Re: Access map element wrong using operator []

[Thomas Maeder]

Long Li <lilongll@gmail.com> writes:

> #include <iostream>
> using std::cout;
> using std::endl;

Side note: std::endl is declared in <ostream>, not <iostream> (not
necessarily anyway). You'd have to #include <ostream> as well.


> #include <string>
> using std::string;
> #include <vector>
> using std::vector;
> #include <map>
> using std::map;
> class A{
> public:
>   map<string, vector<string> > MapInA;
> };
> void f1( const map<string, A> &aaa ){
>   map<string, A>::const_iterator it = aaa.begin();

it is a const_iterator, so what it refers to is const ...


>   while( it != aaa.end() ){
>     if( it->second.MapInA.count( "ID" ) ){
>       cout << (*it).second.MapInA[ "ID" ][ 0 ] << endl;

... including members of it, such as it->second.MapInA.

operator[], on the other hand, is non-const for maps, because an
element will be added to the map for the given index if the map
doesn't already contain such an element.

>     }
>     it++;
>   }
> }
> void f2( const map<string, A> &aaa ){
>   map<string, A>::const_iterator it = aaa.begin();
>   map<string, vector<string> >::const_iterator it2;
>   while( it != aaa.end() ){
>     if( it->second.MapInA.count( "ID" ) ){
>       it2 = it->second.MapInA.find( "ID" );

The find() member function is const, so this is accepted.