Olivier Langlois's blog

One usage of them commonly demonstrated in C++ Templates textbooks such as C++ Templates is when you want to control the maximum capacity of a container like this:

template< typename T, int MAXSIZE >
class stack
{
...
   private:
   T m_array[MAXSIZE];
};

I have found another very useful property of integral constant template parameters. You can define a class template with an integral constant template parameter and not use it at all in the declaration:

template<int i>
class Unique
{
  public:
  static int var;
};

template<int i>
int Unique<i>::var;

The property of this template is that a distinct var variable will be created for each instantiation of the template:

Unique<1234>::var = 1;
Unique<0>::var = 2;

and so on. By itself, it is more or less useful but imagine that you are dealing with a function registering a callback function that does not let you provide a parameter that will be passed to the callback function but your callback needs to access variables and you must provide more than 1 callback function and each of them needs to access its own set of variables. You could do it by hand by writing x different callbacks and create x sets of variables with different names but by doing that, you would be polluting your namespace and that would be a very long task for nothing. Instead, a class template with an integral constant parameter is the right tool for this situation:

template<int i>
class CB
{
  public:
  static void SetCBParams(int x1, int x2)
  { m_x1 = x1; m_x2 = x2; }
  static void CBFunc();
  private:
  static int m_x1;
  static int m_x2;
};

template<int i>
int CB<i>::m_x1;
template<int i>
int CB<i>::m_x2;

and then

CB<1>::SetParam(1,2);
registerCB(&CB<1>::CBFunc);
CB<2>::SetParam(3,4);
registerCB(&CB<2>::CBFunc);
...