Editing Struct (C programming language)

{{Short description|C keyword for defining a structured data type}}
{{lowercase}}
In the [[C programming language]], '''struct''' is the keyword used to define a [[composite data type|composite]], a.k.a. [[Record (computer science)|record]], [[data type]] {{endash}} a named set of values that occupy a block of memory. It allows for the different values to be accessed via a single [[identifier]], often a [[pointer (computer programming)|pointer]]. A struct can contain other data types so is used for mixed-data-type records. For example a bank customer struct might contains fields: name, address, telephone, balance.

A struct occupies a ''contiguous block'' of memory, usually delimited (sized) by word-length boundaries. It corresponds to the similarly named feature available in some [[assembly language|assemblers]] for Intel processors. Being a block of contiguous memory, each field within a struct is located at a certain fixed offset from the start.

The [[sizeof]] operator results in the number of bytes needed to store a particular struct, just as it does for a [[primitive data type]]. The alignment of particular fields in the struct (with respect to [[Word (computer architecture)|word]] boundaries) is implementation-specific and may include padding. Modern compilers typically support the <code>#pragma pack</code> directive, which sets the size in bytes for alignment.<ref>{{Cite web|url=https://stackoverflow.com/questions/2748995/struct-memory-layout-in-c|title=Struct memory layout in C|website=Stack Overflow}}</ref>

The C struct feature was derived from the same-named concept in [[ALGOL 68]].<ref name="sigplan">{{cite journal | first = Dennis M.| last = Ritchie | authorlink = Dennis Ritchie | title = The Development of the C Language | date = March 1993 | journal = ACM SIGPLAN Notices | volume = 28 | issue = 3 | pages = 201–208 | url = http://www.bell-labs.com/usr/dmr/www/chist.html | doi = 10.1145/155360.155580 | quote = The scheme of type composition adopted by C owes considerable debt to Algol 68, although it did not, perhaps, emerge in a form that Algol's adherents would approve of. The central notion I captured from Algol was a type structure based on atomic types (including structures), composed into arrays, pointers (references), and functions (procedures). Algol 68's concept of unions and casts also had an influence that appeared later.| doi-access = free }}</ref>

== Declaration ==

The syntax for a struct declaration is shown by this simple example:

<syntaxhighlight lang="C">
struct tag_name {
   type member1;
   type member2;
};
</syntaxhighlight>

The <code>tag_name</code> is optional in some contexts.

==Typedef==

Via the keyword [[Typedef|<code>typedef</code>]], a struct type can be referenced without using the <code>struct</code> keyword. However, some{{who|date=October 2021}} programming style guides advise against this, claiming that it can obfuscate the type.

For example:
<syntaxhighlight lang="C">
typedef struct tag_name {
   type member1;
   type member2;
} thing_t;
thing_t thing;
</syntaxhighlight>

In C++ code, typedef is not needed because types defined via <code>struct</code> are part of the regular namespace, so the type can be referred to as either <code>struct thing_t</code> or <code>thing_t</code>.

== Initialization ==

There are three ways to initialize a structure. 

For the type:

<syntaxhighlight lang="c">
struct point_t {
   int x;
   int y;
};
</syntaxhighlight>

''C89-style initializers'' are used when contiguous members may be given.<ref>{{cite book | first1 = Al | last1 = Kelley | first2 = Ira | last2 = Pohl | year = 2004 | url = https://archive.org/details/bookoncprogrammi00kell/page/418 | edition = Fourth | title = A Book On C: Programming in C | isbn = 0-201-18399-4 | pages = [https://archive.org/details/bookoncprogrammi00kell/page/418 418] | url-access = registration }}</ref> For example:

<syntaxhighlight lang="c">
struct point_t a = { 1, 2 };
</syntaxhighlight>

For non contiguous or out of order members list, ''designated initializer'' style may be used.<ref>{{cite web | url=https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.3.0/com.ibm.zos.v2r3.cbclx01/strin.htm | title=IBM Linux compilers. Initialization of structures and unions}}</ref> For example:

<syntaxhighlight lang="c">
struct point_t a = { .y = 2, .x = 1 };
</syntaxhighlight>

If an initializer is given or if the object is [[Static memory allocation|statically allocated]], omitted elements are initialized to 0.

A third way of initializing a structure is to copy the value of an existing object of the same type. For example:

<syntaxhighlight lang="c">
struct point_t b = a;
</syntaxhighlight>

== Copy ==

The state of a struct can be copied to another instance. A compiler might use <code>memcpy()</code> to copy the bytes of the memory block.

<syntaxhighlight lang="c">
struct point_t a = { 1, 3 };
struct point_t b;
b = a;
</syntaxhighlight>

== Pointers ==

Pointers can be used to refer to a <code>struct</code> by its address. This is useful for passing a struct to a function to avoid the overhead of copying the struct. The <code>-></code> operator [[Dereference operator|dereferences]] the pointer (left operand) and accesses the value of a struct member (right operand).

<syntaxhighlight lang="c">
struct point_t point = { 3, 7 };
int x = point.x;
point.x = 10;
struct point_t *pp = &point;
x = pp->x;
pp->x = 8;
</syntaxhighlight>

== In other languages ==

===C++===

In [[C++]], struct is essentially the same as for C. Further, a [[C++ class|class]] is the same as a struct but with different default [[Access modifiers|visibility]]: class members are private by default, whereas struct members are public by default.

===.NET===

[[.NET]] languages have a feature similar to struct in C {{endash}} called <code>struct</code> in [[C Sharp (programming language)|C#]] and <code>Structure</code> in [[Visual Basic .NET]]). This construct provides many features of a class, but acts as a [[value type]] instead of a [[reference type]]. For example, when passing a .NET struct to a function, the value is copied so that changes to the input parameter do not affect the value passed in.<ref>{{Cite web|url=http://yoda.arachsys.com/csharp/parameters.html|title=Parameter passing in C#}}</ref>

== See also ==
* {{Annotated link|Bit field}}
* {{Annotated link|Flexible array member}}
* {{Annotated link|Passive data structure}}
* {{Annotated link|Union type}}

== References ==
{{reflist}}

[[Category:C (programming language)]]