Composite data type

In computer science, a composite data type or compound data type is any data type which can be constructed in a program using the programming language's primitive data types and other composite types. It is sometimes called a structure or aggregate data type, although the latter term may also refer to arrays, lists, etc. The act of constructing a composite type is known as composition. Composite data types are often contrasted with scalar variables.
C/C++ structures and classes.
A codice_1 is C's and C++'s notion of a composite type, a datatype that composes a fixed set of labeled fields or members. It is so called because of the codice_1 keyword used in declaring them, which is short for "structure" or, more precisely, "user-defined data structure".
In C++, the only difference between a codice_1 and a class is the default access level, which is "private" for classes and "public" for codice_1s.
Note that while classes and the codice_5 keyword were completely new in C++, the C programming language already had a crude type of codice_1s. For all intents and purposes, C++ codice_1s form a superset of C codice_1s: virtually all valid C codice_1s are valid C++ codice_1s with the same semantics.
Declaration.
A codice_1 declaration consists of a list of fields, each of which can have any type. The total storage required for a codice_1 object is the sum of the storage requirements of all the fields, plus any internal padding.
For example:
struct Account {
 int account_number;
 char *first_name;
 char *last_name;
 float balance;
defines a type, referred to as codice_13. To create a new variable of this type, we can write codice_14
which has an integer component, accessed by codice_15, and a floating-point component, accessed by codice_16, as well as the codice_17 and codice_18 components. The structure codice_19 contains all four values, and all four fields may be changed independently.
Since writing codice_13 repeatedly in code becomes cumbersome, it is not unusual to see a codice_21 statement in C code to provide a more convenient synonym for the codice_1.
For example:
typedef struct Account_ {
 int account_number;
 char *first_name;
 char *last_name;
 float balance;
} Account;
In C++ code, the codice_21 is not needed because types defined using codice_1 are already part of the regular namespace, so the type can be referred to as either codice_13 or simply codice_26.
As another example, a three-dimensional Vector composite type that uses the floating point data type could be created with:
struct Vector {
 float x;
 float y;
 float z;
A variable named codice_27 with a codice_28 composite type would be declared as codice_29 Members of the codice_27 would be accessed using a dot notation. For example, codice_31 would set the codice_32 component of codice_27 equal to 5.
Likewise, a color structure could be created using:
struct Color {
 unsigned int red;
 unsigned int green;
 unsigned int blue;
In 3D graphics, you usually must keep track of both the position and color of each vertex. One way to do this would be to create a codice_34 composite type, using the previously created codice_28 and codice_36 composite types:
struct Vertex {
 Vector position;
 Color color;
Instantiation.
Create a variable of type codice_37 using the same format as before: codice_38
Member access.
Assign values to the components of codice_39 like so:
v.position.x = 0.0;
v.position.y = 1.5;
v.position.z = 0.0;
v.color.red = 128;
v.color.green = 0;
v.color.blue = 255;
Primitive subtype.
The primary use of codice_1 is for the construction of complex datatypes, but sometimes it is used to create primitive structural subtyping. For example, since Standard C requires that if two structs have the same initial fields, those fields will be represented in the same way, the code
struct ifoo_old_stub {
 long x, y;
struct ifoo_version_42 {
 long x, y, z;
 char *name;
 long a, b, c;
void operate_on_ifoo(struct ifoo_old_stub *);
struct ifoo_version_42 s;
operate_on_ifoo(&s);
will work correctly.
Type signature.
Type signatures (or Function types) are constructed from primitive and composite types, and can serve as types themselves when constructing composite types:
typedef struct {
 int x;
 int y;
} Point;
typedef double (*Metric) (Point p1, Point p2);
typedef struct {
 Point centre;
 double radius;
 Metric metric;
} Circle;