Re: upcasting in vector

From:
"Jim Langston" <tazmaster@rocketmail.com>
Newsgroups:
comp.lang.c++
Date:
Fri, 4 Jan 2008 04:15:45 -0800
Message-ID:
<Xrpfj.5$8P2.3@newsfe02.lga>
majsta@majsta.net wrote:

Hello, I have the following code.

#include <vector>
#include <iostream>

class Foo {
public:
 Foo(){}
 virtual void print() const { std::cout << "foo" << std::endl;}
};

class Boo:public Foo {
public:
 Boo():Foo(){}
 virtual void print() const { std::cout << "boo" << std::endl;}
};

int main(void) {
 std::vector< Foo > vec;
 Foo ff;
 Boo bb;
 vec.push_back(ff);
 vec.push_back(bb);
 std::vector< Foo >::iterator Ivec = vec.begin();
 while(Ivec != vec.end()){
(*Ivec++).print();
 }
 std::cout << "===========" << std::endl;
 std::vector< Foo* > vec2;
 vec2.push_back(&ff);
 vec2.push_back(&bb);
 std::vector< Foo* >::iterator Ivec2 = vec2.begin();
 while(Ivec2 != vec2.end()){
(*Ivec2++)->print();
 }
   return 0;
};

the output is:

foo
foo
===========
foo
boo

so when I create a vector of pointers to Foo, the upcasting of print()
works fine, but it does not for a vector of Foo.

Could you, please, tell me what am I missing here?

Thank you very much for help and your time


Slicing. In your vec you are storing Foo. When you push_back it is dony by
copy, so your Boo is copied to a Foo. Only the Foo part of Boo is taken.
So your vec contains instances of Foo with none of the Bar part.

Polymorphism is done by pointers. In your vec2 you are pushing pointers,
the pointer values are copied, the isntances of Foo and Boo are left alone,
they never get copied or changed.

--
Jim Langston
tazmaster@rocketmail.com

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