Item 36: Never redefine an inherited non-virtual function
Suppose I tell you that a class D is publicly derived from a class B and that there is a public member function mf defined in class B. The parameters and return type of mf are unimportant, so let's just assume they're both void. In other words, I say this:
class B {
public:
void mf();
...
};
class D: public B { ... };
Even without knowing anything about B, D, or mf, given an object x of type D,
D x; // x is an object of type D
you would probably be quite surprised if this,
B *pB = &x; // get pointer to x
pB->mf(); // call mf through pointer
behaved differently from this:
D *pD = &x; // get pointer to x
pD->mf(); // call mf through pointer
That's because in both cases you're invoking the member function mf on the object x. Because it's the same function and the same object in both cases, it should behave the same way, right?
Right, it should. But it might not. In particular, it won't if mf is non-virtual and D has defined its own version of mf:
class D: public B {
public:
void mf(); // hides B::mf; see Item33
...
};
pB->mf(); // calls B::mf
pD->mf(); // calls D::mf
The reason for this two-faced behavior is that non-virtual functions like B::mf and D::mf are statically bound (see Item 37). That means that because pB is declared to be of type pointer-to-B, 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, as it does in this example.
Virtual functions, on the other hand, are dynamically bound (again, seeItem 37), so they don't suffer from this problem. If mf were a virtual function, a call to mf through either pB or pD would result in an invocation of D::mf, because what pB and pD really point to is an object of type D.
If you are writing class D and you redefine a non-virtual function mf that you inherit from class B, D objects will likely exhibit inconsistent behavior. In particular, any given D object may act like either a B or a D when mf is called, and the determining factor will have nothing to do with the object itself, but with the declared type of the pointer that points to it. References exhibit the same baffling behavior as do pointers.
But that's just a pragmatic argument. What you really want, I know, is some kind of theoretical justification for not redefining inherited non-virtual functions. I am pleased to oblige.
Item 32 explains that public inheritance means is-a, and Item 34 describes why declaring a non-virtual function in a class establishes an invariant over specialization for that class. If you apply these observations to the classes B and D and to the non-virtual member function B::mf, then
Everything that applies to B objects also applies to D objects, because every D object is-a B object; Classes derived from B must inherit both the interface and the implementation of mf, because mf is non-virtual in B.
Now, if D redefines mf, there is a contradiction in your design. If D really needs to implement mf differently from B, and if every B object — no matter how specialized — really has to use the B implementation for mf, then it's simply not true that every D is-a B. In that case, D shouldn't publicly inherit from B. On the other hand, if D really has to publicly inherit from B, and if D really needs to implement mf differently from B, then it's just not true that mf reflects an invariant over specialization for B. In that case, mf should be virtual. Finally, if every D really is-a B, and if mf really corresponds to an invariant over specialization for B, then D can't honestly need to redefine mf, and it shouldn't try to.
Regardless of which argument applies, something has to give, and under no conditions is it the prohibition on redefining an inherited non-virtual function.
If reading this Item gives you a sense of déjà vu, it's probably because you've already read Item 7, which explains why destructors in polymorphic base classes should be virtual. If you violate that guideline (i.e., if you declare a non-virtual destructor in a polymorphic base class), you'll also be violating this guideline, because derived classes would invariably redefine an inherited non-virtual function: the base class's destructor. This would be true even for derived classes that declare no destructor, because, as Item 5 explains, the destructor is one of the member functions that compilers generate for you if you don't declare one yourself. In essence, Item 7 is nothing more than a special case of this Item, though it's important enough to merit calling out on its own.
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