Accessibility of Derived-to-Base conversion in protected inheritance.
这是一个粑粑一样的feature。
class B;
class C;
class D;
class A{
public:
virtual ~A(){}
friend void gg(D* d);
void ga(B *b,C* c,D* d);
int mem;
};
class B:protected A{
public:
void gb(B *b,C* c,D* d);
};
class C:protected B{
public:
void gc(B *b,C* c,D* d);
};
class D:protected C{
public:
void gd(B *b,C* c,D* d);
};
void A::ga(B *b,C*c,D*d){
int tmpa0 = b->mem; // illegal
int tmpa1 = c->mem; // illegal
int tmpa2 = d->mem; // illegal
A *a1=b; // 'A' is an inaccessible base of 'B'
A *a2=c; // 'A' is an inaccessible base of 'C'
A *a3=d; // 'A' is an inaccessible base of 'D'
}
void B::gb(B *b,C*c,D*d){
int tmpb1 = b->mem;
int tmpb2 = c->mem; // illegal
int tmpb3 = d->mem; // illegal
A *a1=b; // no problem here
A *a2=c; // illegal
A *a3=d; // illegal
}
void C::gc(B *b,C*c,D*d){
int tmpc1 = b->mem; // illegal
int tmpc2 = c->mem; // no problem here
int tmpc3 = d->mem; // illegal
A *a1=b; // no problem here
A *a2=c; // no problem here
A *a3=d; // illegal
}
void D::gd(B *b,C*c,D*d){
int tmpd1 = b->mem; // illegal
int tmpd2 = c->mem; // illegal
int tmpd3 = d->mem; // illegal
A *a1=b; // no problem here
A *a2=c; // no problem here
A *a3=d; // no problem here
}
void gg(D* d){
A* a=d; // illegal
}
int main(){
B b;
C c;
D d;
A a;
gg(&d); // 'A' is an inaccessible base of 'D'
a.ga(&b,&c,&d);
b.gb(&b,&c,&d);
A a1(d); // illegal
A a4=d; // illegal
return 0;
}
继承树全部用protected,基类无法访问派生类对象,如B中无法访问C中的mem。
如果派生类能够进行隐式转换,即Derived-to-Base conversion,那么它的直接基类和间接基类直到root,都可以进行这个转换。
class B;
class C;
class D;
class A{
public:
virtual ~A(){}
friend void gg(D* d);
void ga(B *b,C* c,D* d);
int mem;
};
class B:protected A{
public:
void gb(B *b,C* c,D* d);
};
class C:public B{
public:
void gc(B *b,C* c,D* d);
};
class D:protected C{
public:
void gd(B *b,C* c,D* d);
};
void A::ga(B *b,C*c,D*d){
int tmpa0 = b->mem; // illegal
int tmpa1 = c->mem; // illegal
int tmpa2 = d->mem; // illegal
A *a1=b; // 'A' is an inaccessible base of 'B'
A *a2=c; // 'A' is an inaccessible base of 'C'
A *a3=d; // 'A' is an inaccessible base of 'D'
}
void B::gb(B *b,C*c,D*d){
int tmpb1 = b->mem;
int tmpb2 = c->mem; // no problem
int tmpb3 = d->mem; // illegal
A *a1=b; // no problem here
A *a2=c; // no problem here
A *a3=d; // illegal
}
void C::gc(B *b,C*c,D*d){
int tmpc1 = b->mem; // illegal
int tmpc2 = c->mem; // no problem here
int tmpc3 = d->mem; // illegal
A *a1=b; // no problem here
A *a2=c; // no problem here
A *a3=d; // illegal
}
void D::gd(B *b,C*c,D*d){
int tmpd1 = b->mem; // illegal
int tmpd2 = c->mem; // illegal
int tmpd3 = d->mem; // no problem here
A *a1=b; // no problem here
A *a2=c; // no problem here
A *a3=d; // no problem here
}
void gg(D* d){
A* a=d; // illegal
}
int main(){
B b;
C c;
D d;
A a;
gg(&d); // 'A' is an inaccessible base of 'D'
a.ga(&b,&c,&d);
b.gb(&b,&c,&d);
A a1(d); // illegal
A a4=d; // illegal
return 0;
}
修改C:protected B 为C: public B.
在B中可以访问C中的mem,即gb函数中,c->mem是可一访问的,于是可以在B中完成C to A Conversion
如过继续修改D: protected C为D :public B,那么在B中c->mem 和 d->mem都可以访问,所以D to A Conversion也能够实现。
总结
在protected和public混合继承中,如果当前的派生类可以实现Derived-to Base Conversion,那么它的直接基类和间接基类都可以实现Derived-to Base Conversion。
如果想要在当前派生类实现它派生类的Derived-to Base Conversion,例如在B中实现C to A 或者D to A,那么需要在B中能够访问C或者D中继承自A的member,简单来说就是C or D的继承必须用public。