C++11: What's the difference between "parameter pack" and "variadic template"?

Question

Seen from the introduction http://en.cppreference.com/w/cpp/language/parameter_pack, seems the concept of "parameter pack" is something the same as "variadic template". So what's the conceptual difference between them?

Thanks!

Answer 1

Syntactically, parameter pack is a part of a variadic template. For example,

template<class ... Types> struct Tuple {};

is a variadic structure template, and Types is a parameter pack in it. Tuple itself is not a parameter pack.

You may pass any set of type parameters to Tuple, and they will be packed to Types pack. For example in the code

Tuple<int, float, std::string> foo;

we instantiated a Tuple template with int, float and string types and got a structure of actual type Tuple<int, float, std::string>. The code of Tuple may now expand Types pack.