Constructor (object-oriented programming): Difference between revisions

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In [[class-based programming|class-based]] [[object-oriented programming]], a '''constructor''' (sometimes shortened to '''ctor''') in a [[Class (computer programming)|class]] is a special type of [[subroutine]] called to [[object creation|create an object]]. It prepares the new object for use, often accepting [[argument]]s that the constructor uses to set required [[member variable]]s.

'''Instance constructors''', sometimes referred to as '''.ctor''', are used to create and initialize any instance member variables when the new expression is used to create an object of a class. To initialize a static class, or static variables in a non-static class, a static constructor must be defined. Static constructors are sometimes referred to as '''.cctor'''.<ref>{{cite web|url=http://msdn.microsoft.com/en-us/library/k6sa6h87.aspx|title=Instance Constructors (C# Programming Guide) |publisher=Microsoft Developers Networks |accessdate=2014-04-05}}</ref><ref>{{cite web|url=http://books.google.com/books?id=oAcCRKd6EZgC&pg=PA222&lpg=PA222&dq=ctor+cctor&source=bl&ots=KAYfbDu_ay&sig=gLj6X9SpWXz86GHMvljkfb-D8Oc&hl=en&sa=X&ei=fUFAU--WJOewsASQ54GQAw&ved=0CGoQ6AEwCQ#v=onepage&q=ctor%20cctor&f=false |title=Expert .NET 2.0 IL Assembler |publisher=APress |date=2006-01-01|accessdate=2014-04-05}}</ref>

A constructor resembles an [[method (computer science)|instance method]], but it differs from a method in that it has no explicit [[return type]], it is not implicitly [[inheritance (object-oriented programming)|inherited]] and it usually has different rules for scope modifiers. Constructors often have the same name as the declaring [[class (computer science)|class]]. They have the task of [[initialization (computing)|initializing]] the object's [[data member]]s and of establishing the [[invariant (computer science)|invariant]] of the class, failing if the invariant is invalid. A properly written constructor leaves the resulting [[object (computer science)|object]] in a ''valid'' state. [[Immutable object]]s must be initialized in a constructor.