AFAIK we cannot change a value of a constant variable in C. But i faced this interview question as below:
In C++, we have procedure to change the value of a constant variable. Could anybody tell me how could we do it?
6 Answers
You can modify mutable data members of a const-qualified class-type object:
struct awesome_struct { awesome_struct() : x(0) { } mutable int x; }; int main() { const awesome_struct a; a.x = 42; } The behavior here is well-defined.
1 Comment
Sarfaraz Nawaz
I had this in mind when writing my own post, but didn't know how to put it without using const member function. Good Job. +1!
Under the circumstances, I think I'd have explained the situation: attempting to change the value of a variable that's const gives undefined behavior. What he's probably asking about is how to change a variable that isn't itself const, but to which you've received a pointer or reference to const. In this case, when you're sure the variable itself is not const-qualified, you can cast away the const-ness with const_cast, and proceed to modify.
If you do the same when the variable itself is const-qualified the compiler will allow the code to compile, but the result will be undefined behavior. The attempt at modifying the variable might succeed -- or it might throw an exception, abort the program, re-format your NAS appliance's hard drives, or pretty much anything else.
It's probably also worth mentioning that when/if a variable is likely to need to be used this way, you can specify that the variable itself is mutable. This basically means that the variable in question is never const qualified, even if the object of which it's a part is const qualified.
Comments
There is no perfect way to cast away const-ness of a variable(which is const by definition) without invoking UB
Most probably the interviewers have no idea what they are talking about. ;-)
Comments
You cannot change the value of a constant variable. Trying to do so by cheating the compiler, invokes undefined behavior. The compiler may not be smart enough to tell you what you're doing is illegal, though!
Or maybe, you mean this:
const char *ptr= "Nawaz"; ptr = "Sarfaraz"; ??
If so, then it's changing the value of the pointer itself, which is not constant, rather the data ptr points to is constant, and you cannot change that for example by writing ptr[2]='W';.
Comments
The interviewer was looking for const_cast. As an added bonus, you can explain scenarios where const_cast make sense, such as const_cast<MyClass *>(this)->doSomething() (which can be perfectly valid, although IMO not very clean), and scenarios where it can be dangerous , such as casting away constness of a passed const reference and therefore breaking the contract your functions signature provides.
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If a const variable requires a change in value, then you have discovered a flaw in the design (under most, if not all, situations). If the design is yours, great! - you have an opportunity to improve your code.
If the design is not yours, good luck because the UB comment above is spot-on. In embedded systems, const variables may be placed in ROM. Performing tricks in an attempt to make such a variable writable lead to some spectacular failures. Furthermore, the problem extends beyond simple member variables. Methods and/or functions that are marked const may be optimized in such away that "lack of const-ness" becomes nonsensical.
(Take this "comment-as-answer" as a statement on the poor interview question.)
const_cast<>so rarely (I have used it only once) that I would have to look up the C++ standard every time I use it to make sure it doesn't invoke undefined behavior, despite the fact that I know very well what it does. And when I do use it, it's buried deep in library code.