Detecting Compile-time vs Runtime in C++: if consteval vs std::is_constant_evaluated()

C++ has been steadily adding features to let programmers distinguish code that runs at compile time from code that runs at runtime. Two important tools for this are the function std::is_constant_evaluated() (C++20) and the language-level if consteval (C++23). This post explains both, shows practical examples, compares their guarantees and trade-offs, and suggests when to use each. Both techniques…