Using std::unique_ptr for managing COM objects

Question

I'm trying to use smart pointers to hold COM objects in my class while avoiding ComPtr. Is it possible to use unique_ptr for this purpose?

I'm quite new to smart pointers and so far I'm a bit confused. Please consider the following simplified code:

class Texture
{
private:
    struct ComDeleter
    {
        operator() (IUnknown* p)
        {
            p.Release();
            delete p;
        }
    }

    ID3D11Texture* m_dumbTexture;
    std::unique_ptr<ID3D11Texture, ComDeleter> m_smartTexture;

public:
    ID3D11Texture* getDumbTexture() const { return m_dumbTexture; }
    ID3D11Texture* getSmartTexture() const { return m_smartTexture.get(); } // what to return here?
}

Texture::Texture() :
    dumbTexture  (NULL),
    smartTexture (nullptr)
{
}

Texture::init()
{
    D3DX11CreateTexture(&m_dumbTexture);
    D3DX11CreateTexture(&m_smartTexture.get());  // error: '&' requires r-value
}

So my problems are: what should the getter return (raw pointer or unique_ptr instance) and how can I pass the unique_ptr to function which creates the resource?

This falls into the re-inventing the wheel category. Use ComPtr and move on. — Peter R
p.Release(); delete p; is wrong: COM objects destroy themselves on the last Release(). — James McNellis
So no delete p necessary? Looks like I've been reading some wrong tutorials. — jaho
delete p is very wrong. What guarantee do you have that the COM object was allocated with C++'s new, and out of the same heap that your C++ library sets up for you? (hint: definitely not the case for DX objects). A major reason why COM objects manage their own lifetimes is because they are the only ones that know the correct way to deallocate themselves. — Euro Micelli
@PeterR Have found this question and upvoted it, I suppose one of my first thoughts were about code reuse and portability. It would make sense to want to avoid Windows specific code for those purposes. — Charlie

James McNellis James McNellis · Accepted Answer · 2014-02-17T04:10:17

I'm trying to use smart pointers to hold COM objects in my class while avoiding ComPtr.

The solution is to use ComPtr. unique_ptr is a poor substitute for ComPtr if you're working with COM objects.

ComPtr provides specialized functionality for working with COM objects and IUnknown. For example, it has built in support for safe interface querying (via the As member function). It is also usable with out parameters, which are common in COM (via the GetAddressOf functions).

Using std::unique_ptr for managing COM objects

2 Answers