How to use C++ standard smart pointers with Windows HANDLEs?

Question

I was wondering if there is a way to use unique_ptr<T> with Windows HANDLEs?

I was thinking to replace the std::default_delete with specific handle_trats that calls CloseHandle. The problem is that HANDLE is defined as void* unique_ptr<void> won't compile as sizeof(void) is not defined.

So far I see only two possibilities:

1. Create a wrapper class for HANDLEs and use it like this: unique_ptr<new CHandle(h)>. This pretty much makes the unique_ptr<T> itself useless.
2. Use HANDLE specific smart pointer class that resembles unique_ptr<T>.

What do you think is better choice? What would you suggest?

The question can be extended for COM IUnknown pointers - can CComPtr be replaced by any of the standard smart pointers?

Answer 1

The question can be extended for COM IUnknown pointers - can CComPtr be replaced by any of the standard smart pointers?

Yes. You don't specialize std::default_deleter, you simply replace the deleter type.

struct COMDeleter {
    template<typename T> void operator()(T* ptr) {
        ptr->Release();
    }
};
unique_ptr<IUnknown, COMDeleter> ptr; // Works fine

The same principle applies to shared_ptr and indeed, to HANDLE.

Answer 2

Create a specific smart pointer class, won't take long. Don't abuse library classes. Handle semantics is quite different from that of a C++ pointer; for one thing, dereferencing a HANDLE makes no sense.

One more reason to use a custom smart handle class - NULL does not always mean an empty handle. Sometimes it's INVALID_HANDLE_VALUE, which is not the same (actually -1).

Answer 3

inspired by Alexander Drichel's solution, here is even shorter

std::unique_ptr< HANDLE, decltype(&CloseHandle) > uniqueHandle( nullptr, CloseHandle );

Works in MSVC 2010. Note that you need to specify '&' for the function name in decltype() to deduce a pointer-to-function type.