operator const void*()

// Example with smart pointers:
     
template<typename T>class SmartPointer
{
    public:
    SmartPointer( T* );

    operator const void*() const;
};

// Here the designer added conversion to void
// const* to help comparison to the null pointer.
// However a bad side effect to this feature
// is that it is possible to generate equality
// comparison between two smart pointers on
// different types:

void doIt( AA* pa, BB* pb )
{
    SmartPointer <aa> 
spa( pa );
    SmartPointer <bb> 
spb( pb );
    if (spa == spb)  // problem! comparing different pointer types
    {}
}