Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...
In object-oriented programming, a class is a construct that is used as a blueprint to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior...
In computer programming, a variable is a symbolic name given to some known or unknown quantity or information, for the purpose of allowing the name to be used independently of the information it represents...
In object-oriented programming, a member variable is a variable that is associated with a specific class, and accessible for all its methods. If there is only one copy of the variable shared with all instances of the class, it is called a class variable or static member variable...
) of which a single copy exists, regardless of how many instance
Instance (computer science)
In object-oriented programming an instance is an occurrence or a copy of an object, whether currently executing or not. Instances of a class share the same set of attributes, yet will typically differ in what those attributes contain....
In object-oriented programming with classes, an instance variable is a variable defined in a class , for which each object of the class has a separate copy. They live in memory for the life of the object....
. It is a special type of class attribute (or class property, field, or data member).
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
In computer programming, a keyword is a word or identifier that has a particular meaning to the programming language. The meaning of keywords — and, indeed, the meaning of the notion of keyword — differs widely from language to language....
static, and may therefore be referred to as static member variables.
The same dichotomy between instance and class members applies to method
Method (computer science)
In object-oriented programming, a method is a subroutine associated with a class. Methods define the behavior to be exhibited by instances of the associated class at program run time...
s ("member functions") as well; a class may have both instance methods and class methods. Again, Java, C#, and C++ use the keyword static to indicate that a method is a class method ("static member function").
Example
struct Request {
static int count;
int number;
Request {
number = count; // modifies the instance variable "this->number"
++count; // modifies the class variable "Request::count"
}
};
int Request::count = 0;
In this C++ example, the class variable Request::count is increment
Increment
An increment is an increase of some amount, either fixed or variable. For example one's salary may have a fixed annual increment or one based on a percentage of its current value...
In object-oriented programming, a constructor in a class is a special type of subroutine called at the creation of an object. It prepares the new object for use, often accepting parameters which the constructor uses to set any member variables required when the object is first created...
, so that Request::count always holds the number of Requests that have been constructed, and each new Request object is given a number in sequential order. Since count is a class variable, there is only one object Request::count; in contrast, each Request object contains its own distinct number field.
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