In computer programming

Computer programming

Computer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...

, an automatic variable is a lexically-scoped

Scope (programming)

In computer programming, scope is an enclosing context where values and expressions are associated. Various programming languages have various types of scopes. The type of scope determines what kind of entities it can contain and how it affects them—or semantics...

 variable

Variable (programming)

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...

 which is allocated and de-allocated automatically when program flow enters and leaves the variable's scope

Scope (programming)

In computer programming, scope is an enclosing context where values and expressions are associated. Various programming languages have various types of scopes. The type of scope determines what kind of entities it can contain and how it affects them—or semantics...

. The term local variable

Local variable

In computer science, a local variable is a variable that is given local scope. Such a variable is accessible only from the function or block in which it is declared. In programming languages with only two levels of visibility, local variables are contrasted with global variables...

is usually synonymous with automatic variable, since these are the same thing in many programming languages.
Automatic variables may be allocated in the stack frame of the procedure in which they are declared; this has the useful effect of allowing recursion

Recursion

Recursion is the process of repeating items in a self-similar way. For instance, when the surfaces of two mirrors are exactly parallel with each other the nested images that occur are a form of infinite recursion. The term has a variety of meanings specific to a variety of disciplines ranging from...

 and re-entrancy. (For efficiency, the optimizer

Compiler optimization

Compiler optimization is the process of tuning the output of a compiler to minimize or maximize some attributes of an executable computer program. The most common requirement is to minimize the time taken to execute a program; a less common one is to minimize the amount of memory occupied...

 will try to allocate some of these variables in processor register

Processor register

In computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are addressed by mechanisms other than main memory and can be accessed more quickly...

s.)

C

C (programming language)

C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....

, C++

C++

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...

(Called automatic variables.)

All variables declared within a block of code are automatic by default, but this can be made explicit with the auto keyword. An uninitialized automatic variable has an undefined value until it is assigned a valid value of its type.

Using the storage class register instead of auto is a hint to the compiler to cache the variable in a processor register. Other than not allowing the referencing operator (&) to be used on the variable or any of its subcomponents, the compiler is free to ignore the hint.

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 RAII.

Java

Java (programming language)

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...

(Called local variables.)

Similar to C and C++, but there is no auto or register 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. This differs from instance variables, which are implicitly initialized with default values (which are 0 for numbers and null for objects).

Perl

(Called lexical, my or private variables.)

In Perl, local variables are declared using the my operator. Uninitialized scalars will have the value undef; uninitialized arrays or hashes will be .

Perl also has a local

Local variable

In computer science, a local variable is a variable that is given local scope. Such a variable is accessible only from the function or block in which it is declared. In programming languages with only two levels of visibility, local variables are contrasted with global variables...

operator that does not create automatic variables, instead giving global (package) variables a temporary value, which is dynamically scoped to the enclosing block. When the scope of the variable is left, the old value is restored.

See also

• C syntax#Storage duration specifiers
• Variable-length array
  Variable-length array
  In programming, a variable-length array is an array data structure of automatic storage duration whose length is determined at run time ....
  
   (C99
  C99
  C99 is a modern dialect of the C programming language. It extends the previous version with new linguistic and library features, and helps implementations make better use of available computer hardware and compiler technology.-History:...
  
  new feature)