Closure (computer programming): Difference between revisions

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This is most often achieved by a function return, since the function must be defined within the scope of the non-local variables, in which case typically its own scope will be smaller.

This can also be achieved by [[variable shadowing]] (which reduces the scope of the [[non-local variable]]), though this is less common in practice, as it is less useful and shadowing is discouraged. In this example <code>f</code> can be seen to be a closure because <code>x</code> in the body of <code>f</code> is bound to the <code>x</code> in the global namespace, not the <code>x</code> local to <code>g</code>:

<syntaxhighlight lang="python">

x = 0

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</syntaxhighlight>

The arrow operator <code>=></code> is used to define an [https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow%20functionsArrow_functions arrow function expression], and an <code>Array.filter</code> method<ref>{{cite web |url=https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Global_Objects/Array/filter |title=array.filter |work=Mozilla Developer Center |date=10 January 2010 |access-date=2010-02-09}}</ref> instead of a global <code>filter</code> function, but otherwise the structure and the effect of the code are the same.

A function may create a closure and return it, as in this example:

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===Example 2: Accidental reference to a bound variable===

For this example the expected behaviour would be that each link should emit its id when clicked; but because the variable 'e' is bound to the scope above, and lazy evaluated on click, what actually happens is that each on click event emits the id of the last element in 'elements' bound at the end of the [[for loop]].<ref>{{cite web |title=Closures |url=https://developer.mozilla.org/en-US/docs/Web/JavaScript/Closures#Creating_closures_in_loops_A_common_mistake |website=MDN Web Docs |access-date=20 November 2018}}</ref>

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var elements = document.getElementsByTagName('a');

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=== Function objects (C++) ===

[[C++]] enables defining [[function object]]s by overloading <code>operator()</code>. These objects behave somewhat like functions in a functional programming language. They may be created at runtime and may contain state, but they do not implicitly capture local variables as closures do. As of [[C++11|the 2011 revision]], the C++ language also supports closures, which are a type of function object constructed automatically from a special [[language construct]] called ''lambda-expression''. A C++ closure may capture its context either by storing copies of the accessed variables as members of the closure object or by reference. In the latter case, if the closure object escapes the scope of a referenced object, invoking its <code>operator()</code> causes undefined behavior since C++ closures do not extend the lifetime of their context.{{main|Anonymous functionExamples_of_anonymous_functions#C++ _(since Csince_C++11)}}

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|date=2007-01-28

|title=A Definition of Closures

|url=httphttps://gafter.blogspot.com/2007/01/definition-of-closures.html

}}

* {{cite web