What is the meaning of template<> with empty angle brackets in C++?

Question

template<>
class A{
//some class data
};

I have seen this kind of code many times. what is the use of template<> in the ab

Answer 1

Doesn't look right. Now, you might have instead written:

template<>
class A<foo> {
// some stuff
};

... which would be a template specialisation for type foo.

Answer 2

template<> tells the compiler that a template specialization follows, specifically a full specialization. Normally, class A would have to look something like this:

template<class T>
class A{
  // general implementation
};

template<>
class A<int>{
  // special implementation for ints
};

Now, whenever A<int> is used, the specialized version is used. You can also use it to specialize functions:

template<class T>
void foo(T t){
  // general
}

template<>
void foo<int>(int i){
  // for ints
}

// doesn't actually need the <int>
// as the specialization can be deduced from the parameter type
template<>
void foo(int i){
  // also valid
}

Normally though, you shouldn't specialize functions, as simple overloads are generally considered superior:

void foo(int i){
  // better
}

---

And now, to make it overkill, the following is a partial specialization:

template<class T1, class T2>
class B{
};

template<class T1>
class B<T1, int>{
};

Works the same way as a full specialization, just that the specialized version is used whenever the second template parameter is an int (e.g., B<bool,int>, B<YourType,int>, etc).

Answer 3

template<> introduces a total specialization of a template. Your example by itself isn't actually valid; you need a more detailed scenario before it becomes useful:

template <typename T>
class A
{
    // body for the general case
};

template <>
class A<bool>
{
    // body that only applies for T = bool
};

int main()
{
    // ...
    A<int>  ai; // uses the first class definition
    A<bool> ab; // uses the second class definition
    // ...
}

It looks strange because it's a special case of a more powerful feature, which is called "partial specialization."