Making a HANDLE RAII-compliant using shared_ptr with a custom deleter

Question

I\'ve recently posted a general question about RAII at SO. However, I still have some implementation issues with my HANDLE example.

A HANDLE is typedeff

Answer 1

Example 1 is OK

Example 2 is wrong. By blindly casting to PHANDLE, the shared_ptr logic is bypassed. It should be something like this instead:

HANDLE h;
OpenProcessToken(...., &h);
shared_ptr safe_h(h, &::CloseHandle);

or, to assign to a pre-exising shared_ptr:

shared_ptr safe_h = ....
{
  HANDLE h;
  OpenProcessToken(...., &h);
  safe_h.reset(h, &::CloseHandle);
}//For extra safety, limit visibility of the naked handle

or, create your own, safe, version of OpenProcessToken that returns a shared handle instead of taking a PHANDLE:

// Using SharedHandle defined at the end of this post
SharedHandle OpenProcess(....)
{
    HANDLE h = INVALID_HANDLE_VALUE;
    ::OpenProcessToken(...., &h);
    return SharedHandle(h);
}

Example 3: No need to take these detours. This should be ok:

Process32First(h.get(), ...);

Example 4: Again, no detour:

if (h.get() == INVALID_HANDLE){...}

To make things nicer, you could typedef something like:

typedef shared_ptr SharedHandle;

or better yet, if all handles are to be closed with CloseHandle(), create a SharedHandle class wrapping a shared_ptr and automatically providing the right deleter:

// Warning: Not tested. For illustration purposes only
class SharedHandle
{
public:
  explicit SharedHandle(HANDLE h) : m_Handle(h, &::CloseHandle){};
  HANDLE get()const{return m_Handle.get();}

  //Expose other shared_ptr-like methods as needed
  //...

private:
  shared_ptr m_Handle;
};