Question

When declaring a template, I am used to having this kind of code:

template <class T> 

But in this question, they used:

template <unsigned int N> 

I checked that it compiles. But what does it mean? Is it a non-type parameter? And if so, how can we have a template without any type parameter?

Answer 1

It’s perfectly possible to template a class on an integer rather than a type. We can assign the templated value to a variable, or otherwise manipulate it in a way we might with any other integer literal:

unsigned int x = N; 

In fact, we can create algorithms which evaluate at compile time (from Wikipedia):

template <int N> struct Factorial  {      enum { value = N * Factorial<N - 1>::value }; };  template <> struct Factorial<0>  {     enum { value = 1 }; };  // Factorial<4>::value == 24 // Factorial<0>::value == 1 void foo() {     int x = Factorial<4>::value; // == 24     int y = Factorial<0>::value; // == 1 }