Struct (C programming language)

A struct in the C programming language (and many derivatives) is a composite data type (or record) declaration that defines a physically grouped list of variables under one name in a block of memory, allowing the different variables to be accessed via a single pointer or by the struct declared name which returns the same address. The struct data type can contain other data types so is used for mixed-data-type records such as a hard-drive directory entry (file length, name, extension, physical address, etc.), or other mixed-type records (name, address, telephone, balance, etc.).

The C struct directly references a contiguous block of physical 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.

Because the contents of a struct are stored in contiguous memory, the sizeof operator must be used to get the number of bytes needed to store a particular type of struct, just as it can be used for primitives. The alignment of particular fields in the struct (with respect to word boundaries) is implementation-specific and may include padding, although modern compilers typically support the #pragma pack directive, which changes the size in bytes used for alignment.^[1]

In the C++ language, a struct is identical to a C++ class but has a different default visibility: class members are private by default, whereas struct members are public by default.

In other languages

The struct data type in C was derived from the ALGOL 68 struct data type.^[2]

Like its C counterpart, the struct data type in C# (Structure in Visual Basic .NET) is similar to a class. The biggest difference between a struct and a class in these languages is that when a struct is passed as an argument to a function, any modifications to the struct in that function will not be reflected in the original variable (unless pass-by-reference is used).^[3]

This differs from C++, where classes or structs can be statically allocated or dynamically allocated either on the stack (similar to C#) or on the heap, with an explicit pointer. In C++, the only difference between a struct and a class is that the members and base classes of a struct are public by default. (A class defined with the class keyword has private members and base classes by default.)

Declaration

The general syntax for a struct declaration in C is:

struct tag_name {
   type member1;
   type member2;
   /* declare as many members as desired, but the entire structure size must be known to the compiler. */
};

Here tag_name is optional in some contexts.

Such a struct declaration may also appear in the context of a typedef declaration of a type alias or the declaration or definition of a variable:

typedef struct tag_name {
   type member1;
   type member2;
} struct_alias;

Initialization

There are three ways to initialize a structure. For the struct type

/* Declare the struct with integer members x, y */
struct point {
   int x;
   int y;
};

C89-style initializers are used when contiguous members may be given.^[4]

/* Define variable 'a' of type point, and initialize its first two members in place */
struct point a = { 1, 2 };

For non contiguous or out of order members list, designated initializer style^[5] may be used

/* Define variable 'a' of type point, and set members using designated initializers */
struct point 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

/* Define variable 'b' of type point, and set members to the same values as those of 'a' */
struct point b = a;

Assignment

A struct may be assigned to another struct. A compiler might use memcpy() to perform such an assignment.

struct point {
    int x;
    int y;
};

int main(void)
{
    struct point a = { 1, 3 }; /* initialized variable */
    struct point b;            /* uninitialized */
    b = a;                     /* copy member values from 'a' into 'b' */
    return 0;
}

Pointers to struct

Pointers can be used to refer to a struct by its address. This is useful for passing structs to a function. The pointer can be dereferenced using the * operator. The -> operator dereferences the pointer to struct (left operand) and then accesses the value of a member of the struct (right operand).

struct point {
   int x;
   int y;
};
struct point my_point = { 3, 7 };
struct point *p = &my_point;  /* 'p' is a pointer to my_point */
(*p).x = 8;                   /* set the first member of the struct */
p->x = 8;                     /* equivalent method to set the first member of the struct */

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.
^ "Parameter passing in C#".
^ 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".