Template specialization with empty brackets and struct

There are different levels of template specialization:

1) Template declaration (no specialization)

template <class Key, class Value>
struct Foo {};

2) Partial specialization

template <class Key>
struct Foo<Key, int> {};

3) Full/explicit specialization

template <>
struct Foo<std::string, int> {};

Then, when instantiating the templates, the compiler will choose the most specialized definition available:

Foo<std::string, std::string> f1; // Should match #1
Foo<int,         int>         f2; // Should match #2
Foo<std::string, int>         f3; // Should match #3

#1 and #3 work for template functions as well.