std::unique_ptr, deleters and the Win32 API

Question

In VC2012, I want to create a mutex in a constructor using a unique pointer and a deleter, so that I don\'t need to create a destructor just to call CloseHandle.

I w

Answer 1

The problem is you actually define unque_ptr that holds pointer to handle (HANDLE*) type, but you pass just HANDLE, not pointer to it.

Answer 2

Forget about the custom deleter for now. When you say std::unique_ptr<T>, the unique_ptr constructor expects to receive a T*, but CreateMutex returns a HANDLE, not a HANDLE *.

There are 3 ways to fix this:

std::unique_ptr<void, deleter> m_mutex;

You'll have to cast the return value of CreateMutex to a void *.

Another way to do this is use std::remove_pointer to get to the HANDLE's underlying type.

std::unique_ptr<std::remove_pointer<HANDLE>::type, deleter> m_mutex;

Yet another way to do this is to exploit the fact that if the unique_ptr's deleter contains a nested type named pointer, then the unique_ptr will use that type for its managed object pointer instead of T*.

struct mutex_deleter {
  void operator()( HANDLE h ) 
  {
    ::CloseHandle( h );
  }
  typedef HANDLE pointer;
};
std::unique_ptr<HANDLE, mutex_deleter> m_mutex;
foo() : m_mutex(::CreateMutex(NULL, FALSE, NULL), mutex_deleter()) {}

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Now, if you want to pass a pointer to function type as the deleter, then when dealing with the Windows API you also need to pay attention to the calling convention when creating function pointers.

So, a function pointer to CloseHandle must look like this

BOOL(WINAPI *)(HANDLE)

Combining all of it,

std::unique_ptr<std::remove_pointer<HANDLE>::type, 
                BOOL(WINAPI *)(HANDLE)> m_mutex(::CreateMutex(NULL, FALSE, NULL),
                                                &::CloseHandle);

I find it easier to use a lambda instead

std::unique_ptr<std::remove_pointer<HANDLE>::type, 
                void(*)( HANDLE )> m_mutex;
foo() : m_mutex(::CreateMutex(NULL, FALSE, NULL), 
                []( HANDLE h ) { ::CloseHandle( h ); }) {}

Or as suggested by @hjmd in the comments, use decltype to deduce the type of the function pointer.

std::unique_ptr<std::remove_pointer<HANDLE>::type, 
                decltype(&::CloseHandle)> m_mutex(::CreateMutex(NULL, FALSE, NULL),
                                                  &::CloseHandle);

Answer 3

Others have pointed out how the whole HANDLE/HANDLE* issue works. Here's a much cleverer way to deal with it, using interesting features of std::unique_pointer.

struct WndHandleDeleter
{
  typedef HANDLE pointer;

  void operator()(HANDLE h) {::CloseHandle(h);}
};

typedef std::unique_ptr<HANDLE, WndHandleDeleter> unique_handle;

This allows unique_handle::get to return HANDLE instead of HANDLE*, without any fancy std::remove_pointer or other such things.

This works because HANDLE is a pointer and therefore satisfies NullablePointer.