How to use const_cast?

Question

I have a private variable in my Student class defined as:

const int studentNumnber;

I am trying to write a copy constructor for the Student and I need to cast away the constness to do this. Unfortunately, I don't understand how to use std::const_cast.

This is what I am trying to do in my copy constructor:

    Student(const Student & s) 
        : Person(p.getName(), p.getEmailAddress(), p.getBirthDate()), school(0), studentNumber(0) {
        school = new char[strlen(s.school) + 1];
        strcpy_s(school, strlen(s.school) + 1, s.school);
        const_cast<int*>(this)->studentNumber = s.studentNumber;
        //studentNumber = s.studentNumber);
    }

That doesn't work... I am unsure of the syntax.

Lone

Does this answer your question? Is it allowed to cast away const on a const-defined object as long as it is not actually modified? – Worsley

Answer 1

You are not allowed to const_cast and then modify variables that are actually const. This results in undefined behavior. const_cast is used to remove the const-ness from references and pointers that ultimately refer to something that is not const.

So, this is allowed:

int i = 0;
const int& ref = i;
const int* ptr = &i;

const_cast<int&>(ref) = 3;
*const_cast<int*>(ptr) = 3;

It's allowed because i, the object being assigned to, is not const. The below is not allowed:

const int i = 0;
const int& ref = i;
const int* ptr = &i;

const_cast<int&>(ref) = 3;
*const_cast<int*>(ptr) = 3;

because here i is const and you are modifying it by assigning it a new value. The code will compile, but its behavior is undefined (which can mean anything from "it works just fine" to "the program will crash".)

You should initialize constant data members in the constructor's initializers instead of assigning them in the body of constructors:

Student(const Student & s) 
    : Person(p.getName(), p.getEmailAddress(), p.getBirthDate()),
      school(0),
      studentNumber(s.studentNumber)
{
    // ...
}

Dudleyduds

"You should initialize data members..." not just const data – Distrustful

What is the part that is not allowed about casting away constness from objects that are actually (defined) const? The cast itself, or the subsequent modification? – Cordillera

"You are not allowed to const_cast variables that are actually const" This is not true. Writing to such a variable has undefined behaviour, but the cast itself is 100% legal (but a bad idea because it creates the possibility to reach said UB!) – Snakeroot

See https://mcmap.net/q/23051/-is-it-allowed-to-cast-away-const-on-a-const-defined-object-as-long-as-it-is-not-actually-modified for confirmation that this answer is wrong. – Charnel

what's the point of language having a const_cast if it can result in undefined behavior? makes no sense. – Lasting

Then why this works? const char* s = "abc"; char*p = const_cast<char*>(s); the s points to a constant, and yet const_cast didn't complain. – Kopeisk

@metablaster, See "Lightness Races in Orbit" and Alexey Romanov's comment – Honestly

@timrau: would you mind modifying the beginning of your answer from You are not allowed to const_cast variables [...] to You are not allowed to const_cast and then modify variables [...] ? I think this was your intention. And I think that's what's actually correct according to the N4860 draft, "expr.const.cast" chapter 7.6.1.10 (6). The note says "[...] a write operation through [...]". I can do the edit for you, if you like. Just drop me a comment. – Worsley

@timrau: see also https://mcmap.net/q/23051/-is-it-allowed-to-cast-away-const-on-a-const-defined-object-as-long-as-it-is-not-actually-modified – Worsley

Answer 2

In your code you are trying cast this pointer instead of variable. You can try the following:

Student(const Student & s)
    : Person(p.getName(), p.getEmailAddress(), p.getBirthDate()), school(0), studentNumber(0) {
    school = new char[strlen(s.school) + 1];
    strcpy_s(school, strlen(s.school) + 1, s.school);
    *const_cast<int*>(&studentNumber) = s.studentNumber;
}

Margherita