Writing to class member through const &

Question

In this example, is the c-style cast to int& followed by an assignment to kind of hack the interface of class A undefined behavior?

class A
{
public:
    A()
    : x(0)
    {
    }

    ~A()
    {
        std::cout << x << std::endl;
    }

    const int& getX()
    {
        return x;
    }

private:
    int x;
};

int main()
{
    A a;
    int& x = (int&)a.getX();
    x = 17;
    std::cout << x << std::endl;
}

Output:

17
17

If so, what part of the standard can i refer to? Also, is there any reason why this compiles without warnings? (i tested with c++14 on cpp.sh with -Wall, -Wextra and -Wpedantic)

I would be UB with `const A a;`, else it is a valid `const_cast`. — Jarod42, Apr 28 '17 at 16:21
afaik if you know for sure that it isnt `const` you can change it without any problem. Compare this for example: http://stackoverflow.com/a/856839/4117728 — 463035818_is_not_an_ai, Apr 28 '17 at 16:21
just realized that the answer I linked is almost a perfect match for this question, even if the question isnt a dupe of the linked one — 463035818_is_not_an_ai, Apr 28 '17 at 16:23
If you are expecting the compiler to diagnose problems here, you are going to be disappointed. — , Apr 28 '17 at 16:26
If you are casting `const` away, you have to ask yourself "What's wrong with this design?" — Khouri Giordano, Apr 28 '17 at 16:38

score 8 · Accepted Answer · answered Apr 28 '17 at 16:33

const int& getX() { return x; }

Since this method is not marked const, x is a mutable int. A reference is taken and cast to a const int& at the point of return. Note that although the reference is to a const int, the actual referee int is mutable. This is important.

int& x = (int&)a.getX();

This line takes the returned const int reference and const_cast's it to an int reference. This is legal in c++, full stop. [expr.const.cast]

However, writing through this reference is only legal if the original object being referenced is mutable.

In this case, it is.

You will find the details in [dcl.type.cv]

All that being said, please PLEASE don't write code like this. You better have a really REALLY good reason for casting away a const. And even then, think twice. Or three times. A +1 for the correct answer, but... — Jeff Loughlin, Apr 28 '17 at 16:54

Writing to class member through const &

1 Answers1

Linked