18.5. Default Arguments for Template Type Parameters

In addition, a type parameter can specify a default type argument. For example, the C++ standard’s stack container adapter class template begins with:

which specifies that a stack uses a deque by default to store the stack’s elements of type T. The declaration

creates a stack of ints class-template specialization (behind the scenes) and uses it to instantiate the object named values. The stack’s elements are stored in a deque<int>.

Default type parameters must be the rightmost (trailing) parameters in a template’s type-parameter list. When you instantiate a template with two or more default arguments, if an omitted argument is not the rightmost, then all type parameters to the right of it also must be omitted. As of C++11, you can now use default type arguments for template type parameters in function templates.