<rest...>(c);
}

Won’t work obviously, All<rest...> with rest... being empty won’t expand to a call to a non-template function.

Idea #3:

Let’s rebuild the solution a bit.

template<Func* f>
bool All(Param& c) {
    return f(c);
}

template<Func* f, Func* ...rest>
bool All(Param& c) {
    return f(c) && All<rest...>(c);
}

This one is a no-go, because All(c) would be ambiguous. Hence I need to have a 0-arg case and a >0-arg case… Or what about a 1-arg case and a >1-arg case?

Idea #3.5:

template<Func* f>
bool All(Param& c) {
    return f(c);
}

template<Func* f, Func* f2, Func* ...rest>
bool All(Param& c) {
    return f(c) && All<f2, rest...>(c);
}

Yup, works, but contains copypasta (simple in this case but might be bigger!), hence I’d say it’s no better than what I’ve started with. Just another workaround.

Idea #4:

Let’s try #1 but with classes instead of functions.

template<Func* f, Func* ...rest>
struct All {
    static bool func(Param& c) {
        return f(c) && All<rest...>(c);
    }
};
template<>
struct All {
    static bool func(Param& c) {
        return true;
    }
};

This looks promising since I can specialize classes. But hey, what is it?

sorry, unimplemented: cannot expand ‘rest …’ into a fixed-length argument list

Wasn’t this a GCC 4.4 thing? I’m on MinGW GCC 4.6.1 (tdm-1).

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Anyway, should I think that I cannot do such an elementary thing in a straightforward way? Is it required to use the workaround with an additional dummy template parameter to accomplish this task?

Or is there a simple, correct variant to specify the zero-argument case, which would work?

Answer 1

In this question’s case, since template parameters are non-type,
if we prepare a function with default template argument like the
following, Dummy parameter can be saved:

template<typename = void>
bool All(Param& c) {
    return true;
}

template<Func* f, Func* ...rest>
bool All(Param& c) {
    return f(c) && All<rest...>(c);
}

However, I’m not sure this is always applicable.
For more general case, std::enable_if or similar dispatch might be needed
(this will make the code a little lengthy though).