multiple inheritance: downcasting across derived-sibling classes?

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Dear,

Thanks in advance.
I am working on C++ project in DevC++ environment. I am wondering how following program is working fine?

C++

#include <iostream>
using namespace std;

class Base{};
class Derived1:public Base{};
class Derived2:public Base
{
      public:
             float Derived2Data;
             void Derived2Fun(){}
      void Derived2Fun1()
      {
           Derived2Data = 10.9;
      }
};

void main()
{
      Derived1 obj2;
        Base* pBase1 = &obj2;
        ((Derived2*)pBase1)->Derived2Fun(); // no error
        ((Derived2*)pBase1)->Derived2Fun1(); // no error
        cout<<"Value of Derived2Data is:"<<((Derived2*)pBase1)->Derived2Data<<endl;
}

Output: Value of Derived2Data is: 10.9

even though this is a wrong typecasting. upcasting->cross-downcasting. even then there is no run-time as well as compilation error. it may cause disasters, this is very unsafe?

Posted 8-Feb-12 23:24pm

ggupta2009

Updated 9-Feb-12 1:22am

Richard MacCutchan

v2

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Richard MacCutchan 9-Feb-12 7:30am

I don't know the answer, but I wonder if it is because Base and Derived1 are empty classes? What happens if you put some actual code inside them both?

Stefan_Lang 9-Feb-12 7:59am

This is not multiple inheritance at all.

2 solutions

Solution 1

// I am wondering how following program is working fine?

Please put also the sizesof(Derived1 obj1) and sizeof(Derived2 obj2) out ;)

Posted 9-Feb-12 0:47am

Eugen Podsypalnikov

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ggupta2009 9-Feb-12 7:36am

Hi,

I have added following inside main()

Derived2 obj3;
cout<<"size of Derived1 class is: "<<sizeof(Derived1)<<endl;
cout<<"size of Derived1 class object is: "<<sizeof(obj2)<<endl;
cout<<"size of Derived2 class is: "<<sizeof(Derived2)<<endl;
cout<<"size of Derived2 class object is: "<<sizeof(obj3)<<endl;

Output:
Value of Derived2Data is: 10.9
size of Derived1 class is: 1
size of Derived1 class object is: 1
size of Derived2 class is: 4
size of Derived2 class object is: 4

Eugen Podsypalnikov 9-Feb-12 7:47am

So the casting has just "extend" (incorrect)
the boundary of a smaller object by the data member of the bigger type :)

There could lie another application data,
but your test had a lucky end (not a low) :)

When you try to extend the smaller object by a float member as well,
the "casted" set method will set it (not the original member) :)

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Stefan_Lang · Accepted Answer · 2012-02-09T01:57:00

First of all, using C-style typecasts on C++ types is a very bad idea. Use dynamic_cast, static_cast, etc. instead (and make sure you know exactly when to use which: see http://www.cplusplus.com/doc/tutorial/typecasting/[^] )

~~Second, without virtual functions, inheritance doesn't make sense. But I'll assume for now this is just an oversimplifaction on your part to focus on the code in question.~~ [edit]does not relate to the question - please ignore[/edit]

Third, C style typecasts will always be accepted by the compiler: they basically tell the compiler that you know better than the compiler does. (IME this is very often a misconception, and in this case it definitely is)

Last, this works because you were lucky: What happens here is that on the stack, an object of type Derived1 is created, with the appropriate size to fit such an object. Then, you the compiler that in truth this is an object of type Derived2 instead. Because you used C-style typecasting the system will comply to your wish without question and overlay the memory layout of Derived2 over the memory occupied by obj2. This will work for as long as you don't try to use virtual functions, or data members, but when you call Fun1(), you actually write into a data member: Since the memory orginally allocated is not big enough to fit Derived2, you're writing outside the bounds of that memory.

Normally, this causes other variables to be corrupted, but it seems that you were lucky in this case. I believe that internally, the stack variables are stored in reverse order of declaration: in this case pBase first, then obj2. If it had been the other way round, you would have overwritten pBase1 and the last line of your code would likely have crashed,