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===Composition, inheritance, and delegation===
[[Objectspace|Objects]] can contain other objects in their instance variables; this is known as [[object composition]]. For example, an object in the Employee class might contain (either directly or through a pointer) an object in the Address class, in addition to its own instance variables like "first_name" and "position". Object composition is used to represent "has-a" relationships: every employee has an address, so every Employee object has access to a place to store an Address object (either directly embedded within itself, or at a separate ___location addressed via a pointer).
Languages that support classes almost always support [[inheritance (object-oriented programming)|inheritance]]. This allows classes to be arranged in a hierarchy that represents "is-a-type-of" relationships. For example, class Employee might inherit from class Person. All the data and methods available to the parent class also appear in the child class with the same names. For example, class Person might define variables "first_name" and "last_name" with method "make_full_name()". These will also be available in class Employee, which might add the variables "position" and "salary". This technique allows easy re-use of the same procedures and data definitions, in addition to potentially mirroring real-world relationships in an intuitive way. Rather than utilizing database tables and programming subroutines, the developer utilizes objects the user may be more familiar with: objects from their application ___domain.<ref>{{cite book|last=Jacobsen|first=Ivar|title=Object Oriented Software Engineering|year=1992|publisher=Addison-Wesley ACM Press|isbn=978-0-201-54435-0|pages=[https://archive.org/details/objectorientedso00jaco/page/43 43–69]|author2=Magnus Christerson|author3=Patrik Jonsson|author4=Gunnar Overgaard|url=https://archive.org/details/objectorientedso00jaco/page/43}}</ref>
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