Re: Overhead of subscript operator for STL maps

James Kanze <>
Fri, 17 Oct 2008 10:08:47 -0700 (PDT)
On Oct 17, 5:27 pm, Erik Wikstr=F6m <> wrote:

On 2008-10-17 09:43, Stephen Horne wrote:

On Thu, 16 Oct 2008 20:32:13 +1300, Ian Collins
<> wrote:

The subscript operator creates a default constructed object
then copies the passed object to it. So you will see a copy
for the operator call and one for the copy in.


I always assumed that using [] for a key that isn't already
in the container would throw an exception. It seems like an
obvious case of most-likely-a-mistake to me.

I realise that scripting languages do it, but they get to
handle the [] differently depending on whether its on the
left side of an assignment or not. They still complain if
you try to read a non-existent key.

Do they? The ones I use don't. (You can get this behavior in
C++, if you want it, by having operator[] return a proxy.)

And even with the [] on the left of an assignment, I still
think it's a bit bad, since to me the obvious intent is to
overwrite the data for an existing key.

Personally I consider it a good idea, it can simplify a lot of
code (such as this one, written by Fred Swartz):

Sure, it simplifies some things. And makes others more
complicated. The fundamental problem is that it means that
operator[] can't be used on a const map, which is exceedingly

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

int main() {
    map<string, int> freq; // map of words and their frequencies
    string word; // input buffer for words.

    //--- Read words/tokens from input stream
    while (cin >> word) {

    //--- Write the count and the word.
    map<string, int>::const_iterator iter;
    for (iter=freq.begin(); iter != freq.end(); ++iter) {
        cout << iter->second << " " << iter->first << endl;
    return 0;
}//end main

Notice how little code is dedicated to the actual counting of
the words.

And consider the more common use of using an std::map to look up
a factory, dynamically. If the list of classes is fixed, the
map will typically be const. There's no problem with operator[]
returning a value constructed with the default constructor,
since the mapped_type is normally a pointer, and a null pointer
is a good signal for a missing element. But you really want the
instance to be const.

James Kanze (GABI Software)
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