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:

structtag_name{typemember1;typemember2;/* 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:

typedefstructtag_name{typemember1;typemember2;}struct_alias;

Initialization

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

/* Declare the struct with integer members x, y */structpoint{intx;inty;};

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

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

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

/* Define a variable p of type point, and set members using designated initializers */structpointp={.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 a variable q of type point, and set members to the same values as those of p */structpointq=p;

Assignment

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

structpoint{intx;inty;};intmain(void){structpointp={1,3};/* initialized variable */structpointq;/* uninitialized */q=p;/* copy member values from p into q */return0;}

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).

structpoint{intx;inty;};structpointmy_point={3,7};structpoint*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 */

See also

• Bit field
• Flexible array member
• Passive data structure
• Union type

References

1. "Struct memory layout in C". Stack Overflow.
2. 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.
3. "Parameter passing in C#".
4. Kelley, Al; Pohl, Ira (2004). A Book On C: Programming in C (Fourth ed.). pp.  418. ISBN   0-201-18399-4.
5. "IBM Linux compilers. Initialization of structures and unions".