1. Constructor - idea and syntax #

Created Wednesday 08 January 2020

Idea of a constructor #

• A newly created object **should have **some initial value. Garbage values are undesired.
• Constructor do exactly this - they initialize data.
• Constructors are called automatically - this relieves us of initializing objects using setters.

Properties of a constructor #

1. It is a function
2. Name is the same as that of the class.
3. Has no return type, not even void.
4. Called only once in the life-time of an object.

Types of constructors #

1. Default constructor - No arguments. It is named default coz it takes no argument.
2. Parameterized constructor - takes some argument.
3. Copy constructor - Takes only one object of the same class as argument.

Notes:

• If no constructor(i.e. of any type) is defined, then and only then a default constructor will be supplied by the compiler(actually just an empty body).

// compiler provided constructor
class_name()
{

}

• If we define a constructor of any type, then this 'default’ constructor goes out of scope for us.
• If a type of constructor is called, but not defined, although atleast one constructor(of some other type) has been defined. Then this is a compilation error, as no matching constructor could be found.

http://www.cplusplus.com/doc/tutorial/classes2/#default_constructor

• Tip: A parameterized constructor can be passed default arguments, so that there’s no need of a separate default constructor. Takes care of the ambiguity, which can be a runtime error.
• A constructor can be called explicitly, because this is useful.

int main()
{
	Car c;
	string name = "Batman";
	c.ride(Passenger(name)); // explicit call without declaration
}

• A constructor can call other functions from inside it, no problem.
• A program will compile and run, even if only a (non default param) parameterized constructor is defined. But it’ll give runtime error if no params are given during its creation. This is because we may not need a default constructor at all, due to presence of a parameterized constructor(which is not a default).