3 ms·

> With a const pointer the callee can not mutate the pointee. But it can still be mutated from the outside That's incorrect. In c semantics, at least, it is l

> With a const pointer the callee can not mutate the pointee. But it can still be mutated from the outside That's incorrect. In c semantics, at least, it is legal to take a pointer to non-const, cast it to a pointer to const, pass it back, and mutate the object pointed to. It is only illegal to mutate if the pointer actually points to const memory. Which means that, in a general sense, if you're given a const pointer, since you can't know if it actually points at const memory, you shouldn't mutate through it. But if you're handing out const pointers to non-const memory, you shouldn't count on the memory not being changed through those pointers. IOW const is all but useless in c (except as a declaration of intent). > An ABI like this would probably be much more complex to implement for a small gain Why? Mechanically, it's almost the same as the ms/arm/riscv abi, except that a copy is made by the callee rather than the caller.

gpderetta 5y ago
Const matters to the caller, if it calls a function that takes a pointer to const it can generally reasonably expect that the object wont be changed. It is not about enabling compiler optimisations but about preserving invariants.
TwoBit 5y ago
Are you saying that in C a callee can cast a const pointer to non-const and write to it and that's sanctioned by the Standard?
- moonchild 5y ago
  Yes, as long as the original pointer was to a non-const location. That is: void mutate(const int *x) { *(int*)x = 5; } int main() { int a; mutate(&a); // ok const int b; mutate(&b); // undefined behaviour }
  - TwoBit 5y ago
    > Yes, as long as the original pointer was to a non-const location. So then actually no - it's not guaranteed to be portable. That non-const location qualification of course means portable code cannot just cast const pointers to non-const.

gpderetta 5y ago

Const matters to the caller, if it calls a function that takes a pointer to const it can generally reasonably expect that the object wont be changed. It is not about enabling compiler optimisations but about preserving invariants.

TwoBit 5y ago

Are you saying that in C a callee can cast a const pointer to non-const and write to it and that's sanctioned by the Standard?

moonchild 5y ago
Yes, as long as the original pointer was to a non-const location. That is: void mutate(const int *x) { *(int*)x = 5; } int main() { int a; mutate(&a); // ok const int b; mutate(&b); // undefined behaviour }
- TwoBit 5y ago
  > Yes, as long as the original pointer was to a non-const location. So then actually no - it's not guaranteed to be portable. That non-const location qualification of course means portable code cannot just cast const pointers to non-const.