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C ++ polymorphism ((X *) y) & # 8594; foo () vs ((X) * y) .foo ()

Suppose Y is a derived class from class X, and X declares foo virtual. Let y be of type (Y *). Then ((X *) y) → foo () will execute the Y version of foo (), but ((X) * y) .foo () will execute the X version. Can you tell me why polymorphism is not applied in case of dereferencing? I expect any syntax to give a version of Y foo ().

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You cut a part of the object Yand copy the object to the object X. The called function is called on the object X, and therefore the function is called X.

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The code (X *) y is like creating a pointer to an object, and the compiler knows that the specified object is X or a subclass of X. At run time, when you look up the pointer and call foo using "-> foo ()", the object class is defined and the correct function is used.

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