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.