c++ how to initialise params in derived classes

Question

class car
{
    string carID
    string carName
};

class electric : public car
{
    string battery
    string model
};

base class is car. electric is a derived class that inherits from car. What would need to be included in the constructor to initialise them and then set the values when the user inputs a new car with the data for all the attributes?

possible duplicate of http://stackoverflow.com/questions/120876/c-superclass-constructor-calling-rules — frickskit, May 06 '13 at 17:17
@frickskit: Not sure this is the same question.. he is asking about simply initializing the fields in his derived class. — , May 06 '13 at 17:17
@BartlomiejLewandowski Why? Class capitalization is just a question of personal style, or corporate style guides, or similar. — Some programmer dude, May 06 '13 at 17:18
@JoachimPileborg I do understand that it is a matter of personal style, but i believe that it makes code much more readable, and it stands out from variables and other language constructs. — Bartlomiej Lewandowski, May 06 '13 at 18:13

score 1 · Answer 1 · answered May 06 '13 at 17:20

1

You need to define constructors and subsequently use them properly. During initialization of a derived class, you should always call the appropriate constructor of the base class:

Derived::Derived(...):Base(...),<optional initizaliation of Derived members>{...}

For example:

class car
{
    string carID;
    string carName;
public:
    car(string carid, string carname):carID(carid),carName(carname){}
};

class electric : public car
{
    string battery;
    string model;
public:
    electric(string carid, string carname, string battery, string model)
    :car(carid,carname),battery(battery),model(model){}
};

answered May 06 '13 at 17:20

Marc Claesen

16,778
6
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thank you for your reply. How would i define the set function to set the values of a particular electric car, will i have to call the car constructor then the electric? – user2355449 May 06 '13 at 17:26
No, you always call the derived class constructor. As you can see in the code example I listed, the derived class's constructor calls the base class constructor during initialization. – Marc Claesen May 06 '13 at 17:29
You're welcome, if it is clear to you now, please mark this question as answered. – Marc Claesen May 06 '13 at 17:34

score 0 · Answer 2 · answered May 06 '13 at 17:18

0

In your current model, you actually can't access any of the fields of car, because by default in C++ when using the class keyword, all fields are private. They need to be protected or public in order for a sub-class to access them.

answered May 06 '13 at 17:18

Kevin Anderson

6,850
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score 0 · Answer 3 · answered May 06 '13 at 17:21

You should initialize base class members by calling the base class constructor at the derived class's constructor initialization list.

 class car
 {
    string carID
    string carName
    public:
       car (string id, string name): carID(id), carName(name) {}
 };

 class electric : public car
 {
   string battery
   string model
   public:
      electric (string id, string name, string b, string m): car(id, name), 
                                                battery(b), model(m){}
 };

c++ how to initialise params in derived classes

3 Answers3