Struct (C programming language)
In the C programming language, struct is the keyword used to define a composite, a.k.a. record, data type – 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. 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 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 boundaries) is implementation-specific and may include padding. Modern compilers typically support the #pragma pack
directive, which sets the size in bytes for alignment.[1]
The C struct feature was derived from the same-named concept in ALGOL 68.[2]
Declaration
The syntax for a struct declaration is shown by this simple example:
struct tag_name { type member1; type member2; };
The tag_name
is optional in some contexts.
Typedef
Via the keyword typedef
, a struct type can be referenced without using the struct
keyword. However, some[who?] programming style guides advise against this, claiming that it can obfuscate the type.
For example:
typedef struct tag_name { type member1; type member2; } thing_t; thing_t thing;
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
.
Initialization
There are three ways to initialize a structure.
For the type:
struct point_t { int x; int y; };
C89-style initializers are used when contiguous members may be given.[3] For example:
struct point_t a = { 1, 2 };
For non contiguous or out of order members list, designated initializer style may be used.[4] For example:
struct point_t a = { .y = 2, .x = 1 };
If an initializer is given or if the object is 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:
struct point_t b = a;
Copy
The state of a struct can be copied to another instance. A compiler might use memcpy()
to copy the bytes of the memory block.
struct point_t a = { 1, 3 }; struct point_t b; b = a;
Pointers
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 dereferences the pointer (left operand) and accesses the value of a struct member (right operand).
struct point_t point = { 3, 7 }; int x = point.x; point.x = 10; struct point_t *pp = &point; x = pp->x; pp->x = 8;
In other languages
C++
In C++, struct is essentially the same as for C. Further, a class is the same as a struct but with different default 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 – called struct
in C# and Structure
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.[5]
See also
- Bit field – Data structure that maps one or more adjacent bits
- Flexible array member – C language feature in which a struct may contain as its last member an array with no specified size
- Passive data structure – Another term for record
- Union type – Data type that allows for values that are one of multiple different data types
References
- ^ "Struct memory layout in C". Stack Overflow.
- ^ 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.
- ^ Kelley, Al; Pohl, Ira (2004). A Book On C: Programming in C (Fourth ed.). pp. 418. ISBN 0-201-18399-4.
- ^ "IBM Linux compilers. Initialization of structures and unions".
- ^ "Parameter passing in C#".