Re: STL vector Problem: push_back derived object but get base object

From:

Francis Glassborow <francis.glassborow@btinternet.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 11 Mar 2009 10:18:41 CST

Message-ID:

<3NydnQN2X6HgWCrUnZ2dnUVZ8giWnZ2d@bt.com>

gph wrote:

Hello, I have a problem about STL vector. I expect vector can hold my
derived objects. But it doesn't. My program is like this:
==================================
#include <vector>
using namespace std;

class CBase{
public:
virtual void print(){ printf("Base\n");}
};
class CDerived : public CBase{
public:
void print(){ printf("Derived\n");}
};

main(){
vector<CBase> v;
CDerived d;
v.push_back(d);

vector<CBase>::iterator i;
for(i = v.begin(); i != v.end(); ++i){
(*i).print();
}
}
==================================

I expected it to output "Derived", however, it outputs "Base". Could
anyone help me? Thank you very much.

I do not know how helpful this will be for you but all C++ containers
are designed to handle single homogeneous types. IOWs you cannot do waht
you are trying to do. There are excellent reasons why this is the case.
One of them being that containers require to know how much space an
object will take.

The way to achieve your desired objective is not to store objects of the
type base or derived in your container but to have a container of
shared_ptr<Cbase> (note do not try using auto_ptr for this purpose as
the ownership semantics of auto_ptr are wrong for a container) and
attach a the CBase/CDerived objects to those.

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