Accessing structure members in C++

Introduction

Structures group related data together. Accessing members lets you use or change each piece inside the group.

You want to store information about a person, like name and age, in one place.

You need to keep details of a book, such as title and author, together.

You are working with a point in 2D space and want to access its x and y coordinates.

You want to organize data about a car, like model and year, in a single variable.

Syntax

C++

struct StructureName {
    dataType member1;
    dataType member2;
    // more members
};

StructureName variableName;
variableName.member1 = value;
value = variableName.member2;

Use the dot . operator to access members of a structure variable.

Each member can be accessed or changed individually.

Examples

Defines a Person structure and sets age and height using dot operator.

C++

struct Person {
    int age;
    float height;
};

Person p;
p.age = 25;
p.height = 5.9f;

Creates a Point with x=10 and y=20, then reads x value.

C++

struct Point {
    int x;
    int y;
};

Point pt = {10, 20};
int xValue = pt.x;

Sample Program

This program creates a Car structure, assigns values to its members, and prints them.

C++

#include <iostream>
#include <string>
using namespace std;

struct Car {
    string model;
    int year;
};

int main() {
    Car myCar;
    myCar.model = "Toyota";
    myCar.year = 2020;

    cout << "Car model: " << myCar.model << "\n";
    cout << "Car year: " << myCar.year << "\n";
    return 0;
}

Important Notes

Remember to include #include <iostream> and use std::cout or using namespace std; for printing.

Structure members are accessed with the dot operator only when you have a structure variable, not a pointer.

Summary

Structures group related data under one name.

Use the dot operator . to access or change members.

Each member can be used like a normal variable.

Practice

1. What operator is used to access members of a structure variable in C++?

easy

A. Double colon (::)

B. Arrow operator (->)

C. Colon operator (:)

D. Dot operator (.)

Solution

1. Step 1: Recall structure member access syntax

   In C++, to access a member of a structure variable, the dot operator (.) is used.

2. Step 2: Differentiate from pointer access

   The arrow operator (->) is used when accessing members through a pointer, not a direct variable.

3. Final Answer:

   Dot operator (.) -> Option D

4. Quick Check:

   Structure member access = dot operator (.) [OK]

Hint: Use dot (.) for variables, arrow (->) for pointers [OK]

Common Mistakes:

• Confusing dot (.) with arrow (->)
• Using colon (::) which is for scope resolution
• Using arrow (->) on non-pointer variables

2. Which of the following is the correct syntax to access member age of a structure variable person?

easy

A. person->age;

B. person::age;

C. person.age;

D. person:age;

Solution

1. Step 1: Identify the variable type

   Since person is a structure variable (not a pointer), use the dot operator to access members.

2. Step 2: Check syntax correctness

   Correct syntax is person.age;. The arrow operator (->) is for pointers, colons are invalid here.

3. Final Answer:

   person.age; -> Option C

4. Quick Check:

   Direct struct variable uses dot (.) [OK]

Hint: Use dot (.) for struct variables, arrow (->) for pointers [OK]

Common Mistakes:

• Using arrow (->) on non-pointer variables
• Using colon (:) or double colon (::) incorrectly
• Missing semicolon at end

3. What will be the output of the following code?

#include <iostream>
struct Point {
  int x;
  int y;
};

int main() {
  Point p = {3, 4};
  std::cout << p.x + p.y << std::endl;
  return 0;
}

medium

A. 7

B. 34

C. 0

D. Compilation error

Solution

1. Step 1: Understand structure initialization

   The structure Point is initialized with x=3 and y=4.

2. Step 2: Calculate the output expression

   The code outputs p.x + p.y which is 3 + 4 = 7.

3. Final Answer:

   7 -> Option A

4. Quick Check:

   3 + 4 = 7 [OK]

Hint: Add members accessed by dot operator for result [OK]

Common Mistakes:

• Concatenating numbers as strings (34)
• Assuming default zero initialization
• Confusing pointer access syntax

4. Identify the error in the following code snippet:

struct Car {
  int speed;
};

int main() {
  Car c;
  c->speed = 100;
  return 0;
}

medium

A. Missing semicolon after struct definition

B. Using arrow operator (->) on a non-pointer variable

C. Incorrect struct member name

D. No error, code is correct

Solution

1. Step 1: Check variable type and operator

   The variable c is a struct variable, not a pointer.

2. Step 2: Identify correct member access

   Using c->speed is invalid here; should use c.speed.

3. Final Answer:

   Using arrow operator (->) on a non-pointer variable -> Option B

4. Quick Check:

   Non-pointer struct uses dot (.) [OK]

Hint: Use dot (.) for variables, arrow (->) only for pointers [OK]

Common Mistakes:

• Using arrow (->) on struct variables
• Assuming arrow works for all struct access
• Ignoring pointer vs variable difference

5. Given the following code, how can you correctly access the member name of the structure pointer ptr?

struct Student {
  std::string name;
  int id;
};

Student s = {"Alice", 101};
Student* ptr = &s;
// Access name here

hard

A. ptr->name;

B. ptr.name;

C. ptr::name;

D. *ptr.name;

Solution

1. Step 1: Understand pointer member access

   When accessing members through a pointer, you can use the arrow operator (->) or dereference then dot ( (*ptr).member ).

2. Step 2: Check each option

   ptr.name; is invalid syntax for a pointer. ptr->name; is the correct syntax using the arrow operator. ptr::name; incorrectly uses scope resolution operator. *ptr.name; is wrong due to operator precedence; it should be (*ptr).name;.

3. Final Answer:

   ptr->name; -> Option A

4. Quick Check:

   Pointer member access = arrow (->) or (*ptr).member [OK]

Hint: Use arrow (->) or dereference then dot ( (*ptr).member ) [OK]

Common Mistakes:

• Using dot (.) directly on pointer
• Forgetting to dereference pointer
• Confusing pointer and variable access