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V1114. Suspicious use of 'dynamic_cast' when working with COM interfaces. Consider using the 'QueryInterface' member function.
The analyzer has detected a suspicious use of the 'dynamic_cast' operator when working with COM interfaces. This can result in scenarios where the object reference counter is not incremented, leading to the handling of destroyed objects or other issues.
Let's examine the synthetic example. The code contains the COM interfaces, 'IDraw' and 'IShape', responsible for handling a certain geometric shape:
interface IDraw : public IUnknown
{
....
virtual HRESULT Draw() = 0;
....
};
interface IShape : public IUnknown
{
....
virtual HRESULT GetArea(double *area) = 0;
....
};There is also the 'Circle' COM object that implements the 'IDraw' and 'IShape' interfaces:
class Circle : public IDraw, public IShape
{
....
};Look at the example of the incorrect COM object handling via the 'IDraw' interface:
void foo(IDraw *ptrIDraw)
{
IShape *ptrIShape = dynamic_cast<IShape*>(ptrIDraw);
....
if (ptrIShape)
ptrIShape->GetArea(area);
....
}In the above example, the reference counter for the 'Circle'-typed object does not increment. To increment and decrement the counter, call the 'AddRef' and 'Release' functions respectively.
To work with COM interfaces correctly, use the 'QueryInterface' function, specifically designed for this purpose.
The 'QueryInterface' function should:
- check that the requested interface is accessible;
- return a pointer to the requested interface;
- increment the object reference counter.
Here is the fixed code:
void foo(IDraw *ptrIDraw)
{
IShape *ptrIShape = nullptr;
....
if (SUCCEEDED(ptrIDraw->QueryInterface(IID_IShape, &ptrIShape))
....
}This diagnostic is classified as:
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