C ++中的struct模板

Question

I've learned about function templates, class templates, templates specialization but now I've came across this example which blows everything else out of the water because it's so different from the other uses I just can't understand how it was used in this example :

template <int n> struct Fibo
{
    static int val; 
};

template <> int Fibo<0>::val = 1;

template <> int Fibo<1>::val = 1;

template <int n> int Fibo<n>::val = Fibo<n-1>::val + Fibo<n-2>::val;

int CalcFibo(int n)
{ 
    return Fibo<n>::val;
}

Not sure I understand what was done here (of-course I can see it's fibonacci related, but the way it was used compared to class templates or function templates.. I just can't understand what's going on in here and I would be thankful if someone could help me understand)

Answer 1

There is no difference between struct templates and class templates, except the difference between struct s and class es themselves (ie all members in structs are public by default, and all members in classes are private by default.)

This classic example of template metaprogramming is not specific to struct s; it can as easily be accomplished using classes as well. You just need a bit more typing to specify that the val member is public .

As noted in the comments on your question, your code won't compile. The problem is in the CalcFibo function, where you are using a runtime value (the parameter n ) to instantiate a template, which always requires a value that is known at compile-time. You can change your "function" to something like this:

template <int N> int CalcFibo () {return Fibo<N>::val;}

But I doubt that it's what you want.