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Definition: constexpr (C++11)

Meaning in English

I COULD calculate this at compile time, if you give me constant data. If you give me dynamic variables, I will act as a normal function/variable and calculate it on the fly.

Time Dependency (Execution)

Optionally Compile-Time, otherwise Run-Time

Memory Position (ROM/RAM)

Yes, if used in a constant context. Values are placed as hardcoded Assembly.

Example C++ Code

constexpr int square(int x) {
    return x * x;
}

constexpr int a = square(5); // Computed at Compile-Time! a=25

int b = 10;
int c = square(b); // Computed at Run-Time! (because b is not constexpr)

📖 C++ Meta-Programming for Embedded

Using C++ (instead of pure C) in microcontrollers was once infamous (mainly due to alleged memory overhead of abstractions). The modern C++11 standard, and especially C++20, changes the game by introducing the aforementioned keywords.

Thanks to consteval you can write giant mathematical calculators with FOR loops that the compiler will resolve in the background on your PC, and flash into the microcontroller only a single, computed constant number.
Whenever possible for global constants (e.g. prescalers) use in C++ constexpr instead of old-fashioned C preprocessor macros #define, because constexpr is strongly typed and GDB debugger friendly!