I saw a question on StackOverflow asking about why one would use const_cast. Because I’ve thought about that question lately I glossed over that they were asking specifically about applications of casting away volatile…which I didn’t even know you could use const_cast to do!

So I learned something. But I thought the answer I wrote to why you would use const_cast at all was pretty decent, so I was going to leave it there. But then I decided it was too off topic so I’d move it here.

The most common use of const_cast I’ve seen (with const) is this scenario:

• You’re calling a library that has something like a printFoo(Foo* fooPointer) function, which clearly requires a non-const Foo.
• There’s good reason to be “sure” this function doesn’t modify the Foo, but you’re unable to change the prototype or the source to express that knowledge for some reason.
• Your code has made an effort to only use Foo* in contexts where writability is needed and const Foo* in all other contexts.

…that means that as a stopgap measure, you’ll have to const_cast in order to cross this divide and call the routine.

OTOH, the most legitimate use I know of is in subsystems that own objects in a non-const sense, yet have cases where they hand back const pointers to callers. The subsystem natively has more privileges on those objects, so if a caller passes one of those const pointers back it can “upgrade” the privileges.

Sure, the subsystem could store a useless map from non-const pointers to const ones…but const_cast is more time and space efficient:

const SubsystemObject* Subsystem::getReadOnlyObjectById(int id) {
     SubsystemObject* subsystemObject = getObjectCore(id);
 
#ifdef WORLD_WITHOUT_CONST_CAST
     constMap[subsystemObject] = subsystemObject;
#endif
 
     return subsystemObject;
}
 
void Subsystem::someMethod(const SubsystemObject* constSubsystemObject) {
     SubsystemObject* subsystemObject;
 
#ifdef WORLD_WITHOUT_CONST_CAST
     subsystemObject = constMap[constSubsystemObject];
#else
     subsystemObject = const_cast<SubsystemObject*>(constSubsystemObject);
#endif
 
     doSomethingNeedingNonConstAccess(subsystemObject);
}

This entry was posted on Saturday, June 12th, 2010 at 11:35 pm and is filed under C++. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.