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c++templatesvariadic-templates
Denis Sheremet
Denis Sheremet

Reputation: 2583

Variadic template for both typenames and scalar constants

I'm trying to implement a generic class template which forwards it's template arguments to another class, like this:

template <typename... Args>
class A : public B<Args...>
{
    ...

The problem is what B can have scalar constant template like this:

template <size_t N>
class B
{
    ...

or even like this:

template <typename T, size_t N>
class B
{
    ...

So, is there any way to accept mixed up typenames and scalar constants as a variadic template parameters?

Upvotes: 4

Views: 109

Answers (1)

Passer By
Passer By

Reputation: 21130

You can embed values into types. This is a technique long used by boost if I recalled correctly, and was added to the standard library in C++11 as std::integral_constant (indicative of its usefulness).

With C++17, things got even simpler.

template<auto val>
struct constant : std::integral_constant<decltype(val), val> {};

In conjunction with an alias B2 for B, you use it as

template<typename T, typename N>
struct B2 { using type = B<T, N::value>; };

template<typename... Args>
struct A : public B2<Args...>::type {};

A<int, constant<42>> a;

Upvotes: 2

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