Class templates (C++ only)

The relationship between a class template and an individual class is like the relationship between a class and an individual object. An individual class defines how a group of objects can be constructed, while a class template defines how a group of classes can be generated.

Note the distinction between the terms class template and template class:

Class template: is a template used to generate template classes. You cannot declare an object of a class template.
Template class: is an instance of a class template.

A template definition is identical to any valid class definition that the template might generate, except for the following:

The class template definition is preceded by
```
  template< template-parameter-list >
```
where template-parameter-list is a comma-separated list of one or more of the following kinds of template parameters:
- type
- non-type
- template
Types, variables, constants and objects within the class template can be declared using the template parameters as well as explicit types (for example, int or char).

Template parameter packs can also be a kind of parameter for class templates. For more information, see Variadic templates (C++11).

A class template can be declared without being defined by using an elaborated type specifier. For example:

template<class L, class T> class Key;

This reserves the name as a class template name. All template declarations for a class template must have the same types and number of template arguments. Only one template declaration containing the class definition is allowed.

By using template parameter packs, template declarations for a class template can have fewer or more arguments than the number of parameters specified in the class template.

Note: When you have nested template argument lists, you must have a separating space between the > at the end of the inner list and the > at the end of the outer list. Otherwise, there is an ambiguity between the extraction operator >> and two template list delimiters >.

template<class L, class T> class Key { /* ... */};
template<class L> class Vector { /* ... */ };

int main ()
{
   class Key <int, Vector<int> > my_key_vector;
   // implicitly instantiates template
}

When the right angle bracket feature is enabled, the >> token is treated as two consecutive > tokens if both the following conditions are true:

The >> token is in a context where one or more left angle brackets are active. A left angle bracket is active when it is not yet matched by a right angle bracket.
The >> token is not nested within a delimited expression context.

If the first > token is in the context of a template_parameter_list, it is treated as the ending delimiter for the template_parameter_list. Otherwise, it is treated as the greater-than operator. The second > token terminates an enclosing template_id construct or a different construct, such as the const_cast, dynamic_cast reinterpret_cast, or static_cast operator. For example:

template<typename T> struct list {};
template<typename T>

struct vector
{
  operator T() const;
};

int main()
{
  // Valid, same as vector<vector<int> > v;
  vector<vector<int>> v; 
  
  // Valid, treat the >> token as two consecutive > tokens.
  // The first > token is treated as the ending delimiter for the
  // template_parameter_list, and the second > token is treated as
  // the ending delimiter for the static_cast operator. 
  const vector<int> vi = static_cast<vector<int>>(v);
}

A parenthesized expression is a delimited expression context. To use a bitwise shift operator inside template-argument-list, use parentheses to enclose the operator. For example:

template <int i> class X {};
template <class T> class Y {};

Y<X<(6>>1)>> y;          //Valid: 6>>1 uses the right shift operator

Objects and function members of individual template classes can be accessed by any of the techniques used to access ordinary class member objects and functions. Given a class template:

template<class T> class Vehicle
{
public:
    Vehicle() { /* ... */ }    // constructor
    ~Vehicle() {};             // destructor
    T kind[16];
    T* drive();
    static void roadmap();
    // ...
};

and the declaration:

Vehicle<char> bicycle; // instantiates the template

the constructor, the constructed object, and the member function drive() can be accessed with any of the following (assuming the standard header file string.h is included in the program file):

constructor	`Vehicle<char> bicycle; // constructor called automatically, // object bicycle created`
object `bicycle`	`strcpy (bicycle.kind, "10 speed"); bicycle.kind[0] = '2';`
function `drive()`	`char* n = bicycle.drive();`
function `roadmap()`	`Vehicle<char>::roadmap();`