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Objective

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  • Learn about structs and how to use them.
    • Declaring structs.
    • Defining structs.

Lesson

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Introduction

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Structures allow objects of different types to be stored within one object. Let's say that you want to store a car's colour, maximum speed and registration number. We can define a structure that includes all that information.

First, include the header files required for printf and strcpy: 

#include<stdio.h>
#include<string.h>

Define the structure: 

structcar
{
unsignedintcolour;
unsignedintmax_speed;
charreg_num[10];
};

Example of the structure being used: 

intmain(void)
{

/* define two car objects */
structcarcar_1,car_2;

car_1.colour=1;/* initialize car_1's colour */
car_1.max_speed=200;/* initialize car_1's speed */

/* initialize the reg number */
strcpy(car_1.reg_num,"MG 100");

/* copy car_1 data to car_2 */
car_2=car_1;

/* print the values */
printf("Colour = %u\n",car_2.colour);
printf("Speed = %u\n",car_2.max_speed);
printf("Reg = %s\n",car_2.reg_num);

return0;
}

Dynamic Structures

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Along with being static, structures can also be dynamic. This means that they can be created and deleted at run time. An example is given below. 

#include<stdio.h>

// This is a basic dog structure.
structdog
{
char*name;
char*breed;
char*coat;
intage;
};

// Describes the dog.
voiddescribe(structdog*d)
{
printf("This is a %s dog. The %s's name is %s. %s has a nice %s coat. %s is %d years old.\n\n",
d->breed,d->breed,d->name,d->name,d->coat,d->name,d->age);
}

intmain(void)
{
// Allocate memory for all of the dogs.
// Notice that size for malloc is struct dog and it's casted as struct dog*.
structdog*dog1=(structdog*)malloc(sizeof(structdog));
structdog*dog2=(structdog*)malloc(sizeof(structdog));
structdog*dog3=(structdog*)malloc(sizeof(structdog));

// Give the dogs some personality.
dog1->name="Foxtrot";
dog1->breed="American Eskimo";
dog1->coat="white";
dog1->age=6;

dog2->name="Delta";
dog2->breed="German Shepherd";
dog2->coat="tan with a black saddle";
dog2->age=3;

dog3->name="Whiskey";
dog3->breed="Tamaskan";
dog3->coat="wolf grey";
dog3->age=2;

// Tell the user about each dog.
describe(dog1);
describe(dog2);
describe(dog3);

// Free the allocated memory.
free(dog1);
free(dog2);
free(dog3);

return0;
}

OUTPUT 

This is a American Eskimo dog. The American Eskimo's name is Foxtrot. Foxtrot has a nice white coat. Foxtrot is 6 years old.

This is a German Shepherd dog. The German Shepherd's name is Delta. Delta has a nice tan with a black saddle coat. Delta is 3 years old.

This is a Tamaskan dog. The Tamaskan's name is Whiskey. Whiskey has a nice wolf grey coat. Whiskey is 2 years old.

It is now possible to do dynamic things with each dog structure, although those operations could be a little advanced. Also notice the use of the arrow operator (->). This is a cleaner way of using pointers when working with structures. For example dog1->name is equal to *(dog1).name. The arrow operator makes it easier when accessing nested structures.

Assignments

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