Classes and objects in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a class in C++?

A class is a blueprint or template for creating objects. It defines properties (data members) and behaviors (member functions) that the objects created from the class will have.

beginner

What is an object in C++?

An object is an instance of a class. It holds actual values for the properties defined by the class and can use the behaviors (functions) defined in the class.

beginner

How do you define a simple class with one integer property and a function to set it?

Example:
class MyClass {
public:
  int value;
  void setValue(int v) {
    value = v;
  }
};

intermediate

What is the difference between public and private members in a class?

Public members can be accessed from outside the class, while private members can only be accessed from within the class itself.

beginner

How do you create an object of a class and call its member function?

Example:
MyClass obj;  // create object
obj.setValue(10);  // call member function
int x = obj.value;  // access property

What keyword is used to define a class in C++?

A define

B object

C struct

D class

The keyword 'class' is used to define a class in C++.

Which of the following is true about objects?

A Objects are templates for classes

B Objects are instances of classes

C Objects cannot have functions

D Objects are the same as classes

Objects are instances created from classes.

What access specifier allows members to be accessed outside the class?

A private

B protected

C public

D internal

Public members can be accessed from outside the class.

How do you call a member function named 'setValue' on an object 'obj'?

A obj.setValue()

B setValue.obj()

C obj->setValue()

D obj:setValue()

Use dot notation: obj.setValue() to call a member function.

Which of these is NOT a member of a class?

A Objects

B Member functions

C Data members

D Constructors

Objects are instances of classes, not members inside a class.

Explain what a class and an object are in your own words.

Think about how a cookie cutter (class) makes cookies (objects).

Describe how access specifiers like public and private affect class members.

Imagine a house with a front door (public) and a locked safe inside (private).

Practice

1. What is the main purpose of a class in C++?

easy

A. To group data and functions that represent a real-world concept

B. To execute a program faster

C. To store only numbers

D. To replace variables with constants

Solution

1. Step 1: Understand what a class represents

   A class groups related data (variables) and functions (methods) to model real-world things.

2. Step 2: Compare options with the definition

   Only To group data and functions that represent a real-world concept correctly describes this purpose; others are unrelated to classes.

3. Final Answer:

   To group data and functions that represent a real-world concept -> Option A

4. Quick Check:

   Class = group data + functions [OK]

Hint: Classes bundle data and actions together [OK]

Common Mistakes:

• Thinking classes only store numbers
• Confusing classes with program speed
• Believing classes replace variables

2. Which of the following is the correct way to declare a class named Car in C++?

easy

A. class Car { };

B. Car class { };

C. class = Car { };

D. class Car();

Solution

1. Step 1: Recall C++ class declaration syntax

   The correct syntax is: class ClassName { }; with a semicolon at the end.

2. Step 2: Check each option

   class Car { }; matches the correct syntax. Options A, C, and D have syntax errors or wrong order.

3. Final Answer:

   class Car { }; -> Option A

4. Quick Check:

   class ClassName { }; syntax [OK]

Hint: Remember: class name follows 'class' keyword, ends with semicolon [OK]

Common Mistakes:

• Writing 'Car class' instead of 'class Car'
• Missing semicolon after class body
• Using parentheses after class name

3. What will be the output of this C++ code?

class Box {
public:
  int length;
  void setLength(int len) { length = len; }
  int getLength() { return length; }
};

int main() {
  Box box1;
  box1.setLength(10);
  std::cout << box1.getLength();
  return 0;
}

medium

A. length

B. 0

C. Compilation error

D. 10

Solution

1. Step 1: Understand the class methods

   The method setLength sets the length to 10 for box1. The method getLength returns this value.

2. Step 2: Trace the output statement

   std::cout << box1.getLength(); prints the value 10 stored in length.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   setLength(10) then getLength() = 10 [OK]

Hint: Setter sets value, getter returns it [OK]

Common Mistakes:

• Forgetting to call setLength before getLength
• Expecting default value 0 without initialization
• Confusing method names

4. Identify the error in this class definition:

class Person {
  string name;
  int age;
public:
  void setName(string n) { name = n; }
  void setAge(int a) { age = a; }
};

medium

A. Missing semicolon after class definition

B. Missing public: before data members

C. Missing #include <string> header

D. Incorrect function return types

Solution

1. Step 1: Check class syntax

   The class ends with a semicolon, and functions have correct return types (void). Data members are private by default, which is allowed.

2. Step 2: Check for missing includes

   The code uses string but does not include <string> header, which is required for std::string.

3. Final Answer:

   Missing #include <string> header -> Option C

4. Quick Check:

   Use #include <string> for string type [OK]

Hint: Always include headers for standard types [OK]

Common Mistakes:

• Assuming private members need public keyword
• Forgetting to add semicolon after class
• Confusing function return types

5. Given this class:

class Student {
  int id;
  string name;
public:
  Student(int i, string n) : id(i), name(n) {}
  void display() {
    std::cout << "ID: " << id << ", Name: " << name << std::endl;
  }
};

int main() {
  Student s1(101, "Alice");
  s1.display();
  return 0;
}

What concept does this code demonstrate?

hard

A. Inheritance between classes

B. Constructor initializing object data

C. Function overloading

D. Static class members

Solution

1. Step 1: Identify the special function in the class

   The function Student(int i, string n) : id(i), name(n) {} is a constructor initializing the object's data members.

2. Step 2: Check other options

   There is no inheritance, no function overloading, and no static members shown.

3. Final Answer:

   Constructor initializing object data -> Option B

4. Quick Check:

   Constructor sets initial values [OK]

Hint: Look for function named as class for constructor [OK]

Common Mistakes:

• Confusing constructor with regular method
• Thinking this shows inheritance
• Assuming static members without keyword