std::enable_if second ask

Question

I am quite new to std::enable_if and wondering how to use it. I have a template class:

template<int a, int b>
class foo {
  int c;
}

I only want the template to have member c when

a = 5. 

How do I do that using std::enable_if? Is this one the correct case to use std::enable_if?

Answer 1

You can use partial specialization. No need of std::enable_if.

//primary template
template<int a, int b>
class foo 
{
      //whatever 
};

//partial specialization
template<int b>
class foo<5,b>  //when a = 5, this specialization will be used!
{
  int c;  //it has member c
};

Usage:

foo<1,3>  f1; //primary template is used
foo<5,3>  f2; //partial specialization is used