Is const_cast on this pointer undefined behavior?

Question

In another question I ran across this code:

Real StatData::mean(Real trim) const
{
   // trim, pun not intended
   const_cast<StatData&>(*this).items.sort();
   // trim
}

cppreference also has an example on their page:

struct type {
    type() :i(3) {}
    void m1(int v) const {
        // this->i = v;                 // compile error: this is a pointer to const
        const_cast<type*>(this)->i = v; // OK
    }
    int i;
};

Aside from the obvious question of why this would ever be practical, is it unsafe? Does it matter of the object created is const or not and more importantly, is the this pointer safe from undefined behavior because it's only marked const for that one function?

Answer 1

Does it matter of the object created is const or not

Yes. If the object was created non-const, then no matter what you do the const_cast will be safe. Just a bad idea (because in general you don't know what the object was created non-const).

the this pointer safe from undefined behavior because it's only marked const for that one function?

That's the wrong question really, but the point is that until you try to modify a const object you're safe. That means the const_cast itself is perfectly legal and well-defined, but if items.sort() modifies stuff (and we have to assume it does) then that operation results in UB.

Finally, although you've tried to gloss over this point in your question, the practicality of this is actually fundamental to the scenario: it's remarkably difficult to guarantee, even in this seemingly specific case, that the code is safe. So, don't do it. Despite your attempts at abstracting it away for the purposes of this question, I can't stress that enough.