Problem with C++ Constructor. What exactly is getting initialized here?

Question

#include<bits/stdc++.h>
using namespace std;


class Vehicle
{
private:
int speed;

public:
Vehicle(int x)
{
   speed=x;
   cout << "Vehicle's parametrized constructor called" << endl;
}
};

class Car : public Vehicle
{ 
   public:
   Car() : Vehicle(5)
   {
        cout << "Car's constructor called" << endl;
   }
};

int main()
{
    Car a;
}

Output- Vehicle's parametrized constructor called
Car's constructor called

Since the access specifier is public, speed is not inherited. What does 5 get assigned to as there is no speed member in Car?

*"Since the access specifier is public, speed is not inherited"*... what? I suggest you read a bit more about inheritance, derived classes contain *all members* of their base classes. — Cory Kramer, Apr 09 '20 at 16:49
They most certainly are inherited. You cannot access them from derived classes though. — ChrisMM, Apr 09 '20 at 16:50
`private` vs `protected` specifies *access* to the members, but does not prohibit them from existing — Cory Kramer, Apr 09 '20 at 16:51
@jamesgem private members are inherited, you just aren't allowed to reference them unless you are a friend of the class where they were declared. — Jacob Dlougach, Apr 09 '20 at 16:51
[Why should I not #include ?](https://stackoverflow.com/q/31816095/5910058) — Jesper Juhl, Apr 09 '20 at 17:08

score 4 · Accepted Answer · answered Apr 09 '20 at 16:51

Since the access specifier is public, speed is not inherited.

This is a misunderstanding. A derived class does inherit all members from the base class. What access specifier control is only whether the inherited class can access the members. The Car cannot directly access speed but the member is there.

Remember that public inheritance models a "is-a" relationship. A Car is a Vehicle. If Car didn't have the speed member it would not be a Vehicle.

score 2 · Answer 2 · 2020-04-09T17:13:07.060

You can use a display function to test that for yourself.

Add this function to your class Vehicle under public access specifier:

void display() { std::cout<<speed; }

Now from an object of derived class Car, call this function in main():

Car a; a.display();

It will output 5, which is the value of the variable speed you have initialzed in derived class Car, from base class Vehicle.

Since the access specifier is public, speed is not inherited.

Irrespective of access specifiers imposed, all variables are inherited. The difference is that they are not accessible directly. For private members like speed, you'll need to access them via a public member function, just like I did above.

What does 5 get assigned to as there is no speed member in Car?

It gets assigned to the derived variable speed in objects of Car.

score 2 · Answer 3 · answered Apr 09 '20 at 17:14

Class Car is derived from Vehicle. when you create an object of class Car, it contains the memory for Vehicle also. it means object of class Car has Vehicle as a member. Hope this link will give a good understanding.

when you are creating an object of Car, it will call constructor of class Car which internally call constructor of class Vehicle before executing the body of Car constructor.

 Car() : Vehicle(5)

This is a way of initializing the members of Vehicle class. There is a concept of Initializer List in C++ and you can read here when it is better to use.

Value 5 is passed to Vehicle's constructor and eventually stored in class member speed .In your code if you print speed, you will see, whatever value you are passing is getting stored there.

Problem with C++ Constructor. What exactly is getting initialized here?

3 Answers3