Declaring class templates is similar to declaring function templates: Before the declaration, a statement declares an identifier as a type parameter. Again, T is usually used as an identifier:
template <typename T>
class Stack {
…
};
Here again, the keyword class can be used instead of typename:
template <class T>
class Stack {
…
};
Inside the class template, T can be used just like any other type to declare members and member functions. In this example, T is used to declare the type of the elements as vector of Ts, to declare push() as a member function that gets a constant T reference as an argument, and to declare top() as a function that returns a T:
To define a member function of a class template, you have to specify that it is a function template, and you have to use the full type qualification of the class template. Thus, the implementation of the member function push() for type Stack<T> looks like this:
template <typename T>
void Stack<T>::push (T const& elem)
{
elems.push_back(elem); // append copy of passed elem
}
In this case, push_back() of the element vector is called, which appends the element at the end of the vector.
Note that pop_back() of a vector removes the last element but doesn't return it. The reason for this behavior is exception safety. It is impossible to implement a completely exception-safe version of pop() that returns the removed element (this topic was first discussed by Tom Cargill in [CargillExceptionSafety] and is discussed as Item 10 in [SutterExceptional]). However, ignoring this danger, we could implement a pop() that returns the element just removed. To do this, we simply use T to declare a local variable of the element type:
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