Answer to Question #20263 in C++ for roneey

Question

when the constructor for a derived class compulsary?

Answer 1

The constructor of a derived class is compulsory in the following cases:
1. If the class (it could be a simple class, not a child) has no default constructor
but has a constructor with parameters. For example the following code
will produce compilation error:
class Parent {};
class Child : Parent
{
public:
Child(int x) { ... }
};

void main()
{
Child a;
}
If you don't define any class constructors, compiler will create the default one for you,
which takes no arguments and initializes all class fields with default values.
But if you define a constructor with parameters you must also define your own
default constructor implementation.

2. If parent class has no default constructor but has only constructors with parameters.
So the following code will also produce an error:
class Parent
{
public:
Parent(int x) { ... }
};

class Child : Parent
{
public:
Child() { ... }
Child(int y) { ... }
};

void main()
{
Child b;
Child c(4);
}
because the Child() and Child(int y) constructors try to call the parent class default constructor
implicitly and parent class has no default constructor defined (see case 1).
So you have to call the available parent class constructor explicitly:
class Parent
{
public:
Parent(int x) { ... }
};

class Child : Parent
{
public:
Child() : Parent(5) { ... }
Child(int y) : Parent(y) { ... }
};
or just define the parent class default constructor:
class Parent
{
public:
Parent() { ... }
Parent(int x) { ... }
};

class Child : Parent
{
public:
Child() { ... }
Child(int y) { ... }
};