Designated Initializers, the best feature of C++20

To be honest, I like that it enforces ordering. It avoids inconsistent ordering, keeping ordering consistent is just an extra cognitive overhead while the freedom to have inconsistent ordering gives me nothing.

The only annoying thing is that Apple Clang doesn't enforce ordering; it doesn't even have an option to enable warnings about breaking C++ ordering rules. So I'll be writing C++ on my Mac, not noticing my inconsistent ordering, then realize my mistake only when I later try to compile the project on my Linux desktop (or when it fails in CI, depending on the project).

We use these a bit in snmalloc to provide API-stable functions with a bunch of optional parameters. The last argument is a struct that has default values for everything and you can use this syntax to override the ones you want to change. This is a C++ localisation of a common Python idiom.

Speaking as someone who doesn't really do C++, that article was not just about designated initialisers, but in the function call instances (from a C perspective) about compound literals as well. That being how one can also provided 'named arguments' in C.

One often underappreciated aspect of C compound literals is that they're not simply restricted to structures but to all (I've not tried all) types. So one can do the following:

int main() {
        auto arr = (int[1]){ 1 };
        const int a0v = arr[0];
        auto larr = (long long[2]){ 5, 7};
        float pi = (float){ 3.14 };

        printf("arr[0] = %d\n", a0v);
        printf("larr[0] = %lld; larr[1] = %lld\n", larr[0], larr[1]);
        printf("pi = %f\n", pi);

        return 0;
}

While those may look a bit weird, they can come in handy when building certain forms of macro based facility.

Next: The array variant of designated initializers.