constexpr functions

constexpr functions

The real power is when you use constexpr for functions.

    const float SQRTPI = sqrtf(3.14f);   // Works?
    constexpr float SQRTPI = sqrtf(3.14f); // Works?

Oh, dear! I just tested this code snippet, and the const version works, whereas the constexpr version fails to compile, which is the opposite of what I was expecting. According to an informed source that was trained on Internet scrapings, sqrtf is not going to be declared as a “constexpr” function until C++26. Alas, by then all C++ programmers will have been replaced by robots, so feel free to skip this section.

The apparently futuristic idea is that sqrtf should have a “constexpr” keyword in its declaration, because the function return value can be computed at compile-time if you pass it a constant argument. In other words, the compiler can evaluate “sqrtf(3.14f)” at compile-time. Hence, the whole function should be declared “constexpr” in the standard library header file. The const version is also probably not evaluating the sqrtf function at compile-time, but just calling it dynamically whenever the const variable is first initialized (this non-compile-time initialization is allowed for const variables, provided you don't later attempt to change its value).

Anyway, you can already declare your own function with the “constexpr” specifier.

    constexpr int twice(int x)
    {
        return x + x;
    }

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