CODEWITHSUNDEEP
c++listdictionarystlkey
Laura Galera
Laura Galera

Reputation: 119

C++: How initialize a map that contains a list of pointers with a given key?

I am doing a practice in C++ based on data structures.

My class contains an attribute like this map<int, list <Route *>>. My question is, what happens when I add a key to the map. Must I initialize the list or not? My wish is to have an empty file in the description of that key.

Something like this is what I thought:

map<int, list<Route* > >::iterator it= _mapRoutesAirline.find(IDAirline);
        if(it == _mapRoutesAirline.end())
            _mapRoutesAirline[IDAirline] = list<Route*>();

Upvotes: 2

Views: 145

Answers (3)

Hrishikesh
Hrishikesh

Reputation: 376

You can create an empty list when you define a new key, no need to define the data type of the list.

_mapRoutesAirline[IDAirline] = {};

Also, you don't need to define an iterator just to find and insert element It can be done in this way

if(_mapRoutesAirline.find(IDAirline) == _mapRoutesAirline.end()){
       _mapRoutesAirline[IDAirline] = {};
}

Upvotes: 1

dhavenith
dhavenith

Reputation: 2048

Yes, you can simply assign to the map element (syntax assuming C++11 or above):

 _mapRoutesAirline[IDAirline] = {};

Note that in many use cases you don't have to assign an empty list, because any attempt to access a key that isn't present in the map yet will automatically create a default constructed list for that key, which is an empty list. This means that, for example, _mapRoutesAirline[42] would return an empty list if you hadn't assigned anything to the _mapRoutesAirline[42] yet.

Upvotes: 2

Oblivion
Oblivion

Reputation: 7374

Yes you can add an empty list to the map as value e.g.:

if(it == _mapRoutesAirline.end())
    _mapRoutesAirline.insert(std::make_pair(IDAirline, list<Route*>{}));

Upvotes: 1

Related Questions