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 contain fields for the customer's name, address, telephone number, and 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]

The syntax for a struct declaration is shown by this simple example:

struct TagName {
    Type member1;
    Type member2;
};

The TagName is optional in some contexts.

Via the keyword typedef, a struct type can be referenced without using the struct keyword. However, some^[3] programming style guides advise against this, claiming that it can obfuscate the type.

For example:

typedef struct TagName {
   Type member1;
   Type member2;
} Thing;

// struct TagName can now be referred to as Thing
Thing 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 or Thing. typedef in C++ is also superseded by the using statement, which can alias types that have templates.

There are three ways to initialize a structure.

For the type:

struct Point {
    int x;
    int y;
};

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

struct Point a = { 1, 2 };

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

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. For example:

struct Point b = a;

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 a = { 1, 3 };
struct Point b;
b = a;

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 p = { 3, 7 };
int x = p.x;
p.x = 10;
struct Point* pp = &p;
x = pp->x;
pp->x = 8;

D,^[6] Go, Julia,^[7] Rust, Swift and Zig^[8] have structs.

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 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.^[9]

• Bit field – Data structure that maps one or more adjacent bits
• Flexible array member – C language feature
• Passive data structure – Another term for record
• Union type – Data type that allows for values that are one of multiple different data types