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 above code? And what are the cases where we need mandate the use of it?

Answer 1

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 } 

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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 2

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."