any potential multi-heritance fault?

[Patricia]

Hi all,

Could you tell me any potential fault in the following codes if I run
them on UNIX? BTW, class B could contain a lot of data.

#include <iostream.h>

class A{
public:
A(){}
~A(){}

int size;
int a;
};

class B : public A{
public:
B(){}
~B(){}
int b;
};

class C{
public:
C(){c = 100;}
~C(){}
int c;
};

class D : public C, public A{
public:
D():C(){}
~D(){}
};

int main()
{
B *bp = new B();
bp->b = 10;
bp->size = sizeof(B);

int size = sizeof(C) + bp->size;

D *dp;
dp = (D*) new char[size];
new (dp)D();

memcpy((A*)dp, bp, bp->size);

cout << "b:" << dp->size << endl;

delete []((char*)dp);
delete bp;
}

Best Regards,

Patricia

[mlimber]

Patricia wrote:

Hi all,

Could you tell me any potential fault in the following codes if I run
them on UNIX?
The platform should be irrelevant.
BTW, class B could contain a lot of data.
I trust that your example below actually demonstrates your question,
then. If the actual code differs too much in principle, then our
answers won't help.
#include <iostream.h>
#include <iostream> // iostream.h is deprecated
#include <new> // for placement new used in main
#include <cstring> // for memcpy
using namespace std;

class A{
public:
A(){}
~A(){}

int size;
int a;
};
Generally it's a poor design to have public data, but I suspect this is
only for purposes of this example.

class B : public A{
public:
B(){}
~B(){}
int b;
};

class C{
public:
C(){c = 100;}
// See http://www.parashift.com/c++-faq-lit....html#faq-10.6
C() : c(100) {}
~C(){}
int c;
};

class D : public C, public A{
public:
D():C(){}
~D(){}
};

int main()
{
B *bp = new B();
Prefer attaching a smart pointer (e.g. std::auto_ptr,
boost::shared_ptr, etc.) to any object allocated with new. It provides
exception safety, ease of use (i.e., you don't have to remember to
delete it), and no performance overhead.
bp->b = 10;
bp->size = sizeof(B);

int size = sizeof(C) + bp->size;

D *dp;
dp = (D*) new char[size];
new (dp)D();

memcpy((A*)dp, bp, bp->size);
Bad bad bad. Don't do this. Your objects should define copy
constructors or assignment operators if you want to convert between
them. The appearance of memcpy in C++ programs should set off alarms
and raise danger flags.

cout << "b:" << dp->size << endl;

You have a destructor for D, which I presume means you want to use it,
but since you used placement new, you must explicitly call the
destructor before you delete its allocated memory (see
http://www.parashift.com/c++-faq-lit...html#faq-11.10 et al.):

dp->~D();
delete []((char*)dp);
delete bp;
return 0;
}

Best Regards,

Patricia

Cheers! --M