map::size test

#include <map>
#include <iostream>
using namespace std;
main(){
    map<int,int>m;
    cout<<m.size()<<endl; //0
    m[0]=1;
    cout<<m.size()<<endl; //1
    cerr<<m[1]<<endl;     //0
    cout<<m.size()<<endl; //2 ^^;;;
}