Defining alias templates

In C++, an alias is a name used to refer to a type that has been previously defined, whether a built-in type or a user-defined type. The primary purpose of aliases is to give shorter names to types that have a long name or provide semantically meaningful names for some types. This can be done either with a typedef declaration or with a using declaration (the latter was introduced in C++11). Here are several examples using typedef:

typedef int index_t;
typedef std::vector<
           std::pair<int, std::string>> NameValueList;
typedef int (*fn_ptr)(int, char);
template <typename T>
struct foo
{
   typedef T value_type;
};

In this example, index_t is an alias for int, NameValueList is an alias for std::vector<std::pair<int, std::string>>, while fn_ptr is an alias for the type of a pointer to a function that returns an int and has two parameters of type int...