Initialization (programming): Difference between revisions

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{{Short description|Assignment of an initial value for variable}}

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In [[computer programming]], '''initialization''' (or '''initialisation)''' is the [[Assignment (computer science)|assignment]] of an initial value for a [[data object]] or variable. The manner in which initialization is performed depends on the [[programming language]], as well as the type, storage class, etc., of an object to be initialized. Programming constructs which perform initialization are typically called '''initializers''' and '''initializer lists'''. Initialization is distinct from (and preceded by) [[declaration (computer programming)|declaration]], although the two can sometimes be conflated in practice. The complement of initialization is [[finalization]], which is primarily used for objects, but not variables.

Initialization is done either by statically embedding the value at compile time, or else by assignment at [[Run time (program lifecycle phase)|run time]]. A section of code that performs such initialization is generally known as "initialization code" and may include other, one-time-only, functions such as opening files; in [[object-oriented programming]], initialization code may be part of a ''[[Constructor (object-oriented programming)|constructor]]'' (class method) or an ''initializer'' (instance method). Setting a memory ___location to [[hexadecimal]] zeroes is also sometimes known as "clearing" and is often performed by an [[exclusive or]] instruction (both operands specifying the same variable), at [[machine code]] level, since it requires no additional memory access.

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===Initializer===

In C/C99/C++, an '''initializer''' is an optional part of a [[declarator (computing)|declarator]]. It consists of the '=' character followed by an [[expression (programming)|expression]] or a comma-separated list of expressions placed in curly brackets (braces). The latter list is sometimes called the "initializer list" or "initialization list" (although the term "initializer list" is formally reserved for initialization of class/struct members in C++; [[#Initializer list|see below]]).

A declaration which includescreates initializationa data object, instead of merely describing its existence, is commonly called a '''definition'''.

Many find it convenient to draw a distinction between the terms "declaration" and "definition", as in the commonly seen phrase "the distinction between a ''declaration'' and ''definition''...", implying that a declaration merely designates a data object (or function). In fact, according to the [[C++ standard]], a definition ''is'' a declaration. Still, the usage "declarations and definitions", although formally incorrect, is common.<ref>''C++ FAQs'', by Cline, Lomow, and Girou, Addison-Wesley, 1999, {{ISBN|0-201-30983-1}}.</ref> Although all definitions are declarations, not all declarations are definitions.

C examples:

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and not to turn this page into a C++ primer

-->

<sourcesyntaxhighlight lang="c">

int i = 0;

int k[4] = {0, 1};

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char ty[2] = 'f';

struct Point {int x; int y;} p = { .y = 13, .x = 7 };

</syntaxhighlight>

</source>

C++ examples:

<sourcesyntaxhighlight lang="cpp">

int i2(0);

int j[2] = {rand(), k[0]};

MyClass* xox = new MyClass (0, "zaza");

point q = {0, i + 1};

</syntaxhighlight>

</source>

===Initializer list===

In C++, a [[constructor (object-oriented programming)|constructor]] of a class/struct can have an '''initializer list''' within the definition but prior to the constructor body. It is important to note that when you use an initialization list, the values are not assigned to the variable. They are initialized. In the below example, 0 is initialized into re and im.

Example:

<sourcesyntaxhighlight lang="c">

struct int_complexIntComplex {

IntComplex() int: re(0), im;(0) {}

int_complex() : re(0), im(0) { }

}int re;

int im;

</source>

};

</syntaxhighlight>

Here, the construct <code> : re(0), im(0)</code> is the initializer list.

Sometimes the term "initializer list" is also used to refer to the list of expressions in the array or struct initializer.

[[C++11]] provides for a [[C++11#Initializer_lists|more powerful concept of initializer lists]], by means of a template, called <tt>{{mono|std::initializer_list</tt>}}.

===Default initialization===