C++ Primer Plus

Templates As Parameters

You’ve seen that a template can have type parameters, such as typename T, and non-type parameters, such as int n. A template can also have a parameter that is itself a template. Such parameters are yet another template feature addition that is used to implement the STL.

Listing 14.21 shows an example that begins with these lines:

template <template <typename T> class Thing>class Crab

The template parameter is template <typename T> class Thing. Here template <typename T> class is the type, and Thing is the parameter. What does this imply? Suppose you have this declaration:

Crab<King> legs;

For this to be accepted, the template argument King has to be a template class whose declaration matches that of the template parameter ...