Elements are auto inserted to std::map during invalid loop cout operation

Question

#include<iostream>
#include<map>
using namespace std;
int main() 
{
    map<int, string> sample;
    for (int i = 5; i > 0; i--)
        sample[i] = 'i' + i;
    map<int, string>::iterator i = sample.begin();
    for (int j = 1; j <= 10; j++) 
    {
        cout << sample[j] << endl;
    }
    for (; i != sample.end(); i++)
        cout << i->first;
    cout << "Size is :" << sample.size();
}

I ran this program to know more about std::map ie I am getting map size as 10 and pls refer the below screenshot of output.

Could someone pls clarify me why std::map is auto extending in invalid loop display function .

FYI the output you are showing does not match your code. When running I get: http://coliru.stacked-crooked.com/a/3c71efd9ed946206 — NathanOliver, Oct 27 '15 at 13:03
I corrected the second for loop ie updating j value range to be 1 to 10 . — Prakash Kumar, Oct 27 '15 at 13:05

score 4 · Accepted Answer · answered Oct 27 '15 at 13:00

4

std::map::operator [key] is equivalent to (*((this->insert(make_pair(key, mapped_type()))).first)).second, so inserts a default value if element is not present.

You have to use at or find to not insert element in map, or use iterator.

answered Oct 27 '15 at 13:00

Jarod42

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score 1 · Answer 2 · answered Oct 27 '15 at 13:01

Could someone pls clarify me why std::map is auto extending in invalid loop display function .

std::map::operator[]

Returns a reference to the value that is mapped to a key equivalent to key, performing an insertion if such key does not already exist.

Elements are auto inserted to std::map during invalid loop cout operation

2 Answers2