Instance variable

In object-oriented programming with classes, an instance variable is a variable defined in a class (i.e. a member variable), for which each object of the class has a separate copy. They live in memory for the life of the object.

An instance variable contrasts with a class variable, and it is a special type of instance member. An example (C++ or Java) declaration of an instance variable is:

 private double length;

Technically speaking, instance variables are objects stored in individual states in "non-static fields", that is, fields declared without the static keyword. Non-static fields are also known as instance variables because their values are unique to each instance of a class (to each object, in other words); the currentSpeed of one bicycle is independent from the currentSpeed of another. A simpler definition is that instance variables are things an object knows about itself.

A typical declaration in Objective-C would look like:

 @interface CustomClassName : NSObject{
     NSString *ivar;
 }