is c++ Template Metaprogramming a form of functional programming

Question

is c++ Template Metaprogramming a form of functional programming? If it is, do some pitfalls like stackoverflow for non-tail recursion relevant for c++ Template Metaprogramming?

For the factorial template example in this question, I guess it is standard functional programming. Or the similarity is only superficial?

#include <iostream>
using namespace std;

template< int n >
struct factorial { enum { ret = factorial< n - 1 >::ret * n }; };

template<>
struct factorial< 0 > { enum { ret = 1 }; };

int main() {
    cout << "7! = " << factorial< 7 >::ret << endl; // 5040
    return 0;
}

Answer 1

is c++ Template Metaprogramming a form of functional programming?

Yep! Template expansion has no side effects, so it is pure-functional.

If it is, do some pitfalls like stackoverflow for non-tail recursion relevant for c++ Template Metaprogramming?

Absolutely. Factorial isn't a good demonstration of this, since the result will overflow long before your stack does, but long recursions can definitely cause the compiler to error out. Interestingly, however, compilers tend to implement templates in such a way that you get automatic memoization. For instance, a naively written implementation of the Fibonacci series will tend to compile in O(n) time rather than O(2^n).