What happens if I read a map's value where the key does not exist?

Question

map dada;
dada[\"dummy\"] = \"papy\";
cout << dada[\"pootoo\"];

I\'m puzzled because I don\'t know if it\'s considered

Answer 1

If operator [] doesn't find a value for the provided key, it inserts one at that position.

But you should note that if you visit a not exist key and invoke it's member function, like mapKV[not_exist_key].member_fun().The program may crash.

Let me give an example, test class as below:

struct MapValue{
    int val;

    MapValue(int i=0){
        cout<<"ctor: "<

Test code:

cout<<"-------create map-------"< idName{{1, MapValue(1)}, {2, MapValue(2)}};

cout<<"-----cout key[2]-----"<

Output as below:

-------create map-------
ctor: 1
ctor: 2
dtor: 2
dtor: 1
dtor: 2
dtor: 1
-----cout key[2]-----
MapValue: 2

-----cout key[5]-----
ctor: 0
MapValue: 0

-------runs here means, does't crash-------
dtor: 0
dtor: 2
dtor: 1

We can see that: idName[5] invoke map construct {5, MapValue(0)} to insert to idName.

But if, you invoke member function by idName[5], then the program crashes :

cout<<"-------create map-------"< idName{{1, MapValue(1)}, {2, MapValue(2)}};


idName[5].toString();  // get crash here.


cout<<"------- runs here means, doesn't crash-------"<