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

It improves readability of your code, provides extra type safety and save some compiler efforts.

Say you want to print each element of a container, you can use the following code without template template parameter

template  void print_container(const T& c)
{
    for (const auto& v : c)
    {
        std::cout << v << ' ';
    }
    std::cout << '\n';
}

or with template template parameter

template< template class ContainerType, typename ValueType, typename AllocType>
void print_container(const ContainerType& c)
{
    for (const auto& v : c)
    {
        std::cout << v << ' ';
    }
    std::cout << '\n';
}

Assume you pass in an integer say print_container(3). For the former case, the template will be instantiated by the compiler which will complain about the usage of c in the for loop, the latter will not instantiate the template at all as no matching type can be found.

Generally speaking, if your template class/function is designed to handle template class as template parameter, it is better to make it clear.