structure and pointer

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In this article, you’ll learn about structures in C++ programming; what is it, how to define it and use it in your program.

Structure is a collection of variables of different data types under a single name. It is similar to a class in that, both holds a collecion of data of different data types.

For example: You want to store some information about a person: his/her name, citizenship number and salary. You can easily create different variables name, citNo, salary to store these information separately.

However, in the future, you would want to store information about multiple persons. Now, you’d need to create different variables for each information per person: name1, citNo1, salary1, name2, citNo2, salary2

You can easily visualize how big and messy the code would look. Also, since no relation between the variables (information) would exist, it’s going to be a daunting task.

A better approach will be to have a collection of all related information under a single name Person, and use it for every person. Now, the code looks much cleaner, readable and efficient as well.

This collection of all related information under a single name Person is a structure.

How to declare a structure in C++ programming?

The struct keyword defines a structure type followed by an identifier (name of the structure).

Then inside the curly braces, you can declare one or more members (declare variables inside curly braces) of that structure. For example:

struct Person
{
    char name[50];
    int age;
    float salary;
};

Here a structure person is defined which has three members: name, age and salary.

When a structure is created, no memory is allocated.

The structure definition is only the blueprint for the creating of variables. You can imagine it as a datatype. When you define an integer as below:

int foo;

The int specifies that, variable foo can hold integer element only. Similarly, structure definition only specifies that, what property a structure variable holds when it is defined.

Note: Remember to end the declaration with a semicolon (;)

How to define a structure variable?

Once you declare a structure person as above. You can define a structure variable as:

Person bill;

Here, a structure variable bill is defined which is of type structure Person.

When structure variable is defined, only then the required memory is allocated by the compiler.

Considering you have either 32-bit or 64-bit system, the memory of float is 4 bytes, memory of int is 4 bytes and memory of char is 1 byte.

Hence, 58 bytes of memory is allocated for structure variable bill.

How to access members of a structure?

The members of structure variable is accessed using a dot (.) operator.

Suppose, you want to access age of structure variable bill and assign it 50 to it. You can perform this task by using following code below:

bill.age = 50;

Example: C++ Structure

C++ Program to assign data to members of a structure variable and display it.

#include <iostream>
using namespace std;

struct Person
{
    char name[50];
    int age;
    float salary;
};

int main()
{
    Person p1;
    
    cout << "Enter Full name: ";
    cin.get(p1.name, 50);
    cout << "Enter age: ";
    cin >> p1.age;
    cout << "Enter salary: ";
    cin >> p1.salary;

    cout << "\nDisplaying Information." << endl;
    cout << "Name: " << p1.name << endl;
    cout <<"Age: " << p1.age << endl;
    cout << "Salary: " << p1.salary;

    return 0;
}

Output

Enter Full name: Magdalena Dankova
Enter age: 27
Enter salary: 1024.4

Displaying Information.
Name: Magdalena Dankova
Age: 27
Salary: 1024.4

Here a structure Person is declared which has three members name, age and salary.

Inside main() function, a structure variable p1 is defined. Then, the user is asked to enter information and data entered by user is displayed.

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In this tutorial, you’ll learn to use pointers to access members of structs in C programming. You will also learn to dynamically allocate memory of struct types.

Before you learn about how pointers can be used with structs, be sure to check these tutorials:

C Pointers
C struct

C Pointers to struct

Here’s how you can create pointers to structs.

struct name {
    member1;
    member2;
    .
    .
};

int main()
{
    struct name *ptr, Harry;
}

Here, ptr is a pointer to struct.

Example: Access members using Pointer

To access members of a structure using pointers, we use the -> operator.

#include <stdio.h>
struct person
{
   int age;
   float weight;
};

int main()
{
    struct person *personPtr, person1;
    personPtr = &person1;   

    printf("Enter age: ");
    scanf("%d", &personPtr->age);

    printf("Enter weight: ");
    scanf("%f", &personPtr->weight);

    printf("Displaying:\n");
    printf("Age: %d\n", personPtr->age);
    printf("weight: %f", personPtr->weight);

    return 0;
}

In this example, the address of person1 is stored in the personPtr pointer using personPtr = &person1;.

Now, you can access the members of person1 using the personPtr pointer.

By the way,

personPtr->age is equivalent to (*personPtr).age
personPtr->weight is equivalent to (*personPtr).weight

Dynamic memory allocation of structs

Before you proceed this section, we recommend you to check C dynamic memory allocation.

Sometimes, the number of struct variables you declared may be insufficient. You may need to allocate memory during run-time. Here’s how you can achieve this in C programming.

Example: Dynamic memory allocation of structs

#include <stdio.h>
#include <stdlib.h>
struct person {
   int age;
   float weight;
   char name[30];
};

int main()
{
   struct person *ptr;
   int i, n;

   printf("Enter the number of persons: ");
   scanf("%d", &n);

   // allocating memory for n numbers of struct person
   ptr = (struct person*) malloc(n * sizeof(struct person));

   for(i = 0; i < n; ++i)
   {
       printf("Enter first name and age respectively: ");

       // To access members of 1st struct person,
       // ptr->name and ptr->age is used

       // To access members of 2nd struct person,
       // (ptr+1)->name and (ptr+1)->age is used
       scanf("%s %d", (ptr+i)->name, &(ptr+i)->age);
   }

   printf("Displaying Information:\n");
   for(i = 0; i < n; ++i)
       printf("Name: %s\tAge: %d\n", (ptr+i)->name, (ptr+i)->age);

   return 0;
}

When you run the program, the output will be:

Enter the number of persons:  2
Enter first name and age respectively:  Harry 24
Enter first name and age respectively:  Gary 32
Displaying Information:
Name: Harry	Age: 24
Name: Gary	Age: 32

In the above example, n number of struct variables are created where n is entered by the user.

To allocate the memory for n number of struct person, we used,

ptr = (struct person*) malloc(n * sizeof(struct person));

Then, we used the ptr pointer to access elements of person.

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Tag: structure and pointer

C++ Structures

C++ Tutorial

C++ Introduction

C++ Flow Control

C++ Functions

C++ Arrays & String

C++ Structures

C++ Object & Class

C++ Pointers

C++ Inheritance

How to declare a structure in C++ programming?

How to define a structure variable?

How to access members of a structure?

Example: C++ Structure

C Structure and Pointers

C-Tutorial

C Structure and Pointers

C Pointers to struct

Example: Access members using Pointer

Dynamic memory allocation of structs

Example: Dynamic memory allocation of structs