4 ms·

Specifically it's well-defined behavior to mutate an object after const_cast-ing away constness if the object wasn't const to begin with (const references or co

Specifically it's well-defined behavior to mutate an object after const_cast-ing away constness if the object wasn't const to begin with (const references or const pointers can refer to non-const objects). In your first example you have `int x = 1;` which isn't const, so the compiler has to assume that `f` may mutate it after const casting. In your second example you have `const int x = 1;` which is const so the compiler can assume the value will never change.

einpoklum 5y ago
You must be right. But - it's so easy to forget that! Or - never to be told that in the first place.
pjmlp 5y ago
Except that on embedded const objects might land on read only memory and cast-ing away constness will give hours of debugging pleasure.
- MaxBarraclough 5y ago
  They covered that with if the object wasn't const to begin with.
  - pjmlp 5y ago
    Partially "(const references or const pointers can refer to non-const objects)". Here are the references to linker scripts?
    - MaxBarraclough 5y ago
      I'm afraid I'm not following. I believe this account of things is accurate (ignoring C++'s reference types for simplicity): The C and C++ standards are written in such a way as to enable compilers to place constant data on read-only memory. Casting away constness is never illegal in and of itself, but if you do so and then assign to a variable which was declared as const, that is undefined behaviour. Similarly, assigning into the character array of a string literal is undefined behaviour, whether or not const was used. Again that's to enable the compiler to make use of read-only memory. Going in the other direction is safe, as preventing assignments doesn't introduce problems. That is to say, using a pointer-to-const type to point to a non-const variable poses no problem. Related: • https://stackoverflow.com/a/9079161/ https://stackoverflow.com/a/9079161/ • https://wiki.sei.cmu.edu/confluence/display/c/EXP05-C.+Do+not+cast+away+a+const+qualification https://wiki.sei.cmu.edu/confluence/display/c/EXP05-C.+Do+no...

einpoklum 5y ago

You must be right. But - it's so easy to forget that! Or - never to be told that in the first place.

pjmlp 5y ago

Except that on embedded const objects might land on read only memory and cast-ing away constness will give hours of debugging pleasure.

MaxBarraclough 5y ago
They covered that with if the object wasn't const to begin with.
- pjmlp 5y ago
  Partially "(const references or const pointers can refer to non-const objects)". Here are the references to linker scripts?
  - MaxBarraclough 5y ago
    I'm afraid I'm not following. I believe this account of things is accurate (ignoring C++'s reference types for simplicity): The C and C++ standards are written in such a way as to enable compilers to place constant data on read-only memory. Casting away constness is never illegal in and of itself, but if you do so and then assign to a variable which was declared as const, that is undefined behaviour. Similarly, assigning into the character array of a string literal is undefined behaviour, whether or not const was used. Again that's to enable the compiler to make use of read-only memory. Going in the other direction is safe, as preventing assignments doesn't introduce problems. That is to say, using a pointer-to-const type to point to a non-const variable poses no problem. Related: • https://stackoverflow.com/a/9079161/ https://stackoverflow.com/a/9079161/ • https://wiki.sei.cmu.edu/confluence/display/c/EXP05-C.+Do+not+cast+away+a+const+qualification https://wiki.sei.cmu.edu/confluence/display/c/EXP05-C.+Do+no...