In C++ what does template<> mean?

Question

It\'s not a syntax I\'m familiar with, but I saw it in another question, an example being:

template<> struct Allowed { };

Answer 1

It is a template specialization. The typical case would be partial specialization:

#include 

template struct foo
{
  void doStuff() { std::cout << "generic foo "; }
};

template
struct foo
{
 void doStuff() { std::cout << "specific foo with T2=int"; }
};

As you can see, the specialization removes one element from the template parameters and explicitly states a type instead of the removed one. That means if there is only one template type, the <> just become empty:

template struct bar
{
  void doStuff() { std::cout << "generic bar"; }
};

template<>
struct bar
{
 void doStuff() { std::cout << "specific bar with T1=int"; }
};