A generalized basic rule from item 7.

Suppose we have an inheritance hierarchy defined like this:

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class B {
public:
    void mf();
    ...
};

class D: public B {
public: 
    void mf();  // hide B::mf
    ...
};

According to item 33, this will lead to following behavior:

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D x;  // x is an object of type D
B *pB = &x;  // get pointer to x
D *pD = &x;  // get pointer to x

pB->mf();  // calls B::mf
pD->mf();  // calls D::mf

The reason for this two-faced behavior if that non-virtual functions like B::mf and D::mf are statically bound (item 37), which means that non-virtual functions invoked through pB will always be those defined for class B, even if pB points to an object of a class derived from B. This mismatched behavior exhibits inconsistent behavior, since any given D object may act like either a B or a D (depending on the pointer type) when mf is called.

On the other hand, virtual functions are dynamically bound (item 37), so if mf were a virtual function, a call to mf through either pB or pD would result in an invocation of D::mf, because pB and pD really point to is an object of type D.

Worse than the inconsistent behavior problem in the real world, the example above basically shows a contradiction in class design, theoretically:

• public inheritance means is-a (item 32), so everything that applies to B object also applies to D object
• a non-virtual function in the base class establishes an invariant over specialization that classes derived from B must inherit both the interface and the implementation (item 34), and mf is such an non-virtual function in B

As long as we redefine the mf, an inherited non-virtual function, one of the conditions above will break, leading to two solutions: either make the mf virtual, or change the class relationship from ‘is-a’ to something else.

In fact, this is exactly the same argument in item 7, which exlains why destructors in polymophic base classes should be virtual. In essence, item 7 is nothing more than a special case of this item.