Thread: Modifying reference to a const

Originally Posted by kishore

It works on TurboC++ 3.0
which uses a temprory variable if the types of the variable and its reference don't match...........

.....and is practically guaranteed to allow subtle bugs in the program!?

If you have a const, and you try to modify its reference, then you are making a logical error. It's a good thing compilers can catch this.

I use const references very often to pass large classes to functions that shouldn't modify them. If the compiler allowed me to modify the object, that would mean that the function could be seriously flawed without anybody noticing it.

that's actually undefined behaviour according to the standard

Originally Posted by section 7.1.5.1

Except that any class member declared mutable (7.1.1) can be modified, any attempt to modify a const
object during its lifetime (3.8) results in undefined behavior.

Code:

const int* ciq = new const int (3); // initialized as required
int* iq = const_cast<int*>(ciq); // cast required
*iq = 4; // undefined: modifies a const object

this means that while it might work on your particular compiler, another compiler is free to put const values into rom and crash horribly when you try to write to it. Actually undefined means it can actually do anything! (sending porn to your boss, deleting your hard disk, these are perfectly acceptable in the eyes of the standard! )

bottom line: don't use const_cast to modify a const pointer or reference unless you're damn sure that whatever it points to isn't const!