What are some uses of template template parameters?

Question

I\'ve seen some examples of C++ using template template parameters (that is templates which take templates as parameters) to do policy-based class design. What other uses do

Answer 1

Say you're using CRTP to provide an "interface" for a set of child templates; and both the parent and the child are parametric in other template argument(s):

template  class interface {
    void do_something(VALUE v) {
        static_cast(this)->do_something(v);
    }
};

template  class derived : public interface {
    void do_something(VALUE v) { ... }
};

typedef interface, int> derived_t;

Note the duplication of 'int', which is actually the same type parameter specified to both templates. You can use a template template for DERIVED to avoid this duplication:

template