Data members and member functions in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a data member named 'age' of type int inside the class.

C++

class Person {
public:
    [1] age;
};

Drag options to blanks, or click blank then click option'

Avoid

Bint

Cfloat

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'void' as a data member type causes a syntax error.

Using 'string' without including the proper header or namespace.

The data member 'age' should be declared as type int to store whole numbers.

2fill in blank

medium

Complete the code to define a member function named 'getAge' that returns the age.

C++

class Person {
private:
    int age;
public:
    int [1]() {
        return age;
    }
};

Drag options to blanks, or click blank then click option'

AsetAge

Bage

CgetAge

DprintAge

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'setAge' which is usually for setting a value, not getting.

Using the data member name 'age' as a function name causes confusion.

The member function that returns the age should be named getAge.

3fill in blank

hard

Fix the error in the member function definition to correctly set the age.

C++

class Person {
private:
    int age;
public:
    void setAge(int a) {
        [1] = a;
    }
};

Drag options to blanks, or click blank then click option'

Athis->age

Ba

Cage

DsetAge

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning parameter to itself like a = a; which does nothing.

Using just 'age = a;' without this-> can cause ambiguity.

Use this->age to refer to the data member and avoid confusion with the parameter a.

4fill in blank

hard

Fill both blanks to create a member function that increases age by 1 and returns the new age.

C++

class Person {
private:
    int age;
public:
    int [1]() {
        age [2] 1;
        return age;
    }
};

Drag options to blanks, or click blank then click option'

AincrementAge

B+=

C-=

DdecrementAge

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-=' operator which decreases the value.

Naming the function 'decrementAge' which means to reduce age.

The function name should be incrementAge and the operator += adds 1 to age.

5fill in blank

hard

Fill all three blanks to define a constructor that sets the age data member using the parameter.

C++

class Person {
private:
    int age;
public:
    Person(int [1]) {
        [2] = [3];
    }
};

Drag options to blanks, or click blank then click option'

Aa

Bthis->age

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same name for parameter and data member without this-> causes ambiguity.

Assigning parameter to itself like a = a; which does nothing.

The constructor parameter is named a. Use this->age = a; to set the data member.

Practice

1. Which of the following best describes a data member in a C++ class?

easy

A. A keyword used to define a class

B. A function that performs actions on an object

C. A variable that holds information about an object

D. A special type of loop inside a class

Solution

1. Step 1: Understand what data members are

   Data members are variables inside a class that store information about each object created from that class.

2. Step 2: Differentiate from member functions

   Member functions are actions or behaviors, not storage. Keywords and loops are unrelated to data members.

3. Final Answer:

   A variable that holds information about an object -> Option C

4. Quick Check:

   Data member = variable storing object info [OK]

Hint: Data members store data, member functions do actions [OK]

Common Mistakes:

• Confusing data members with member functions
• Thinking data members are keywords
• Mixing data members with control structures

2. Which of the following is the correct way to declare a member function named display inside a C++ class?

easy

A. void display();

B. display void();

C. void display {}

D. function void display();

Solution

1. Step 1: Recall correct member function syntax

   Member functions are declared with return type, name, and parentheses, ending with a semicolon inside the class.

2. Step 2: Check each option

   void display(); matches correct syntax: return type 'void', function name 'display', parentheses, and semicolon. Others are invalid syntax.

3. Final Answer:

   void display(); -> Option A

4. Quick Check:

   Correct function declaration = void display(); [OK]

Hint: Member functions: returnType name(); inside class [OK]

Common Mistakes:

• Omitting parentheses after function name
• Using wrong order of return type and name
• Missing semicolon after declaration

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

#include <iostream>
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. 0

B. Compilation error

C. length

D. 10

Solution

1. Step 1: Understand member function calls

   The function setLength(10) sets the data member length to 10 for object box1.

2. Step 2: Check output from getLength()

   The function getLength() returns the current value of length, which was set to 10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   Set length to 10, get length returns 10 [OK]

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

Common Mistakes:

• Assuming default value 0 is printed
• Confusing function names
• Expecting compilation error due to missing constructor

4. Identify the error in the following C++ class code:

#include <iostream>
class Person {
  int age;
public:
  void setAge(int a) {
    age = a;
  }
  int getAge() {
    return age;
  }
};

int main() {
  Person p;
  p.age = 25;
  std::cout << p.getAge();
  return 0;
}

medium

A. Error: 'age' is private and cannot be accessed directly

B. No error, output is 25

C. Error: Missing return type for getAge

D. Error: setAge function is not called

Solution

1. Step 1: Check access to data member 'age'

   The data member 'age' is private by default because no access specifier is given before it.

2. Step 2: Analyze main function usage

   In main, code tries to access 'p.age = 25;' directly, which is not allowed for private members, causing a compilation error.

3. Final Answer:

   Error: 'age' is private and cannot be accessed directly -> Option A

4. Quick Check:

   Private members can't be accessed outside class [OK]

Hint: Private members need public functions to access [OK]

Common Mistakes:

• Trying to access private data members directly
• Ignoring default private access in classes
• Assuming no error if setter is not called

5. Given the class below, which code snippet correctly increases the count data member by 1 for an object obj?

class Counter {
private:
  int count = 0;
public:
  void increment() {
    count++;
  }
};

hard

A. obj.count = obj.count + 1;

B. obj.increment();

C. Counter::increment();

D. obj.count++; obj.increment();

Solution

1. Step 1: Understand the class design

   The class has a private data member count = 0 and provides a public member function increment() that increases count by 1.

2. Step 2: Check each option for correctness

   obj.count = obj.count + 1; tries to access private count directly, compilation error. obj.increment(); calls the member function correctly. Counter::increment(); tries to call non-static member function without an object, invalid. obj.count++; obj.increment(); first accesses private count, error.

3. Final Answer:

   obj.increment(); -> Option B

4. Quick Check:

   Use member function to change data member [OK]

Hint: Use object's member function to update data members [OK]

Common Mistakes:

• Calling member function without object
• Manually changing data member instead of using function
• Incrementing count twice accidentally