How does the C++ compiler know which implementation of a virtual function to call?

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Each object (that belongs to a class with at least one virtual function) has a pointer, called a vptr. It points to the vtbl of its actual class (which each class with virtual functions has at least one of; possibly more than one for some multiple-inheritance scenarios).

The vtbl contains a bunch of pointers, one for each virtual function. So at runtime, the code just uses the object’s vptr to locate the vtbl, and from there the address of the actual overridden function.

In your specific case, Polygon, Rectangle, and Triangle each has a vtbl, each with one entry pointing to its relevant area method. Your ppoly1 will have a vptr pointing to Rectangle‘s vtbl, and ppoly2 similarly with Triangle‘s vtbl. Hope this helps!

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