Class definition syntax in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of simple class member access

What is the output of this C++ code?

C++

class Box {
public:
    int length;
    Box(int l) { length = l; }
};

int main() {
    Box b(5);
    std::cout << b.length << std::endl;
    return 0;
}

A 5

B 0

C Compilation error

D Undefined behavior

💡 Hint

Look at how the constructor sets the length and how it's accessed.

The constructor sets length to 5, and b.length prints 5.

📝 Syntax

intermediate

Identify the syntax error in class definition

Which option contains a syntax error in the class definition?

C++

class Person {
public
    std::string name;
    int age;
};

A Missing semicolon after class name

B Missing semicolon after public keyword

C Missing return type for constructor

D Missing curly braces around class body

💡 Hint

Check the line with the access specifier.

The 'public' keyword must be followed by a colon ':' not just a newline.

🔧 Debug

advanced

Why does this class cause a compilation error?

Given the code below, why does it cause a compilation error?

C++

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

int main() {
    Counter c;
    c.count = 5;
    c.increment();
    std::cout << c.count << std::endl;
    return 0;
}

A increment() is not declared as static

B Missing semicolon after class definition

C Constructor does not initialize count

D count is private and cannot be accessed directly outside the class

💡 Hint

Check the access level of count and where it is accessed.

The member 'count' is private and cannot be accessed directly in main.

🧠 Conceptual

advanced

Class member initialization order

What will be the output of this code?

C++

class Test {
public:
    int x;
    int y;
    Test() : y(10), x(y + 5) {}
};

int main() {
    Test t;
    std::cout << t.x << ", " << t.y << std::endl;
    return 0;
}

A x=garbage, y=10

B x=5, y=10

C x=garbage value, y=10 (because x initialized before y)

D Uninitialized x, y=10

💡 Hint

Members are initialized in the order they are declared, not the order in initializer list.

x is initialized first with y's value before y is set to 10, so x gets garbage.

❓ Predict Output

expert

Output of class with static member

What is the output of this program?

C++

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

int Sample::count = 0;

int main() {
    Sample a;
    Sample b;
    Sample c;
    std::cout << Sample::count << std::endl;
    return 0;
}

A 3

B 0

C Compilation error due to static member

D 1

💡 Hint

Static members are shared across all instances.

Each constructor call increments count, so after 3 objects, count is 3.

Practice

1. What is the correct way to start defining a class named Car in C++?

easy

A. class Car { };

B. define Car { };

C. Car class { };

D. class = Car { };

Solution

1. Step 1: Recognize the class keyword usage

   In C++, a class is defined using the keyword class followed by the class name and curly braces.

2. Step 2: Check the syntax correctness

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

3. Final Answer:

   class Car { }; -> Option A

4. Quick Check:

   class keyword + name + braces + semicolon = correct [OK]

Hint: Use 'class ClassName { };' to define a class [OK]

Common Mistakes:

• Omitting the semicolon after the class definition
• Using 'define' instead of 'class'
• Placing '=' between class and name

2. Which of the following is the correct syntax to declare a public member function start() inside a class?

easy

A. public void start();

B. void public start();

C. void start() public;

D. public: void start();

Solution

1. Step 1: Understand access specifier placement

   In C++, access specifiers like public: are labels followed by a colon, not keywords before functions.

2. Step 2: Identify correct function declaration

   The function declaration inside the class is void start(); placed under the public: label.

3. Final Answer:

   public: void start(); -> Option D

4. Quick Check:

   Access specifier label + function declaration = correct [OK]

Hint: Use 'public:' label before members, not before return type [OK]

Common Mistakes:

• Writing 'public' before return type without colon
• Placing 'public' after function parentheses
• Using 'void public' which is invalid

3. What will be the output of this code?

class Box {
public:
  int length;
};

int main() {
  Box b;
  b.length = 10;
  std::cout << b.length << std::endl;
  return 0;
}

medium

A. Compilation error

B. 10

C. 0

D. Undefined behavior

Solution

1. Step 1: Analyze class and member access

   The class Box has a public integer member length, so it can be accessed and assigned outside the class.

2. Step 2: Trace the main function execution

   An object b is created, b.length is set to 10, then printed. This will output 10.

3. Final Answer:

   10 -> Option B

4. Quick Check:

   Public member assigned 10, printed 10 [OK]

Hint: Public members can be accessed and printed directly [OK]

Common Mistakes:

• Assuming private by default causes error
• Forgetting to assign value before printing
• Expecting zero because of uninitialized variable

4. Identify the error in this class definition:

class Person {
  string name;
public:
  void setName(string n) {
    name = n;
  }
};

medium

A. Missing semicolon after class definition

B. Member variable 'name' should be public

C. Missing #include <string> and std:: prefix

D. Function setName should return string

Solution

1. Step 1: Check for syntax errors

   The class ends with a semicolon, so no missing semicolon error.

2. Step 2: Check usage of string type

   The code uses string without including <string> header and without std:: prefix, which causes compilation error.

3. Final Answer:

   Missing #include <string> and std:: prefix -> Option C

4. Quick Check:

   Use #include and std::string for string type [OK]

Hint: Always include <string> and use std::string in C++ [OK]

Common Mistakes:

• Assuming string is built-in without include
• Thinking private members must be public to work
• Expecting setName to return a value

5. You want to define a class Rectangle with private members width and height, and a public function area() that returns the area. Which is the correct class definition?

hard

A. class Rectangle {
private:
int width, height;
public:
int area() { return width * height; }
};

B. class Rectangle {
public:
int width, height;
int area() { return width * height; }
};

C. class Rectangle {
int width, height;
int area() { return width * height; }
};

D. class Rectangle {
private:
int width, height;
int area() { return width * height; }
};

Solution

1. Step 1: Identify private members declaration

   Private members must be declared under private: label to restrict access.

2. Step 2: Confirm public function declaration

   The function area() should be public to allow access from outside.

3. Step 3: Check for correct syntax and placement

   class Rectangle { private: int width, height; public: int area() { return width * height; } }; correctly places width and height as private and area() as public with proper syntax.

4. Final Answer:

   class Rectangle { private: int width, height; public: int area() { return width * height; } }; -> Option A

5. Quick Check:

   Private data + public method = correct class design [OK]

Hint: Use 'private:' for data, 'public:' for functions in class [OK]

Common Mistakes:

• Forgetting access specifiers, making members public by default
• Placing function inside private when it should be public
• Omitting semicolon after class definition