转换为派生类的ptr到ptr

Question

如果我没有为Base**指定显式强制转换,为什么会出现编译错误？

处理派生类时,我可以使用指针指针吗？

class Base { };
class Child : public Base { };

void SomeFunction(Base** ppoObj) {}
void SomeFunction(Base* poObj) {}

int main()
{   
    Child *c = new Child();

    // This passed.
    SomeFunction(c);
    SomeFunction((Base**)&c);

    // CompilationError
    SomeFunction(&c);

    return 0;
}

Answer 1

虽然你可以隐式地转换Child*为Base*,但是没有隐式Child**转换Base**,因为它可能被用来违反类型安全性.考虑:

class Base { };
class Child1 : public Base { };
class Child2 : public Base { };

int main() {
    Child1 *c = new Child1();
    Base **cp = &c;  // If this were allowed...
    *cp = new Child2();  // ...then this could happen.  (*cp is a Base*)
}

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