Variadic templates with exactly n parameters

Question

I want to make a variadic template with exactly N arguments, where N is also a template parameter. For example,

template <int N, typename T[N]>
void function(T t[N]) {
    // do stuff with t[0] through t[N-1]
}

(I realize the above is not valid syntax)

I know that one way to achieve this is to use a static_assert on sizeof...(ArgsT) where ArgsT is a variadic template definition (i.e. template <typename ...ArgsT>).

I am just wondering if there is a better way, not necessarily involving static_assert.

static_assert is already a nice failing of compilation (stick to it) — , May 04 '14 at 16:16
In summary, you want a compile time error when the pack size does not equal the int parameter ? `static_assert` is meant for this, why wouldn't it be the best way ? — Chnossos, May 04 '14 at 16:16
You may also use SFINAE with `std::enable_if`. But I'm not sure it is better. — Jarod42, May 04 '14 at 16:21
@Jarod42: The difference is whether you want the overloads to *exist*. Unless it's important not to pollute the overload set, a static assertion may produce easier diagnostics. — Kerrek SB, May 04 '14 at 16:30

score 6 · Accepted Answer · edited May 04 '14 at 16:59

6

You can use std::enable_if instead of static_assert:

template <std::size_t N, typename ...Args>
auto function(Args&&... args)
    -> typename std::enable_if<N == sizeof...(Args), void>::type
{
    ...
}

Update: It's also possible to use it in constructors, where N is a template argument of the class.

template <std::size_t N>
struct foobar
{
    template <typename ...Args, typename = typename std::enable_if<N == sizeof...(Args), void>::type>
    foobar(Args&&... args) { ... }
};

edited May 04 '14 at 16:59

Emilio Garavaglia

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answered May 04 '14 at 16:23

nosid

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Is it possible to do this for a constructor? – May 04 '14 at 16:31
@par how exactly do you plan to pass in `N`? – Yakk - Adam Nevraumont May 04 '14 at 16:41
template class A { /* STUFF */ }; – May 04 '14 at 16:44
As a summary for other people reading this: it should be noted that ```static_assert``` is probably the more semantically meaningful way to achieve the above, but this is a very clever way to accomplish what was asked. – May 04 '14 at 17:26

Variadic templates with exactly n parameters

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