Re: std::map insert element issue

James Kanze <>
Thu, 5 Feb 2009 02:40:26 -0800 (PST)
On Feb 5, 6:51 am, zs0723 <> wrote:

I need to changed the value of a key in std::map,

You can't change the value of a key without incurring undefined
behavior. The key determines where the element is stored in the
map; changing the key would imply changing where the element is

The only way to do this is to remove the element, modify the
key, and reinsert it.

i have written the following code :

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

int main()
   typedef map<string,string> TABLE;
   typedef TABLE::iterator ITERATOR;
   TABLE table;


   if(table.insert(TABLE::value_type(s","xiao v")).second) // this
will failed , i can't overwirte the exist key's value
      cout<<"insert success\n";
      cout<<"insert failied\n";

This looks more like you're trying to change the value of the
mapped object, and not the key. Except that you're requesting
an explicit insertion of a new object.

// but use like array 's [] , it's ok
// table["zs"]="victor";
// table["zs"]="xiao v";

Of course. Are you surprised that two functions with different
names (operator[] and insert) have different semantics?

   for(ITERATOR itr=table.begin();itr != table.end() ; ++itr)
         cout<<itr->first <<":"<<itr->second;


My question is :
Why insert method can't overwrite the value of the existing

Because that's the way it is specified. If it were specified to
change the mapping of an existing entry, of course, "insert"
would be a very, very poor name.

James Kanze (GABI Software)
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