Question

What does the ‘class’ part of a template statement do?

Example:

template <class T> class Something {     public:         Something(const T &something); } 

And what else can go there? I usually only see ‘class’.

Answer 1

The class keyword means the same thing as the typename keyword for the most part. They both indicates that T is a type.

The only difference between the keywords class and typename is that class can be used to provide class template template arguments to a template, whereas typename can’t. Consider:

template<template <class T> class U> // must be 'class' std::string to_string(const U<char>& u) {   return std::string(u.begin(),u.end()); } 

The only other thing you can put in place of the class or typename keywords is an integral type. For example:

template<std::size_t max> class Foo{...}; ... Foo<10> f; 

For a concrete example of this, take a look at std::bitset<N> in the standard library.