why const_cast is not allowed for unique_ptr?

Question

I am trying to const_cast unique_ptr and it is giving me error :

  const std::unique_ptr<int> myptr;
  std::unique_ptr<int> myptr1 = std::move(const_cast<std::unique_ptr<int> >(myptr));

So I want to understand why const_cast doesn't work with unique_ptr if it can work with normal pointers ?

Naturally, you can't move from a const object: move needs to modify the source, in order to "steal" resources from it. — Igor Tandetnik
– Igor Tandetnik, Commented Jul 10, 2016 at 3:11
so this means i cannot have vector of const std::unique_ptr right ? — PapaDiHatti
– PapaDiHatti, Commented Jul 10, 2016 at 3:13
I'm pretty sure you indeed cannot. vector's elements must be either copyable or movable, and const unique_ptr is neither. — Igor Tandetnik
– Igor Tandetnik, Commented Jul 10, 2016 at 3:14

Benjamin Lindley · Accepted Answer · 2016-07-10 03:39:24Z

6

You can const cast a unique_ptr. What you can't do is move from a const unique_ptr, which is what your code attempts to do. You could do this though:

vec.push_back(std::move(const_cast<std::unique_ptr<int>&>(myptr)));

Of course, this is undefined behavior, since your unique_ptr is actually const. If myptr was instead a const reference to a unique_ptr which was not actually const, then the above would be safe.

In your new code

std::unique_ptr<int> myptr1 = std::move(const_cast<std::unique_ptr<int> >(myptr));

The const cast tries to copy myptr before passing the result to std::move. You need to cast it to a reference in order to not copy it.

std::unique_ptr<int> myptr1 = std::move(const_cast<std::unique_ptr<int>& >(myptr));
//                                                                     ^

edited Jul 10, 2016 at 3:39

answered Jul 10, 2016 at 3:13

Benjamin Lindley

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5 Comments

Benjamin Lindley Over a year ago

@Kapil: Yes. We can. What you can't do is copy a unique_ptr, which your new code attempts to do.

PapaDiHatti Over a year ago

no even this std::unique_ptr<int> myptr1 = std::move(const_cast<std::unique_ptr<int> >(myptr)); doesn't compile

Benjamin Lindley Over a year ago

@Kapil: Because that's still attempting to copy the unique_ptr, in the const_cast itself. You need to cast it to a reference.

Benjamin Lindley Over a year ago

@Kapil: i.e. std::unique_ptr<int> myptr1 = std::move(const_cast<std::unique_ptr<int>&>(myptr));

M.M Over a year ago

the "in the new code" suggestion is still undefined behaviour (modifying a const object)

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