Confuse about how the function Test be called

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Hi theres!
Please see my test code :

C++

#include "stdio.h"
class A 
{ 
public: 
	A(){} 
	virtual void Test1(){} 
	virtual void Test2(){}

}; 

class  B : public A 
{ 
public: 
	B(){} 
	virtual void Test(){printf("B:\n");} 
	virtual void Test1(){} 
	virtual void Test2(){}
	void TestVFunctionNotInVtable(){Test();}
}; 
class C : public B
{	
	virtual void Test(){printf("C:\n");} 
};
class D:public C
{

};

int main()
{
	B * p = new C;
	p->TestVFunctionNotInVtable();
}

when i debug this ,i don't see the function Test(first declared in class B ) in the vtable,but after press Ctrl +F5 it shows C:,so it actually call C::Test,my problem is why the function Test isn't in vtable but it can be called . is it the problem of IDE vs2010 ,function Test is in the vtable but it doesn't show it? somebody knows?

Posted 14-Jul-13 16:29pm

xd zong

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2 solutions

Solution 1

That is exactly what the "virtual" is all about. When you create a "C" object, it knows that it is a C, even though you point to it with a B* pointer.

tl;dr: polymorphism

Posted 14-Jul-13 16:38pm

H.Brydon

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xd zong 14-Jul-13 22:53pm

i think the virtual call depends on whether the virtual function is in the vtable. here is not in

H.Brydon 14-Jul-13 22:58pm

If you are asking about TestVFunctionNotInVtable(), it should show up in B, C and D since you implemented it in B. When you call p->TestVFunctionNotInVtable() on the C object, it should see C::Test() (since it is a C object).

xd zong 14-Jul-13 23:15pm

i don't konw why it should see::Test.

H.Brydon 15-Jul-13 0:16am

No that is not what I mean. Look at your code for B.

When calling a C object, B::TestVFunctionNotInVtable() will call C::Test().

Stefan_Lang 17-Jul-13 4:02am

I think he does understand the output. What he doesn't understand is why the function "Test" doesn't show up in the vtable when he inspects the p in the debugger!

I just tested it: When you stop the program before executing the last line, the vtable will show entries for Test1() and Test2(), but not for Test().

I have no explanation for this, other than that it may be an error of the IDE: apparently it only shows entries for those functions declared in the top level class, A.

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Stefan_Lang · Accepted Answer · 2013-07-16T22:19:00

Apparently this is a limitation of the IDE. I've found an article on CodeProject about it:
Displaying vtable when debugging[^]

The variable vt will be interpretet as a pointer to a vt entry in the debugger watch window, but you can view the entire array if you type "vt,3" in the watch window. (see http://stackoverflow.com/questions/972511/view-array-in-visual-studio-debugger[^] )

P.S.: I just found out that you can see the entire vtable without changing the code, if you type: "(void**)(((*(A*)(&*p))).__vfptr), 3" in the watch window!