Question

C++:

1.When and in what order are constructors and destructors called?

2. Create a class without any constructors, and show that you can create objects with the default constructor. Now create a non-default constructor (one with an argument, aka parameterized constructor) for the class, and try compiling again. Explain what happened.

Answer 1

Question 1:

Constructors are called in the order in which they are inherited, and for destructors it is reverse.

For example see this program:

#include <iostream>
using namespace std;

class A
{
public:
    //Making constructor of base class
    A()
    {
        cout << "Base constuctor called\n";
    }
    //Making destructor of base class
    ~A()
    {
        cout << "Base destructor called\n";
    }
};

class B : public A
{

public:
    //Making constructor of derived class
    B()
    {
        cout << "Derived constructor called\n";
    }
    //Making destructor of derived class
    ~B()
    {
        cout << "Derived destructor called\n";
    }
};

int main()
{
    B ob;
    return 0;
}

OUTPUT:

Question 2:

Here is a class without any constructors (ie default), it runs fine.

#include <iostream>
using namespace std;

//Making the class
class Box
{
    int l, b;

public:
    //Function to show area
    void showarea()
    {
        l = 10;
        b = 20;
        cout << "Area of the box is :" << l * b << "\n";
    }
};

int main()
{
    Box ob;
    ob.showarea();
    return 0;
}

Here is the output of above code:

If we make a parametried constructor then we have to give the parameters while making the object of the class, otherwise it will give error. See this code for example:

#include <iostream>
using namespace std;

class Box
{
    int l, b;

public:
    Box(int aa, int bb)
    {
        l = aa;
        b = bb;
    }
    void showarea()
    {
        cout << "Area of the box is :" << l * b << "\n";
    }
};

int main()
{
    // Box ob; //This line will cause error
    //To fix the error uncomment the below line and comment the above line
    Box ob(50, 60);
    ob.showarea();
    return 0;
}

OUTPUT: