C++: What is template metaprogramming in C++?

1// Compile-time factorial
2template<int N>
3struct Factorial {
4    static const int value = N * Factorial<N - 1>::value;
5};
6
7// Base case
8template<>
9struct Factorial<0> {
10    static const int value = 1;
11};
12
13int main() {
14    std::cout << Factorial<5>::value;  // 120 (computed at compile-time)
15}

1constexpr int factorial(int n) {
2    return n <= 1 ? 1 : n * factorial(n - 1);
3}
4
5int arr[factorial(5)];  // arr[120]

1#include <type_traits>
2
3// Check if type is pointer
4static_assert(std::is_pointer_v<int*>);
5
6// Remove reference
7using T = std::remove_reference_t<int&>;  // int

1template<typename T>
2std::enable_if_t<std::is_integral_v<T>>
3func(T value) {
4    // Only for integral types
5}

1template<std::integral T>
2void func(T value) {
3    // Only for integral types
4}