Question

I have the following class template:

template<class T, unsigned N>
class MyClass;

where T is some type, N – number of components. It is possible to initialize the class using MyClass{a1, a2, a3} where the number of arguments is equal to N.

I want to add a member function template (let’s name it foo) of MyClass that would meet the following requirements:

1. It is templated by another type T2 (i.e. template<class T2> void foo(..))
2. It accepts enough data to construct MyClass<T,N>, but not less and not more. Violating this results in a compile-time error.
3. It deduces T2 from the types of the parameters. I.e. I want that it would be possible to call foo({a1, a2, a3}) or foo(a1, a2, a3) or similar, without typing <double> or MyClass<double,N> every time.

Is there a way to implement the function so that the above requirements are satisfied?

I’ve already thought about and/or tried the following solutions:

1) The obvious one:

...
template<class T2>
void foo(MyClass<T2, N> arg);  
...
a.foo({1,2,3}); //compile-time error

Can’t work in principle, because braced initializer lists are a non-deduced context, thus they can’t deduce any types. That’s quite unfortunate, I’d be very happy if this worked.

2) initializer_list

Can’t work in principle, because it can’t check the number of arguments at compile-time.

3) Variadic template magic

Something like the function below would be neat:

template<class...T2, class std::enable_if<sizeof...(T2) == N, int>::type = 0>
void foo(T2... args);
..
foo(1,2,3);

However, I couldn’t get it to work – T2 still couldn’t be deduced. Maybe someone knows why? I used GCC4.7 20120121 snapshot.

4) The ugly one

Essentially this is the same as the above one, just expanded into several overloads for different N. I would better reimplement MyClass as a set of specializations for different Ns than to use this one.

template<class T2, class std::enable_if<N == 1, int>::type = 0>
void fun(T2 a1); //if N == 1
template<class T2, ..>
void fun(T2 a1, T2 a2); //if N == 2
template<class T2, ..>
void fun(T2 a1, T2 a2, T2 a3); //if N == 3
...

Answer 1

Your third variant’s second non-type param should have prefix typename not class :

template<class...T2, typename std::enable_if<sizeof...(T2) == N, int>::type = 0>
void foo(T2... args);
..
foo(1,2,3);

Check it

Gcc 4.7.0 snapshots has some bugs with templates I guess, if you try it with gcc 4.6.2/1 it shall work.