Automatic variable: Difference between revisions

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{{Short description|Local variable in computer programming}}

{{About|Automatic variables|the C++ keyword {{Cpp|auto}} used for [[Typetype inference]]|C++11#Type inference}}

__NOTOC__

In [[computer programming]], an '''automatic variable''' is a local [[Variable (programming)|variable]] which is allocated and deallocated automatically when program flow enters and leaves the variable's scope. The [[Scope (computer science)|scope]] is the lexical context, particularly the function or block in which a variable is defined. Local data is typically (in most languages) invisible outside the function or lexical context where it is defined. Local data is also invisible and inaccessible to a ''called'' function,<ref group="note">unless it is a [[nested function]], which itself is ''defined'' along that local data</ref> but is not deallocated, coming back in scope as the [[execution thread]] returns to the caller.

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==In specific programming languages==

===C, C++===

{{See also|C syntax#Storage class specifiers}}

(Called ''automatic'' variables.)

All variables declared within a [[block (programming)|block]] of code are automatic by default. An uninitialized automatic variable has an [[undefined behavior|undefined]] value until it is assigned a valid value of its type. <ref>{{As of|2008|alt=Current}} {{cite web|url= http://www.open-std.org/JTC1/SC22/WG14/www/docs/n1256.pdf |title=C standard }}&nbsp;{{small|(3.61&nbsp;[[Mebibyte|MiB]])}}: section 6.2.4, Storage durations of objects</ref> The storage-class specifier <code>auto</code> can be added to these variable declarations as well, but as they are all automatic by default, this is entirely redundant and rarely done.

In C, using the storage class <code>register</code> is a hint to the compiler to cache the variable in a processor register. Other than not allowing the referencingaddress-of operator (<code>&</code>) to be used on the variable or any of its subcomponents, the compiler is free to ignore the hint.<ref>{{citation|url=https://en.cppreference.com/w/c/language/storage_duration| title=Storage Duration| website=cppreference.com}}</ref>

In [[C++]], the constructor of automatic variables is called when the execution reaches the place of declaration. The destructor is called when it reaches the end of the given program block (program blocks are surrounded by curly brackets). This feature is often used to manage resource allocation and deallocation, like opening and then automatically closing files or freeing up memory., Seecalled [[Resource Acquisition Is Initialization]] (RAII). Note, C++11 has a new auto specifier[http://en.cppreference.com/w/cpp/language/auto], which is different. In this case, the variable's type is [[type inference|inferred]].

In C, using the storage class <code>register</code> is a hint to the compiler to cache the variable in a processor register. Other than not allowing the referencing operator (<code>&</code>) to be used on the variable or any of its subcomponents, the compiler is free to ignore the hint.

Since C++11, C++ allows variables to be declared with the <code>auto</code> type specifier,<ref>{{citation| url=http://en.cppreference.com/w/cpp/language/auto| title=Placeholder type specifiers| website=cppreference.com}}</ref> but this means that the variable's type is [[type inference|inferred]], and does not refer to the scope of the variable.

In C++, the constructor of automatic variables is called when the execution reaches the place of declaration. The destructor is called when it reaches the end of the given program block (program blocks are surrounded by curly brackets). This feature is often used to manage resource allocation and deallocation, like opening and then automatically closing files or freeing up memory. See [[Resource Acquisition Is Initialization]] (RAII). Note, C++11 has a new auto specifier[http://en.cppreference.com/w/cpp/language/auto], which is different. In this case, the variable's type is [[type inference|inferred]].

===Java===

(Called ''local variables''.)

Similar to C and C++, but there is no <code>auto</code> or <code>register</code> keyword. However, the Java compiler will not allow the usage of a not-explicitly-initialized local variable and will give a compilation error (unlike C and C++ where the compiler will usually only give a warning). The Java standard demands that every local variable must be explicitly initialized before being used.<ref>{{cite web | url=http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html#4.12.5 | title=4.12.5 Initial Values of Variables | publisher=Sun Microsystems | accessdateaccess-date=2008-10-17}}</ref> This differs from instance variables, which are implicitly initialized with default values (which are <tt>{{samp|0</tt>}} for numbers and <tt>{{samp|null</tt>}} for objects).

===Perl===

(Called ''lexical'', ''my'' or ''private'' variables.)

In Perl, local variables are declared using the <code>my</code> operator. Uninitialized scalars will have the value <code>undef</code>; uninitialized arrays or hashes will be <code>()</code>.<ref>{{cite web | url=http://perldoc.perl.org/perlsub.html#Private-Variables-via-my() | title=Private variables via my() - perlsub - perldoc.perl.org | publisher= | accessdateaccess-date=2008-10-17}}</ref>

Perl also has a <code>[[Local variable|local]]</code> operator that does not create automatic variables,<ref>{{cite web | url=http://perldoc.perl.org/perlsub.html#Temporary-Values-via-local%28%29 | title=Temporary values via local() - perlsub - perldoc.perl.org | publisher= | accessdateaccess-date=2011-02-25}}</ref> instead giving global (package) variables a temporary value, which is [[Scope_(computer_science)#Dynamic_scoping|dynamically scoped]] to the enclosing block. When the scope of the variable is left, the old value is restored.

==See also==

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{{DEFAULTSORT:Automatic Variable}}

{{Memory management}}

[[Category:Memory management]]

[[Category:Variable (computer science)]]