Question

C++11 introduced variadic templates

template <typename... Args>
void foo(Args... params) {
    cout << sizeof...(Args) << endl;
}

What are the names of Args and params? I know that one of them (at least?) is called a variadic template pack but which is it? And what is the other called?

Answer 1

Partially quoting the FDIS, §14.5.3:

¹ A template parameter pack is a template parameter that accepts zero or more template arguments.

² A function parameter pack is a function parameter that accepts zero or more function arguments.

³ A parameter pack is either a template parameter pack or a function parameter pack.

⁴ A pack expansion consists of a pattern and an ellipsis, the instantiation of which produces zero or more instantiations of the pattern in a list.

So in your example,

• typename... Args is a template parameter pack (and consequently also a parameter pack)
• Args... params is a function parameter pack (and consequently also a parameter pack)
• sizeof...(Args) is a pack expansion wherein Args is the pattern (an identifier in this context).