What is a structure?
A structure is a user defined data type in C/C++. A structure creates a data type that can be used to group items of possibly different types into a single type.
How to create a structure?
‘struct’ keyword is used to create a structure. Following is an example.
struct address { char name[50]; char street[100]; char city[50]; char state[20]; int pin; }; |
How to declare structure variables?
A structure variable can either be declared with structure declaration or as a separate declaration like basic types.
// A variable declaration with structure declaration. struct Point { int x, y; } p1; // The variable p1 is declared with 'Point' // A variable declaration like basic data types struct Point { int x, y; }; int main() { struct Point p1; // The variable p1 is declared like a normal variable } |
Note: In C++, the struct keyword is optional before in declaration of a variable. In C, it is mandatory.
How to initialize structure members?
Structure members cannot be initialized with declaration. For example the following C program fails in compilation.
struct Point { int x = 0; // COMPILER ERROR: cannot initialize members here int y = 0; // COMPILER ERROR: cannot initialize members here }; |
The reason for above error is simple, when a datatype is declared, no memory is allocated for it. Memory is allocated only when variables are created.
Structure members can be initialized using curly braces ‘{}’. For example, following is a valid initialization.
struct Point { int x, y; }; int main() { // A valid initialization. member x gets value 0 and y // gets value 1. The order of declaration is followed. struct Point p1 = {0, 1}; } |
How to access structure elements?
Structure members are accessed using dot (.) operator.
#include<stdio.h> struct Point { int x, y; }; int main() { struct Point p1 = {0, 1}; // Accesing members of point p1 p1.x = 20; printf ("x = %d, y = %d", p1.x, p1.y); return 0; } |
x = 20, y = 1
What is designated Initialization?
Designated Initialization allows structure members to be initialized in any order. This feature has been added in C99 standard.
#include<stdio.h> struct Point { int x, y, z; }; int main() { // Examples of initializtion using designated initialization struct Point p1 = {.y = 0, .z = 1, .x = 2}; struct Point p2 = {.x = 20}; printf ("x = %d, y = %d, z = %d
", p1.x, p1.y, p1.z); printf ("x = %d", p2.x); return 0; } |
x = 2, y = 0, z = 1 x = 20
This feature is not available in C++ and works only in C.
What is an array of structures?
Like other primitive data types, we can create an array of structures.
#include<stdio.h> struct Point { int x, y; }; int main() { // Create an array of structures struct Point arr[10]; // Access array members arr[0].x = 10; arr[0].y = 20; printf("%d %d", arr[0].x, arr[0].y); return 0; } |
10 20
What is a structure pointer?
Like primitive types, we can have pointer to a structure. If we have a pointer to structure, members are accessed using arrow ( -> ) operator.
#include<stdio.h> struct Point { int x, y; }; int main() { struct Point p1 = {1, 2}; // p2 is a pointer to structure p1 struct Point *p2 = &p1; // Accessing structure members using structure pointer printf("%d %d", p2->x, p2->y); return 0; } |
1 2
What is structure member alignment?
See https://tutorialspoint.dev/slugresolver/structure-member-alignment-padding-and-data-packing/
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