struct (C programming language)

<h2 id="declaration">Declaration</h2>
<p>The syntax for a struct declaration is shown by this simple example:
</p>
struct tag_name {
   type member1;
   type member2;
};

<p>The tag_name is optional in some contexts.
</p>
<h2 id="typedef">Typedef</h2>
<p>Via the keyword <a href="/facts/Typedef/kqSR4VXn">typedef</a>, a struct type can be referenced without using the struct keyword. However, some[<i>who?</i>] programming style guides advise against this, claiming that it can obfuscate the type.
</p><p>For example:
</p>
typedef struct tag_name {
   type member1;
   type member2;
} thing_t;
thing_t thing;

<p>In C++ code, typedef is not needed because types defined via struct are part of the regular namespace, so the type can be referred to as either struct thing_t or thing_t.
</p>
<h2 id="initialization">Initialization</h2>
<p>There are three ways to initialize a structure. 
</p><p>For the type:
</p>
struct point_t {
   int x;
   int y;
};

<p><i>C89-style initializers</i> are used when contiguous members may be given.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> For example:
</p>
struct point_t a = { 1, 2 };

<p>For non contiguous or out of order members list, <i>designated initializer</i> style may be used.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> For example:
</p>
struct point_t a = { .y = 2, .x = 1 };

<p>If an initializer is given or if the object is <a href="/facts/Static_memory_allocation/dYIs2E5p">statically allocated</a>, omitted elements are initialized to 0.
</p><p>A third way of initializing a structure is to copy the value of an existing object of the same type. For example:
</p>
struct point_t b = a;

<h2 id="copy">Copy</h2>
<p>The state of a struct can be copied to another instance. A compiler might use memcpy() to copy the bytes of the memory block.
</p>
struct point_t a = { 1, 3 };
struct point_t b;
b = a;

<h2 id="pointers">Pointers</h2>
<p>Pointers can be used to refer to a struct by its address. This is useful for passing a struct to a function to avoid the overhead of copying the struct. The -> operator <a href="/facts/Dereference_operator/HnG6rwyG">dereferences</a> the pointer (left operand) and accesses the value of a struct member (right operand).
</p>
struct point_t point = { 3, 7 };
int x = point.x;
point.x = 10;
struct point_t *pp = &point;
x = pp->x;
pp->x = 8;

<h2 id="in-other-languages">In other languages</h2>
<h3>C++</h3>
<p>In <a href="/facts/C%252B%252B/Et0F2qn9">C++</a>, struct is essentially the same as for C. Further, a <a href="/facts/C%252B%252B_class/1rWoasWM">class</a> is the same as a struct but with different default <a href="/facts/Access_modifiers/QKGyDmsw">visibility</a>: class members are private by default, whereas struct members are public by default.
</p>
<h3>.NET</h3>
<p><a href="/facts/.NET/PDMQdfFu">.NET</a> languages have a feature similar to struct in C – called struct in <a href="/facts/C_Sharp_(programming_language)/UC2gxeyb">C#</a> and Structure in <a href="/facts/Visual_Basic_.NET/09mPEo4q">Visual Basic .NET</a>). This construct provides many features of a class, but acts as a <a href="/facts/Value_type/0xLHHokA">value type</a> instead of a <a href="/facts/Reference_type/0xLHHokA">reference type</a>. 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.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a>
</p>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Bit_field/fWgTjqvX">Bit field</a> – Data structure that maps one or more adjacent bits</li>
<li><a href="/facts/Flexible_array_member/npL2s4wd">Flexible array member</a> – C language feature in which a struct may contain as its last member an array with no specified size</li>
<li><a href="/facts/Passive_data_structure/10yefr1d">Passive data structure</a> – Another term for record</li>
<li><a href="/facts/Union_type/GSV1DsyV">Union type</a> – Data type that allows for values that are one of multiple different data types</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>"Struct memory layout in C". Stack Overflow. <a href="https://stackoverflow.com/questions/2748995/struct-memory-layout-in-c" target="_blank">https://stackoverflow.com/questions/2748995/struct-memory-layout-in-c</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>Ritchie, Dennis M. (March 1993). "The Development of the C Language". ACM SIGPLAN Notices. 28 (3): 201–208. doi:10.1145/155360.155580. 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. <a href="/wiki/Dennis_Ritchie" target="_blank">/wiki/Dennis_Ritchie</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>Kelley, Al; Pohl, Ira (2004). A Book On C: Programming in C (Fourth ed.). pp. 418. ISBN 0-201-18399-4. <a href="0-201-18399-4" target="_blank">0-201-18399-4</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>"IBM Linux compilers. Initialization of structures and unions". <a href="https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.3.0/com.ibm.zos.v2r3.cbclx01/strin.htm" target="_blank">https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.3.0/com.ibm.zos.v2r3.cbclx01/strin.htm</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
<li id="fn:5"><p>"Parameter passing in C#". <a href="http://yoda.arachsys.com/csharp/parameters.html" target="_blank">http://yoda.arachsys.com/csharp/parameters.html</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li>
</ol>

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struct (C programming language)