Non-copyable deleter in std::unique_ptr

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I've just been looking at solutions to another question (this one). It seems that it should be possible to create a custom deleter for a unique_ptr instance that can unlock a mutex.

For example,

#include <mutex>
#include <memory>

struct LockDeleter
{
  std::unique_lock<std::mutex> lock_;

  FooDeleter(std::mutex& m) : lock_(m) {}
  void operator()(void*) { lock_.unlock(); }
};

int main()
{
  std::mutex moo;

  {
    std::unique_ptr<int, LockDeleter> ptr(new int(42), LockDeleter(moo));
  }
}

Compiling this under VS2013 express, I get

Error 1 error C2280: 'std::unique_lock::unique_lock(const std::unique_lock &)' : attempting to reference a deleted function

and

This diagnostic occurred in the compiler generated function 'LockDeleter::LockDeleter(const LockDeleter &)'

Now, I can't seem to force the lock deleter instance to be moved, rather than copied using std::move... even adding an explicit move constructor to LockDeleter doesn't help, and the deleted copy constructor still gets called.

So, am I doing something silly, or must unique_ptr deleters always be copy constructable?

asked 2 mins ago

Rook

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