There was a proposal for compile-time mutable variables in 2019:
https://github.com/cplusplus/papers/issues/602

Which would be more useful I think. Don't think macros are the right way to
go for something like this.

- Jonas

Frederick Virchanza Gotham via Std-Proposals <std-proposals_at_[hidden]>
schrieb am Mi., 22. Nov. 2023, 13:06:

> Lots of people make use of the __COUNTER__ macro in their code for
> generating unique identifiers and unique types, and for a unique
> incrementing number.
>
> Last week I wrote code that combined __COUNTER__ with __FILE__ to get
> a string which hopefully would be unique for the entire program.
>
> So then I was thinking, what if the preprocessor could help us here
> and give us a 128-Bit random number? What if there were to be five new
> macros that would expand as follows:
>
> #define __UUID__ 0f234c3ac9ec4d7bab8e2fcac3da8a5c
>
> #define __UUIDstr__ "0f234c3ac9ec4d7bab8e2fcac3da8a5c"
>
> #define __UUIDstrH__ "0f234c3a-c9ec-4d7b-ab8e-2fcac3da8a5c"
>
> #define __UUIDint128__ 0x0f234c3ac9ec4d7bab8e2fcac3da8a5c
>
> #define __UUIDint64__ 0x0f234c3ac9ec4d7b, 0xab8e2fcac3da8a5c
>
> Of course __UUIDint128__ can only be used on compilers that have an
> unsigned integer type at least 128 bits wide and which allow 128-Bit
> literals. Here's some sample usages:
>
> #include <cstdint>
> #include <iostream>
> using std::cout, std::endl;
>
> #define CONCATx(a,b) a##b
> #define CONCAT(a,b) CONCATx(a,b)
>
> int main(void)
> {
> unsigned CONCAT(some_variable_,__UUID__) = 68u;
>
> some_variable_0f234c3ac9ec4d7bab8e2fcac3da8a5c = 67u;
>
> cout << "Your build key to give to production is {" <<
> __UUIDstrH__ << "}\n";
>
> std::uint64_t my_uuid[2u] = { __UUIDint64__ };
>
> __uint128_t myuuid = __UUIDint128__;
> }
>
> Furthermore, to do away with the need for the CONCAT macro, I suggest
> that the following line:
>
> unsigned CONCAT(some_variable_,__UUID__) = 68u;
>
> can be simplified to:
>
> unsigned some_variable_###__UUID__### = 68u;
>
> So the three hash symbols (or pound symbols in the US) are used by the
> preprocessor to concatenate two sequences of characters together
> without a space between them. So you could also use this tripple-hash
> in your own code, such as:
>
> #define monkey donkey
> unsigned myvar_###monkey### = 7;
> myvar_donkey = 8;
> --
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>