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.

The C struct feature was derived from the same-named concept in ALGOL 68.

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 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. For example:

struct point_t a = { 1, 2 };

For non contiguous or out of order members list, designated initializer style may be used. 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.

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