Template Metaprogramming Basics

Templates form their own tiny language, evaluated by the compiler at compile time rather than at runtime — historically used for compile-time computation before constexpr (next lesson) made much of it unnecessary:

template<int N>
struct Factorial {
    static const int value = N * Factorial<N - 1>::value;
};

template<>
struct Factorial<0> {
    static const int value = 1; // base case
};

std::cout << Factorial<5>::value << std::endl; // 120 — computed at compile time

<> after the second Factorial is a template specialization — an explicit alternate implementation for one specific type or value, used here as the recursion's base case.

Common mistakes

• Reaching for template metaprogramming where a plain constexpr function (next lesson) would be far more readable — modern C++ only needs the technique above for genuinely type-level computation.