Template Template C++ Function

Question

How do I write a template function that operates on a arbitrary container of a arbitrary type? For example how do I generalize this dummy function

template <typename Element>
void print_size(const std::vector<Element> & a)
{
    cout << a.size() << endl;
}

to

template <template<typename> class Container, typename Element>
void print_size(const Container<Element> & a)
{
    cout << a.size() << endl;
}

Here is a typical usage

std::vector<std::string> f;
print_size(f)

This give error

tests/t_distances.cpp:110:12: error: no matching function for call to ‘print(std::vector<std::basic_string<char> >&)’. I'm guessing I must tell the compiler something more specific about what types that are allowed.

What is this variant of template-use called and how do I fix it?

Answer 1

Is there a specific reason for you to use a template template? Why not just like this?

template <typename Container>
void print_size(Container const& a)
{
    cout << a.size() << endl;
}

In general, template templates aren’t worth the trouble. In your particular case, there certainly is no use for them, and if you really need to access the member type, I suggest you bow to common practice and use a metafunction (typename Container::value_type in this case).

Answer 2

I know that the question was asked long time ago , but i had same problem , and the solution is that you need to write

template <template <typename,typename> class Container, typename element, typename Allocator>
void print_size(Container<element, Allocator> & a)
{
    std::cout << a.size() << std::endl;
}

because the vector has two template parameters