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;