Constant value changing

Question

Suppose I have a struct definition:

struct thing
{
    thing* x;
    int z;

    thing() : x(this), z(0) {}
    void foo() const
    {
        this->x->z++;
    }
};

Note that I create a mutable pointer to myself (evil laugh)

And then I can use this later like this:

int main()
{
    const thing c;
    c.foo();
    assert(c.z == 1);
    c.foo();
    assert(c.z == 2);
    return c.z;
}

And as you can see it seems that I can change a constant value......is this UB?

Answer 1

[dcl.type.cv]p4:

Except that any class member declared mutable ([dcl.stc]) can be modified, any attempt to modify ([expr.ass], [expr.post.incr], [expr.pre.incr]) a const object ([basic.type.qualifier]) during its lifetime ([basic.life]) results in undefined behavior.

[basic.type.qualifier]p1:

A const object is an object of type const T or a non-mutable subobject of such an object.

c.z is a const object, because it is a non-mutable subobject of c. Your code attempts to modify it during its lifetime. It follows that the code has undefined behavior.