const_cast: A Necessary Evil

# A Necessary Evil – Why is a raven like a writing desk? Source: [https://www.elbeno.com/blog/?p=1858](https://www.elbeno.com/blog/?p=1858) We don’t like`const\_cast`much\. It’s ugly, and can even lead to undefined behaviour\. But unfortunately sometimes it’s needed\. Even aside from interacting with C libraries which don’t respect`const`, there is at least one place in the C\+\+ standard that requires you to use it\. If you have a`std::priority\_queue`of move\-only objects, how do you get objects out of it? \(Because It’s not a Roach Motel®, neither is it Hotel California…\) The`pop`function returns`void`\. No help there\. With other container adapters like`std::stack`and`std::queue`, the`top`/`front`/`back`functions return mutable references\. Those container adapters don’t have invariants to uphold that depend on the state of the contained objects\. But the`top`function on`std::priority\_queue`returns a reference\-to\-`const`\.`std::priority\_queue`*can’t*give you a mutable reference to anything inside it, because if it did that, you could alter objects inside it, and that means you could break the heap invariant\. So the only thing you*can*do is use`const\_cast`, temporarily and forcibly break the invariant, and then restore it\. ``` // top can only give us a reference to const const T& obj = q.top(); // we must cast away const to move the object out, // breaking the queue's invariant T exfiltrated_obj = std::move(const_cast<T&>(obj)); // now we should immediately restore the queue invariant q.pop(); ``` Container adapters don’t get much love in the standard: this has been broken since C\+\+11\. Hold your nose, I guess\.