Cast DirectX interfaces to IUnknown pointers - c++

Question

Having quite a few interfaces in my code, I wanted to encapsulate repeating Release code in another method and not in a macro because this is C++ and I hate using macros. My initial attempt was to write a method such as
void SafeRelease(IUnknown **ppInterface) {
if(*ppInterface) {
(*ppInterface)->Release();
(*ppInterface) = nullptr;
}
}
however applying this method to a IDirect3DSurface9 * e.g. like SafeRelease(&mySurface) yields the error IDirect3DSurface9 ** is incompatible with IUnknown **.
What am I doing here wrong?
Is there a better approach (hopefully not using macros) to implement such a function?

Answer 1

Here's my method:
template <typename T> void SafeRelease(T*& ptr)
{
if(ptr)
{
ptr->Release();
ptr = nullptr;
}
}
Example usage:
IDirect3DDevice9 *pD3DDevice = NULL;
d3d->CreateDevice(..., &pD3DDevice);
SafeRelease(pD3DDevice);
You may inline this function if you want.

Answer 2

You could use a template:
template<class DXInterface>
void SafeRelease(DXInterface **ppInterface) {
if(*ppInterface) {
(*ppInterface)->Release();
(*ppInterface) = nullptr;
}
}
You could also use a std::unique_ptr or std::shared_ptr to automatically cleanup:
#include <memory>
#include <iostream>
struct Releaser {
template<class DXInterface>
void operator()(DXInterface *pInterface) const {
if(pInterface) {
pInterface->Release();
}
}
};
// For illustrative purposes only (supplied in DX9 headers)
struct IDirect3DSurface9 { void Release() { std::cout << "Released surface\n";} };
struct IDirect3DTexture9 { void Release() { std::cout << "Released texture\n";} };
void DX9CreateSurface( IDirect3DSurface9** surface )
{
*surface = new IDirect3DSurface9();
}
void DX9CreateTexture( IDirect3DTexture9** texture )
{
*texture = new IDirect3DTexture9();
}
// Your factory functions
IDirect3DSurface9* createSurface( /*init params go here*/ )
{
IDirect3DSurface9* surface;
DX9CreateSurface( &surface );
return surface;
}
IDirect3DTexture9* createTexture( /*init params go here*/ )
{
IDirect3DTexture9* texture;
DX9CreateTexture( &texture );
return texture;
}
int main()
{
typedef std::unique_ptr<IDirect3DSurface9, Releaser> SurfacePtr;
typedef std::unique_ptr<IDirect3DTexture9, Releaser> TexturePtr;
SurfacePtr surface( createSurface() );
TexturePtr texture( createTexture() );
// ... use surface and texture here
// Automatically released here when their lifetimes ends.
}
Note that they use the same Releaser, and note that a call to surface.reset() would also release the interface and would set the pointer inside the unique_ptr to null to boot. These two objects could be members of your class rather than objects in main().

Answer 3

What am I doing here wrong?
I just had the same question, also for COM SafeRelease. So here goes:
void SafeRelease(IUnknown **ppInterface)
...
IDirect3DSurface9 * mySurface = new ...
...
SafeRelease(&mySurface);
IDirect3DSurface9 *, by virtue of inheritance, can be cast to IUnknown *.
But, counterintuitively, IDirect3DSurface9 ** cannot be cast to IUnknown **.
If it were allowed, then inside your SafeRelease(IUnknown**) you could do the following:
// obtain a pointer to an instance of some random subinterface of IUnknown
*ppInterface = pMyRamdomComInterfacePointer;
And thus we would have stored a pointer to some random IUnknown derivative in a pointer to IDirect3DSurface9. That would violate the C++ type system. Which is why casting any other type but T** to T** is not allowed. In other words, a variable of type T** can only be assigned a ppT (a value of type T**) and not a ppSomeSubytpeOfT.
Compare this one: How come a pointer to a derived class cannot be passed to a function expecting a reference to a pointer to the base class? And this one: Casting double pointers of base classes.
For COM SafeRelease, either a template (as suggested here) or a macro will do.