通过纯虚方法覆盖（常规）虚方法有什么影响？

Let's say we have

假设我们有

class A {    
public:    
    virtual int foo() { cout << "foo!"; }    
}

class B : public A {
public:    
    virtual int foo() =0;
}

class C : public B {
public:
    virtual int foo() { cout << "moo!"; }
}
class A {    
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