用子类指针指向父类对象,但只访问父类对象的成员,安全吗?

[menzi11]

比如:
class A { public: int data;}

class B :public A {......}

B* ptr=(B*)new A;
ptr->data=10;

这样做安全吗? 保证不会访问到子类成员,标准中有相关规定吗?

[ri_aje]

B* ptr=(B*)new A;

已经导致为定义行为了，后面的访问都不行。

[「已注销」]

如果要这么做，那么基类中会被调用的所有函数都声明为虚函数，否则看下面

#include <iostream>
using namespace std;

class TestBase
{
private:
	int a;
public:
	TestBase(){ a=0; }
	~TestBase(){}
	void print()
	{
		cout<<"TestBase print:"<<a<<endl;
	}
};

class TestOne:public TestBase
{
private:
	int a;
public:
	TestOne(){ a=1; }
	~TestOne(){}
	void print()
	{
		cout<<"TestOne print"<<a<<endl;
	}
};

int main(int argc, char* argv[])
{
	TestOne* one = (TestOne*)new TestBase;
	one->print();
	return 0;
}

[mujiok2003]

答案：是不一定（考虑一下多继承的情况）.

[Luisfan]

你这样强转过后,肯定是基类的啊.

[max_min_]

显然没有直接对象类型安全！

[Luisfan]

如果B中也有data的话,你这样是会出问题的

[derekrose]

有什么意义呢？倒不如说一下你遇到的问题

[一入程序深似海]

只访问基类的成员，倒是没有什么问题。 但是这样写，可读性不差。 首先，你要注意，访问的到底是不是基类的成员，访问的成员函数，是否被派生类override了？ 与其这样，还不如直接用基类的指针。 起码基类的指针不会让你访问一个不属于基类的成员.

[漫步者、]

[turing-complete]

不安全，标准中没有这样的约定。 而且，我认为多继承的情况下，这很可能就错了。

[worldy]

引用 楼主 menzi11 的回复:

比如: class A { public: int data;} class B :public A {......} B* ptr=(B*)new A; ptr->data=10; 这样做安全吗? 保证不会访问到子类成员,标准中有相关规定吗?

肯定不安全，两个对象占用的空间是不一样的

[ri_aje]

引用 14 楼 menzi11 的回复:

[quote=引用 13 楼 ri_aje 的回复:] [quote=引用 12 楼 menzi11 的回复:] [quote=引用 6 楼 ri_aje 的回复:]

B* ptr=(B*)new A;

已经导致为定义行为了，后面的访问都不行。

这样就UB了吗? 好简单.....[/quote] yep, and if you are interested, here is what the standard has to say. c++11 5.2.9/11 A prvalue of type “pointer to cv1 B,” where B is a class type, can be converted to a prvalue of type “pointer to cv2 D,” where D is a class derived (Clause 10) from B, if a valid standard conversion from “pointer to D” to “pointer to B” exists (4.10), cv2 is the same cv-qualification as, or greater cv-qualification than, cv1, and B is neither a virtual base class of D nor a base class of a virtual base class of D. The null pointer value (4.10) is converted to the null pointer value of the destination type. If the prvalue of type “pointer to cv1 B” points to a B that is actually a subobject of an object of type D, the resulting pointer points to the enclosing object of type D. Otherwise, the result of the cast is undefined.[/quote] 经典!! 此贴终结!也感谢其他人 另外这标准在哪里可以看...[/quote] 这里 (http://www.open-std.org/jtc1/sc22/wg21/) 有的 draft，标准的话，自己找找吧。

[ztenv]

这样做真的没什么意义，

[menzi11]

引用 13 楼 ri_aje 的回复:

[quote=引用 12 楼 menzi11 的回复:] [quote=引用 6 楼 ri_aje 的回复:]

B* ptr=(B*)new A;

已经导致为定义行为了，后面的访问都不行。

这样就UB了吗? 好简单.....[/quote] yep, and if you are interested, here is what the standard has to say. c++11 5.2.9/11 A prvalue of type “pointer to cv1 B,” where B is a class type, can be converted to a prvalue of type “pointer to cv2 D,” where D is a class derived (Clause 10) from B, if a valid standard conversion from “pointer to D” to “pointer to B” exists (4.10), cv2 is the same cv-qualification as, or greater cv-qualification than, cv1, and B is neither a virtual base class of D nor a base class of a virtual base class of D. The null pointer value (4.10) is converted to the null pointer value of the destination type. If the prvalue of type “pointer to cv1 B” points to a B that is actually a subobject of an object of type D, the resulting pointer points to the enclosing object of type D. Otherwise, the result of the cast is undefined.[/quote] 经典!! 此贴终结!也感谢其他人 另外这标准在哪里可以看...

[menzi11]

引用 6 楼 ri_aje 的回复:

B* ptr=(B*)new A;

已经导致为定义行为了，后面的访问都不行。

这样就UB了吗? 好简单.....

[ri_aje]

引用 12 楼 menzi11 的回复:

[quote=引用 6 楼 ri_aje 的回复:]

B* ptr=(B*)new A;

已经导致为定义行为了，后面的访问都不行。

这样就UB了吗? 好简单.....[/quote] yep, and if you are interested, here is what the standard has to say. c++11 5.2.9/11 A prvalue of type “pointer to cv1 B,” where B is a class type, can be converted to a prvalue of type “pointer to cv2 D,” where D is a class derived (Clause 10) from B, if a valid standard conversion from “pointer to D” to “pointer to B” exists (4.10), cv2 is the same cv-qualification as, or greater cv-qualification than, cv1, and B is neither a virtual base class of D nor a base class of a virtual base class of D. The null pointer value (4.10) is converted to the null pointer value of the destination type. If the prvalue of type “pointer to cv1 B” points to a B that is actually a subobject of an object of type D, the resulting pointer points to the enclosing object of type D. Otherwise, the result of the cast is undefined.