Class question

[Pecvx]

I have the following two classes:

class Plant {
private:
    string name;
    string location;
    int sensorId; //sensor ID
    double humidityMin; //Planten trenger vann om den blir tørrere enn denne verdien
    double humidityMax; //Planten er for våt over denne verdien
    double humidity; //Sist leste fuktighetsmåling
    
public:
    Plant(string name, string location, int sensorId, double humidityMin, double humidityMax, double humidity = -1) : name{name}, location{location}, sensorId{sensorId}, humidityMin{humidityMin}, humidity{humidity} {}
    void setName(string plantName) {
        name = plantName;
    }
    string getName() {return name;}
    string getlocation() {return location;}
    //double readHumidity();
    PlantStatus getStatus();
    bool operator==(Plant& rhs);
    string plantStatusText();
    
    
};


class Garden {

private:
    string name;
    vector<Plant> plants;
public:
    Garden(string name);
    void addPlant(Plant p);
    void removePlant(Plant p);
    vector<Plant> needsHelp();
    
};

When looking at the solution manual, there was an entry in the initialization list of the Garden constructor I didn't understand (bold part):

[code]
Garden::Garden(string name) : name(name), plants() {}

Can anyone help?

[keskiverto]

That calls explicitly the default constructor of the std::vector<Plant>. The constructor would do the same thing even if the member initializer list would omit that call.

[Pecvx]

Thank you! but what exactly will it do?

[jonnin]

it constructs a vector. That may or may not allocate some plants as a side effect (not sure if it immediately makes memory or waits on the first element to be added). So it sets the internal size variable to zero and whatever a vector does to itself when created.

try this in your plant constructor.
Plant ( stuff)
{
static int p = 1;
cout << "created plant # " << p++;
}

and in main, make a vector of plants like you do in garden and play with it.
try an empty one, see what it does. then try a push-back on it, see what that did.
try a preallocated one vector<plant> vp(100);
and whatever you want to do to see if you can get enlightenment as to what is going on.

[Pecvx]

Thanks!

when you create a vector in a class, does the constructor of your class always call a default vector constructor?

[keskiverto]

All non-static member variables of a class are default initialized (unless you do something explicit):
https://en.cppreference.com/w/cpp/language/default_initialization
https://en.cppreference.com/w/cpp/language/initializer_list
https://en.cppreference.com/w/cpp/language/default_constructor