C++: What is template metaprogramming?

1template <int N>
2struct Factorial {
3    static constexpr int value = N * Factorial<N - 1>::value;
4};
5
6template <>
7struct Factorial<0> {
8    static constexpr int value = 1;
9};
10
11// Factorial<5>::value == 120

1constexpr bool is_int = std::is_integral_v<int>; // true
2constexpr size_t size = sizeof(double);           // 8