Re: What is wrong with reference into std::map?

From:

"Jim Langston" <tazmaster@rocketmail.com>

Newsgroups:

comp.lang.c++

Date:

Sun, 14 May 2006 11:27:15 -0700

Message-ID:

<fEK9g.31$gb4.25@fe04.lga>

"Jim Langston" <tazmaster@rocketmail.com> wrote in message
news:TBK9g.30$gb4.20@fe04.lga...

Expected output of program:
Key is: 0 String is: Hello
Key is: 1 String is: Goodbye
Key is: 2 String is: The end

Actual output:
Key is: 0 String is: The End
Key is: 1 String is:
Key is: 2 String is:

What am I doing wrong?

If I declare my reference fresh each time, such as putting { } around the
places I insert and doing
std::string& MyString = (*it).second;
each time it will come out right.

Why can't I reuse the references?

It took a long time to find out what was causing this in my program. This
is just a test program showing the issue.

#include <iostream>
#include <string>
#include <map>

std::map<unsigned int, std::string> MyMap;

int main ()
{
   unsigned int ID = 0;
   std::map< unsigned int, std::string>::iterator it = MyMap.insert(
MyMap.end(), std::make_pair< unsigned int, std::string >( ID,
std::string() ) );
   std::string& MyString = (*it).second;
   MyString = "Hello";

   ID = 1;
   it = MyMap.insert( MyMap.end(), std::make_pair< unsigned int,
std::string >( ID, std::string() ) );
   std::string& MyString2 = (*it).second;

This was a broken attempt to diagnose. Even with this line changed to:
std::string& MyString2 = (*it).second

the output is the same.

   MyString = "Goodbye";

   ID = 2;
   it = MyMap.insert( MyMap.end(), std::make_pair< unsigned int,
std::string >( ID, std::string() ) );
   MyString = (*it).second;
   MyString = "The End";

   for ( std::map< unsigned int, std::string >::iterator i =
MyMap.begin(); i != MyMap.end(); ++i )
   {
       std::cout << "Key is: " << (*i).first << " String is: " <<
(*i).second << std::endl;
   }

   std::string wait;
   std::cin >> wait;