Data members and member functions in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of member function modifying data member

What is the output of this C++ program?

C++

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

int main() {
    Counter c;
    c.increment();
    c.increment();
    std::cout << c.count << std::endl;
    return 0;
}

A 2

B 0

C 1

D Compilation error

💡 Hint

The increment function increases the count by one each time it is called.

The count starts at 0. Each call to increment() adds 1. After two calls, count is 2.

❓ Predict Output

intermediate

Output of const member function accessing data member

What will this program print?

C++

class Box {
    int length = 5;
public:
    int getLength() const {
        return length;
    }
};

int main() {
    Box b;
    std::cout << b.getLength() << std::endl;
    return 0;
}

A 5

B 0

C Compilation error due to const function

D Undefined behavior

💡 Hint

Const member functions can read data members but cannot modify them.

The getLength() function returns the value of length which is 5.

❓ Predict Output

advanced

Output when accessing private data member directly

What happens when you try to compile and run this code?

C++

class Sample {
private:
    int value = 10;
public:
    int getValue() {
        return value;
    }
};

int main() {
    Sample s;
    std::cout << s.value << std::endl;
    return 0;
}

A 10

B Runtime error

C 0

D Compilation error: 'value' is private

💡 Hint

Private members cannot be accessed directly outside the class.

The code tries to access private member 'value' directly, which is not allowed.

❓ Predict Output

advanced

Output of static data member shared across objects

What is the output of this program?

C++

class Tracker {
public:
    static int count;
    Tracker() { count++; }
};

int Tracker::count = 0;

int main() {
    Tracker t1;
    Tracker t2;
    Tracker t3;
    std::cout << Tracker::count << std::endl;
    return 0;
}

A 0

B 3

C 1

D Compilation error due to static member

💡 Hint

Static members are shared by all objects of the class.

Each object increments the static count. After three objects, count is 3.

🧠 Conceptual

expert

Effect of const member function on data members

Which statement is true about const member functions in C++?

A They cannot access any data members of the class.

B They can modify any data member of the class.

C They can modify data members marked as mutable.

D They can only be called on non-const objects.

💡 Hint

Const member functions promise not to change the object except for mutable members.

Const member functions cannot modify normal data members but can modify those declared mutable.

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