Enforce variadic template of certain type

Question

I would like to enforce the type of variadic template to be identical to an earlier set template type. In the below example, I\'d like T and U to be the same type.

c

Answer 1

We can use SFINAE to ensure that all U types are the same as T. An important thing to note is that U is not just one type as you imply, but a list of possibly disparate types.

template::value>* = nullptr>
Foo(T first, U... vals) {
    std::cout << "Called multiple argument ctor" << std::endl;
    // [...]   
}

std::enable_if_t is from C++14. If that's not an option for you, just use std::enable_if.

typename std::enable_if::value>::type* = nullptr>

all_same can be implemented in a bunch of different ways. Here's a method I like using boolean packs:

namespace detail
{
    template struct bool_pack;
    template
    //if any are false, they'll be shifted in the second version, so types won't match
    using all_true = std::is_same, bool_pack>;
}
template 
using all_true = detail::all_true;

template 
using all_same = all_true...>;