Ambiguity of 'this' and 'member function' [modified]

[Dean Seo]

This code below occurs no error but the result seems a bit weird.

class KBase
{
public:
void BaseFun()
{
printf( "KBase::BaseFun() this=%p\n", this );
}
};

class KMiddle : public KBase
{
};

class KMiddle2 : public KBase
{
};

class KDerived : public KMiddle, public KMiddle2
{
};

int main()
{
KDerived d;

d.KBase::BaseFun();   // 0012FF63
d.KMiddle::BaseFun();  // 0012FF63
d.KMiddle2::BaseFun();  // 0012FF64 ??

return 0;

}//main()

BaseFun simply shows this pointer and the result goes.. --------------------------- KBase::BaseFun() this=0012FF63 KBase::BaseFun() this=0012FF63 KBase::BaseFun() this=0012FF64 --------------------------- So my question is why KBase::BaseFun() and KMiddle::BaseFun() show the same address and the only KMiddle2::BaseFun() does the different address? I was looking for the answer by reading Stephen Prata's book and Effective C++ but I couldn't. Thanks in advance.

modified on Wednesday, August 24, 2011 9:51 PM

[xrg_soft 163 com]

Which complier do you use? in some complier that follow the standard c++ strictly, like g++, your code will not compiled and got Error Message: `D' is an ambiguous base of `B' To make the multiple inheritance follows the standard c++ well, you should using virtual inheritance in this case.

class KMiddle : virtual public KBase
class KMiddle2 : virtual public KBase

class KDerived : public KMiddle, public KMiddle2

[Dean Seo]

Hi,

xrg_soft@163.com wrote:

Which complier do you use?

I use Visual Studio 2008.

xrg_soft@163.com wrote:

To make the multiple inheritance follows the standard c++ well, you should using virtual inheritance

I know that the multiple inheritance needs virtual inheritance. However, when I don't on purpose, the result seems not understandable. Could you explain to me why those 'this' addresses look unrestrained?

[CPallini]

From "Diamond problem" at Wikipedia[^]:

C++ by default follows each inheritance path separately, so a D object would actually contain two separate A objects, and uses of A's members have to be properly qualified. If the inheritance from A to B and the inheritance from A to C are both marked "virtual" (for example, "class B : virtual public A"), C++ takes special care to only create one A object, and uses of A's members work correctly. If virtual inheritance and nonvirtual inheritance are mixed, there is a single virtual A and a nonvirtual A for each nonvirtual inheritance path to A. Please note that nonvirtual derivation of A in this case will be useless as direct access to any part of class A from class D will practically always end up with compile error.

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[xrg_soft 163 com]

In your case(no virtual inheritance used), the The inside of the KDrived object, there is a KMiddle object followed by a KMiddle2 object. d.KBase::BaseFun(); d.KMiddle::BaseFun(); actually calls to the function of the KMiddle object. and d.KMiddle2::BaseFun(); calls to the function of the KMiddle2 object. As your Kxxx object contains nothing, so sizeof(Kxxx) == 1 That's why the result printed. And if you adding somemember variable into the class, then the result should be: KBase::BaseFun() this= KBase::BaseFun() this= KBase::BaseFun() this=<base> + sizeof(KMiddle)

[Dean Seo]

Thank you so much!! I truly appreciate it!

[Dean Seo]

Thank you!