Re: Explanation for behavior

From:
Kai-Uwe Bux <jkherciueh@gmx.net>
Newsgroups:
comp.lang.c++
Date:
Mon, 16 Jul 2007 20:38:06 +0200
Message-ID:
<f7gdv5$1kg$2@murdoch.acc.Virginia.EDU>
 rogerhillster@gmail.com wrote:

On Jul 16, 11:02 am, red floyd <no.s...@here.dude> wrote:

rogerhills...@gmail.com wrote:

Hi,

I have the below code for printing out the contents of a map.
The output prints the strings contained in the value for each key in
the map.
In the first for loop, I am clearing the list contents and I am
storing the reference to the list in each of the map's entries.
Could somebody let me know as to why even after clearing the contents
of the list, I can see different strings contained in the map for each
key?


Because you're not storing a reference to the list in the map. You're
storing a copy of the list in the map. You can't store references in
Standard Library containers.

[code retained for reference purposes]

#include <iostream>
#include <map>
#include <list>
#include <string>
using namespace std;

int main() {
   typedef list<string> tslist;
   tslist slist;
   map<int, tslist> tmap;
   int i = 0, j =0;
   for (; i < 10; i++) {
           slist.clear();
           char str[10];
           itoa(i, str, 10);
           slist.push_front(string(str));
           tmap[i] = slist;
   }

   i = 0;
   for (; i < 10; i++) {
           tslist temp = tmap[i];
           tslist::iterator iter = temp.begin();
           while(iter!=temp.end()) {
                   string& temp = *iter;
                   cout << temp.c_str() << endl;
                   iter++;
           }
   }
   return 0;
}- Hide quoted text -


- Show quoted text -


Floyd,

I thought that Vector containers store objects by reference.
Am I wrong on that?


Yes, you are wrong.

Indeed, all standard containers store objects by value and make copies of
whatever you push into them. If you need reference semantics for type T
objects in containers, you need to use container<T*> or (as some would
prefer) container< some_smart_pointer<T> > to introduce the necessary level
of indirection.

Best

Kai-Uwe Bux

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