C++ In a Nutshell

Template Declarations

A template declaration begins with a template header followed by a function declaration or definition, or a class declaration or definition. Template declarations can appear only at namespace or class scope. The template name must be unique in its scope (except for overloaded functions).

The template header starts with the template keyword followed by the template parameters enclosed in angle brackets (<>). Multiple parameters are separated by commas. The syntax is:

template <parameter-list > declaration

There are three kinds of template parameters: values, types, and class templates. Similar to a function parameter, a template parameter has an optional name and an optional default argument.

Function templates cannot have default template arguments. If a class template has member definitions that are outside the class definition, only the class template takes default arguments; the individual member definitions do not. If a default argument is present, it is preceded by an equal sign. Only the rightmost parameters can have default arguments. If any parameter has a default argument, all parameters to its right must also have default arguments. Example 7-2 shows valid and invalid member definitions.

Example 7-2. Defining members of a class template

// OK: default argument for template parameter A template<typename T, typename A = std::allocator<T> > class hashset { bool empty( ) const; size_t size( ) const; ... }; // Error: do not use default argument here template<typename ...