C++11 parameters pack overload

Question

I have a function:

template <typename A>
Foo createFoo() {

    std::initializer_list<std::size_t> list = { Object<A>::getIndex() } );
    return Foo(list);
}

It is invoked like:

Foo foo = createFoo<Bar>();

It works correct, returning me an object of class Foo.

Is it possible to write a specialized function in case there're more than one parameter argument, something like this?

template <typename A1, typename A2, typename...Ax>
Foo createFoo() {

    std::initializer_list<std::size_t> list = { Object<A1>::getIndex(), Object<A2>::getIndex()... } );
    return Foo(list);
}

So that I invoke it this way:

Foo foo = createFoo<Bar, Fred, Wilma>();

It gives me an error:

error: expansion pattern ‘Object::getIndex()’ contains no argument packs

and points to the "..." part of the code.

I don't want to have a single function declared as

template <typename ...A>
Foo createFoo() {

    std::initializer_list<std::size_t> list = { Object<A>::getIndex() } );
    return Foo(list);
}

because, let's suppose the Foo class has optimized logics in case there's only one std::size_t item in the initializer_list and I want to utilize that in my API.

I use GCC version 4.8.4 with -std=c++11 option

Thank you.

Answer 1

std::initializer_list<std::size_t> list = { Object<A1>::getIndex(), Object<A2>::getIndex()... } );

when you put ... after a function call, it looks for parameter packs inside of that to expand, but A2 isn't a parameter pack, Ax is.

Why even have the A1 and A2?

template <typename...Ax>
Foo createFoo() {

Then you can just say Object<Ax>::getIndex()...

Or if you must enforce a certain number of parameters, then:

std::initializer_list<std::size_t> list = { Object<A1>::getIndex(),Object<A2>::getIndex(), Object<Ax>::getIndex()... } );

The ... expansion accepts an empty parameter pack, so this will work for 2 or more types.