Effective C++ 笔记 —— Item 36: Never redefine an inherited non-virtual function.

Consider:

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

class D : public B { /*...*/ };

D x; // x is an object of type D

B *pB = &x; // get pointer to x
pB->mf();

D * pD = &x; // get pointer to x
pD->mf();

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 behave the same way.

it won't if mf is nonvirtual and D has defined its own version of mf:

class D : public B 
{
public:
    void mf(); // hides B::mf; see Item 33
    // ...
};

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.

Things to Remember:

• Never redefine an inherited non-virtual function.