c++ - Am I right in saying that const_cast followed by modification on a ref-to-const bound to a temporary is okay?

Question

I would like to check my understanding and conclusions on this matter.

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On IRC, it was asked:

Is it acceptable to const_cast a const reference that's bound to a temporary object?

Translating: he has a ref-to-const bound to a temporary, and he wants to cast away its const-ness to modify it.

My response was that I'd asked a similar question previously, where the consensus seemed to be that temporaries themselves are not inherently const, and thus that you can cast off the const-ness of a reference you have to them, and modify them through the result. And, as long as that original ref-to-const still exists, this won't affect the temporary's lifetime.

That is:

int main()
{
   const int& x = int(3);

   int& y = const_cast<int&>(x);
   y = 4;

   cout << x;
}
// Output: 4
// ^ Legal and safe

Am I right?

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^{(Of course, whether or not such code is actually advisable is another matter entirely!)}

Answer 1

No.

First, as far as I can tell, whether it is a literal or not is irrelevant. Rvalues of non-class types always have non-cv qualified types (§3.10/9), however, in §8.5.3 (initialization of a reference), we have:

A reference to type “cv1 T1” is initialized by an expression of type “cv2 T2” as follows:

[...]

--

Otherwise, a temporary of type “cv1 T1” is created and initialized from the initializer expression using the rules for a non-reference copy initialization (8.5). The reference is then bound to the temporary. If T1 is reference-related to T2, cv1 must be the same cv-qualification as, or greater cvqualification than, cv2; otherwise, the program is ill-formed.

(All of the preceding points concern either lvalues or class types.)

In our case, we have:

int const& x = ...;

So cv1 T1 is int const, and the temporary object we create has type int const. This is a top level const (on the object), so any attempt to modify it is undefined behavior.

At least, that's my interpretation. I wish the standard were a bit clearer about this.