4 ms·

C++’s const was never especially safe — contemplate what mutable does. And that one can cast pointers-to-nonconst to pointer-to-const.

by amluto 3y ago

kimixa 3y ago
It's always been a "programmer aid annotation" rather than anything that really affects code generation. The only possible difference (from the resulting binary POV) I can think of is "static const" variables might be allocated in a read-only executable section?
- throwway120385 3y ago
  Probably not, given the possibility that they can be const_casted back into mutable values. Locating static const values in read-only pages would result in crashes from valid statements in the language. In other words if you put a static const in a read only page, it would probably be considered a bug in your linker.
  - actionfromafar 3y ago
    That doesn't rhyme with my understanding of C++ (which is limited, of course). This sounds like undefined.
  - secondcoming 3y ago
    Aren't they put into the .rodata section of the binary? Crashing is exactly what happens, thankfully https://godbolt.org/z/vKj9dcPMc https://godbolt.org/z/vKj9dcPMc
    - gary_0 3y ago
      Generally yes, if you have a bunch of const data in your C++ code, it will go in .rodata. Depending on the compiler/flags/optimizations, of course. https://stackoverflow.com/a/44938843 https://stackoverflow.com/a/44938843
- PhilipRoman 3y ago
  It doesn't even have to be static. Const is powerful when used on non-pointer types. You can write something like this and the compiler will (on appropriate optimization levels) hard-code return value to 7777 regardless of what "mutate" does: extern void mutate(int *p); int func() { const int foo = 7777; mutate(&foo); return foo; }
- gary_0 3y ago
  const_cast is only for removing const from references or pointers that actually refer to a non-const object. You can't declare a const variable and then modify it with const_cast. See "Notes" at: https://en.cppreference.com/w/cpp/language/const_cast https://en.cppreference.com/w/cpp/language/const_cast The compiler can indeed put const objects in the executable's .rodata section, which means trying to modify it at runtime will probably cause a segfault (but it's all Undefined Behavior so who knows).
bingo3131 3y ago
It is undefined behaviour to modify objects declared as const (except if those objects have mutable members, in which case the mutable members can be modified). const int a = 1; If you try to const_cast away the const of x to change its value, you have undefined behaviour. As for pointers, if you make a const pointer to a non-const object then all it means is that you cannot modify the object via that pointer, not that the object will never be modified. int a = 1; int b = &a; const int c = &a; a = 2; Both b and c are now 2 and this is absolutely fine, and the compiler won't try to optimise out any reads of *c and assume its value is still 1 as const/non-const pointer aliasing is allowed (C has the restrict keyword, it's not in C++).

C++’s const was never especially safe — contemplate what mutable does. And that one can cast pointers-to-nonconst to pointer-to-const.

by amluto 3y ago

C++’s const was never especially safe — contemplate what mutable does. And that one can cast pointers-to-nonconst to pointer-to-const.

kimixa 3y ago

It's always been a "programmer aid annotation" rather than anything that really affects code generation. The only possible difference (from the resulting binary POV) I can think of is "static const" variables might be allocated in a read-only executable section?

throwway120385 3y ago
Probably not, given the possibility that they can be const_casted back into mutable values. Locating static const values in read-only pages would result in crashes from valid statements in the language. In other words if you put a static const in a read only page, it would probably be considered a bug in your linker.
- actionfromafar 3y ago
  That doesn't rhyme with my understanding of C++ (which is limited, of course). This sounds like undefined.
- secondcoming 3y ago
  Aren't they put into the .rodata section of the binary? Crashing is exactly what happens, thankfully https://godbolt.org/z/vKj9dcPMc https://godbolt.org/z/vKj9dcPMc
  - gary_0 3y ago
    Generally yes, if you have a bunch of const data in your C++ code, it will go in .rodata. Depending on the compiler/flags/optimizations, of course. https://stackoverflow.com/a/44938843 https://stackoverflow.com/a/44938843
PhilipRoman 3y ago
It doesn't even have to be static. Const is powerful when used on non-pointer types. You can write something like this and the compiler will (on appropriate optimization levels) hard-code return value to 7777 regardless of what "mutate" does: extern void mutate(int *p); int func() { const int foo = 7777; mutate(&foo); return foo; }
gary_0 3y ago
const_cast is only for removing const from references or pointers that actually refer to a non-const object. You can't declare a const variable and then modify it with const_cast. See "Notes" at: https://en.cppreference.com/w/cpp/language/const_cast https://en.cppreference.com/w/cpp/language/const_cast The compiler can indeed put const objects in the executable's .rodata section, which means trying to modify it at runtime will probably cause a segfault (but it's all Undefined Behavior so who knows).

bingo3131 3y ago

It is undefined behaviour to modify objects declared as const (except if those objects have mutable members, in which case the mutable members can be modified). const int a = 1; If you try to const_cast away the const of x to change its value, you have undefined behaviour. As for pointers, if you make a const pointer to a non-const object then all it means is that you cannot modify the object via that pointer, not that the object will never be modified. int a = 1; int b = &a; const int c = &a; a = 2; Both b and c are now 2 and this is absolutely fine, and the compiler won't try to optimise out any reads of *c and assume its value is still 1 as const/non-const pointer aliasing is allowed (C has the restrict keyword, it's not in C++).