Automatic variable: Difference between revisions

Revision as of 14:07, 13 July 2016

Revision as of 22:18, 26 September 2016

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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}} {{PDFlink|[http://www.open-std.org/JTC1/SC22/WG14/www/docs/n1256.pdf C standard]|3.61&nbsp;[[Mebibyte|MiB]]<!-- application/pdf, 3788603 bytes -->}}: section 6.2.4, Storage durations of objects</ref>

Using the storage class <code>register</code> instead of <code>auto</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.

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 automatically deduced.