structs

Complete

Summary

Struct type

  • grouping heterogenous members
  • not a variable, no memory allocated to types
  • acts as a blueprint for the members associated with the type
c
typedef struct {
	int x;
	int y;
	char *label; // label string
} point; // sizeof(point) = 16

Struct variable

  • similar to creating an instance of a class
c
// declaration
point p1;

// initialization during declaration
point p2 = { 1, 2, "2" };

Assignment

c
// cannot initialize after declaration, similar to arrays
point p3;
p3 = { 1, 3, "3" }; // error

// assignment from a pre-existing struct
point p4 = { 1, 4, "4" }, p5;
p5 = p4;
// or by assigning each member
p5.x = p4.x;
p5.y = p4.y;
p5.label = p4.label;

Concept

Structs as function parameters

c
void inverse(point);

int main(void) {
	point a = { 1, 2, "hello" };
	
	printf("%d, %d, %s\n", a.x, a.y, a.label); // 1, 2, "hello"
	inverse(a);
	printf("%d, %d, %s\n", a.x, a.y, a.label); // 1, 2, "hello"
	return 0; 
}

void inverse(point a) { // a is local to the execution of inverse
	int temp = a.x;
	a.x = a.y;
	a.y = temp;
}

Struct pointers

  • allow a function to make modifications to the original struct
c
void inverse(point *);

int main(void) {
	point a = { 1, 2, "hello" };
	
	printf("%d, %d, %s\n", a.x, a.y, a.label); // 1, 2, "hello"
	inverse(&a);
	printf("%d, %d, %s\n", a.x, a.y, a.label); // 2, 1, "hello"
	return 0; 
}

void inverse(point *a) { // a is a local pointer to the external struct
	int temp = (*a).x; // dereference a and get the member x
	(*a).x = (*a).y;
	(*a).y = temp;
}

the parentheses are required in (*a).x since the . operator has precedence over *

Arrow operator

  • syntactic sugar
c
// rewritten to use the arrow operator
void inverse(point *a) { // (*a).x becomes a->x
	int temp = a->x; // dereference a and get the member x
	a->x = a->y;
	a->y = temp;
}

Application

Nested structs

c
typedef struct {
	int day, month, year;
} date_t;

typedef struct {
	int cardNum;
	date_t expiryDate;
} card_t;

card_t card1 = {888888, {31, 12, 2020}};

Struct array

c
point line[3] = {{ 1, 2, "A" }, { 2, 4, "B" }}; // only 2 points initialised

// pointers
printf("%p\n", line);        // addr
printf("%p\n", &line[0]);    // addr
printf("%p\n", line + 1);    // addr + 1 * sizeof(point)
printf("%p\n", &line[1]);    // addr + 1 * sizeof(point)

// what does the last one get initialized as?
printf("%d\n", line[2]);        // 0
printf("%d\n", line[2].x);      // 0
printf("%d\n", line[2].y);      // 0
printf("%d\n", line[2].label);  // 0
// effectively, all the bytes it would have occupied get initialized to 0

keep in mind that the size of a pointer depends on architecture