C++ 弱问一个 - 未名空间MITBBS历史存档

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C++ 弱问一个

C++ 弱问一个# Programming - 葵花宝典

m*o2007-08-25 07:08

1 楼

请问这个继承最后输出的怎么是BB，而不是CC呢？main函数里的逻辑过程是什么样的？
#include
using namespace std;
class A{
protected:
virtual void print() { cout << "A" << endl; }
void print2() { cout << "AA" << endl; }
};
class B {
public:
virtual void print() { cout << "B" << endl; }
void print2() { cout << "BB" << endl; }
};
class C : public A, public B{
public:
void print2() { cout << "CC" << endl; }
};
int main()
{
C c;
B* p = &c;
p->print2();
};

c*g2007-08-25 07:08

2 楼

print2 virtual?

【在 m*******o 的大作中提到】

: 请问这个继承最后输出的怎么是BB，而不是CC呢？main函数里的逻辑过程是什么样的？
: #include
: using namespace std;
: class A{
: protected:
: virtual void print() { cout << "A" << endl; }
: void print2() { cout << "AA" << endl; }
: };
: class B {
: public:

D*a2007-08-25 07:08

3 楼

print2 is not virtual

【在 m*******o 的大作中提到】

o*o2007-08-25 07:08

4 楼

This is called upcast. You assigned the address of the object to
that of its base class. The function newly defined in derived class
will be un-accessible to that pointer. If a function defined virtual
in base class and you overrided it in derived class, the copy in
derived class will be called. Otherwise, the copy in base class is called.

【在 m*******o 的大作中提到】

s*e2007-08-25 07:08

5 楼

s*e2007-08-25 07:08

6 楼

Name lookup happens at compiling time.
Since the static type of p is B*, p will invoke the print2() defined in
its own scope unless print2() is virtual.
Obviously print2() is not virtual in this case and thus its own print2()
is called. ==> BB

S*X2007-08-25 07:08

7 楼

正解，Thinking in C++好像专门有一章讲的很详细

【在 o*o 的大作中提到】

: This is called upcast. You assigned the address of the object to
: that of its base class. The function newly defined in derived class
: will be un-accessible to that pointer. If a function defined virtual
: in base class and you overrided it in derived class, the copy in
: derived class will be called. Otherwise, the copy in base class is called.

m*o2007-08-25 07:08

8 楼

大概明白了，多谢各位！

m*o2007-08-25 07:08

9 楼

那请教 B* p = &c; 是什么意思？
是定义一个类B的指针p，其地址是&c，那不还是用到了CLASS C吗？？？

t*l2007-08-25 07:08

10 楼

upcast了

【在 m*******o 的大作中提到】

: 那请教 B* p = &c; 是什么意思？
: 是定义一个类B的指针p，其地址是&c，那不还是用到了CLASS C吗？？？

a*s2007-08-25 07:08

11 楼

It seems that for the upcast, there is no need to use static_cast.
but for the downcast. it must.
B* p = &c; //ok
B* p = static_cast(&c) //ok
C* p = &b; //error
C* p = static_cast(&b) //ok
anyone explain it?

【在 t*****l 的大作中提到】
: upcast了