ScopedComPtr

 

// Utility template to prevent users of ScopedComPtr from calling AddRef and/or

// Release() without going through the ScopedComPtr class.

template <class Interface>

class BlockIUnknownMethods : public Interface {

 private:

  STDMETHOD(QueryInterface)(REFIID iid, void** object) = 0;

  STDMETHOD_(ULONG, AddRef)() = 0;

  STDMETHOD_(ULONG, Release)() = 0;

};

 

// A fairly minimalistic smart class for COM interface pointers.

// Uses scoped_refptr for the basic smart pointer functionality

// and adds a few IUnknown specific services.

template <class Interface, const IID* interface_id = &__uuidof(Interface)>

class ScopedComPtr : public scoped_refptr<Interface> {

 public:

  typedef scoped_refptr<Interface> ParentClass;

 

  ScopedComPtr() {

  }

 

  explicit ScopedComPtr(Interface* p) : ParentClass(p) {

  }

 

  ScopedComPtr(const ScopedComPtr<Interface, interface_id>& p)

      : ParentClass(p) {

  }

 

  ~ScopedComPtr() {

    // We don't want the smart pointer class to be bigger than the pointer

    // it wraps.

    COMPILE_ASSERT(sizeof(ScopedComPtr<Interface, interface_id>) ==

                   sizeof(Interface*), ScopedComPtrSize);

  }

 

  // Explicit Release() of the held object.  Useful for reuse of the

  // ScopedComPtr instance.

  // Note that this function equates to IUnknown::Release and should not

  // be confused with e.g. scoped_ptr::release().

  void Release() {

    if (ptr_ != NULL) {

      ptr_->Release();

      ptr_ = NULL;

    }

  }

 

  // Sets the internal pointer to NULL and returns the held object without

  // releasing the reference.

  Interface* Detach() {

    Interface* p = ptr_;

    ptr_ = NULL;

    return p;

  }

 

  // Accepts an interface pointer that has already been addref-ed.

  void Attach(Interface* p) {

    DCHECK(ptr_ == NULL);

    ptr_ = p;

  }

 

  // Retrieves the pointer address.

  // Used to receive object pointers as out arguments (and take ownership).

  // The function DCHECKs on the current value being NULL.

  // Usage: Foo(p.Receive());

  Interface** Receive() {

    DCHECK(ptr_ == NULL) << "Object leak. Pointer must be NULL";

    return &ptr_;

  }

 

  template <class Query>

  HRESULT QueryInterface(Query** p) {

    DCHECK(p != NULL);

    DCHECK(ptr_ != NULL);

    // IUnknown already has a template version of QueryInterface

    // so the iid parameter is implicit here. The only thing this

    // function adds are the DCHECKs.

    return ptr_->QueryInterface(p);

  }

 

  // Queries |other| for the interface this object wraps and returns the

  // error code from the other->QueryInterface operation.

  HRESULT QueryFrom(IUnknown* object) {

    DCHECK(object != NULL);

    return object->QueryInterface(Receive());

  }

 

  // Convenience wrapper around CoCreateInstance

  HRESULT CreateInstance(const CLSID& clsid, IUnknown* outer = NULL,

                         DWORD context = CLSCTX_ALL) {

    DCHECK(ptr_ == NULL);

    HRESULT hr = ::CoCreateInstance(clsid, outer, context, *interface_id,

                                    reinterpret_cast<void**>(&ptr_));

    return hr;

  }

 

  // Checks if the identity of |other| and this object is the same.

  bool IsSameObject(IUnknown* other) {

    if (!other && !ptr_)

      return true;

 

    if (!other || !ptr_)

      return false;

 

    ScopedComPtr<IUnknown> my_identity;

    QueryInterface(my_identity.Receive());

 

    ScopedComPtr<IUnknown> other_identity;

    other->QueryInterface(other_identity.Receive());

 

    return static_cast<IUnknown*>(my_identity) ==

           static_cast<IUnknown*>(other_identity);

  }

 

  // Provides direct access to the interface.

  // Here we use a well known trick to make sure we block access to

  // IUknown methods so that something bad like this doesn't happen:

  //    ScopedComPtr<IUnknown> p(Foo());

  //    p->Release();

  //    ... later the destructor runs, which will Release() again.

  // and to get the benefit of the DCHECKs we add to QueryInterface.

  // There's still a way to call these methods if you absolutely must

  // by statically casting the ScopedComPtr instance to the wrapped interface

  // and then making the call... but generally that shouldn't be necessary.

  BlockIUnknownMethods<Interface>* operator->() const {

    DCHECK(ptr_ != NULL);

    return reinterpret_cast<BlockIUnknownMethods<Interface>*>(ptr_);

  }

 

  // Pull in operator=() from the parent class.

  using scoped_refptr<Interface>::operator=;

 

  // static methods

 

  static const IID& iid() {

    return *interface_id;

  }

};