C++ templates: from basic to SFINAE to concepts_

A function template is just a pattern the compiler stamps out for each type you use it with. template<typename T> T max(T a, T b) { return a > b ? a : b; } generates separate functions for int, double, etc. at compile time. Zero runtime overhead.

Substitution Failure Is Not An Error lets you conditionally enable templates based on type properties. It works but the syntax is genuinely horrible — nested enable_if with decltype and declval. Nobody can read it without a reference.

Concepts let you write: template<std::integral T> T clamp(T val, T lo, T hi); which is readable, gives clear error messages when the constraint is violated, and compiles faster. If you are on C++20, just use concepts and forget SFINAE existed.

Variadic templates power things like std::tuple and printf-safe formatting. The syntax is unusual but the pattern is always the same: base case specialization + recursive expansion. Once you see the pattern it becomes readable.