Writing in a normal way in snake case or camel case or whatever the convention is.

In C++ I do it Google style:

  static constexpr int kConstantName = 42;

This is the old Mac style too. Might just be my upbringing but I prefer it.

What's the point of the `k` prefix?

k for konstant

IMHO that’s much uglier than using caps.

Sometimes when I use C++, I will namespace them however:

    namespace constants {
        static constexpr int name = 42;
    }
    …
    int x = constants::name;

The exact name of the namespace would depend on the use and could be something more descriptive.

Yes, that's what it stands for. What's the point of using any prefix at all?

So you don’t have to wonder if it’s a variable. Old Mac style uses prefixes: kConstant, gGlobalVar, TType, mMemberVar. Remember this was when all coding was done in black and white in a plain text editor.

Hungarian notation

Hungarian notation is for being more specific than the type system allows.

When the type is already "constexpr int", that k isn't helping.

Just use the same style as for variables. What's the point of the distinction? Maybe you'll want to change a constant into a variable later.