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16.1.2. Defining a Class Template

template <class Type> class Queue {

public:

Queue (); // default constructor

Type &front (); // return element from head of Queue

const Type &front () const;

void push (const Type &); // add element to back of Queue

void pop(); // remove element from head of Queue

bool empty() const; // true if no elements in the Queue

private:

// ...

};

A  class template is a template, so it must begin with the keyword templatefollowed by a template parameter list. Our Queuetemplate takes a single template type parameter named Type.

With the exception of the template parameter list, the definition of a class template looks like any other class. A class template may define data, function, and type members; it may use access labels to control access to those members; it defines constructors and destructors; and so on.In the definition of the class and its members, we can use the template parameters as stand-ins for types or values that will be supplied when the class is used.

Using a Class Template

In contrast to calling a function template, when we use a class template, we must explicitly specify arguments for the template parameters:

Queue<int> qi; // Queue that holdsints

Queue< vector<double> > qc; // Queue that holds vectors of doubles

Queue<string> qs; // Queue that holds strings

The compiler uses the arguments to instantiate a type-specific version of the class. Essentially,

the compiler rewrites our Queueclass replacing Typeby the specified actual type provided by the user. In this case, the compiler will instantiate three classes: a version of Queuewith Type replaced by int, a second Queueclass that uses vector<double>in place of  Type, and a third that replaces  Typeby string.

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