Is this a fail-safe way to check if the code is compiled on a 32bit/64bit system?

Let's say I want to define size_t:

size_t is a typedef/type alias for unsigned long long on 64bit systems and for unsigned int on 32bit systems.

Now we know that sizeof(void*) will produce 8 on 64bit and 4 on 32bit systems.

using size_t = std::conditional_t<sizeof(void*) == 8, unsigned long long, unsigned int>;

My question is: Is this safe to do? I would appreciate if someone with more knowledge about operating systems and such share if this could cause cross-compatibility issues on different operating systems! Thanks!