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c++templatesnon-type-template-parameter

What does template <unsigned int N> mean?

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?

Solution

  • 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
}