Default constructor in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of default constructor initialization

What is the output of this C++ program?

C++

#include <iostream>
class Box {
public:
    int length;
    Box() { length = 5; }
};
int main() {
    Box b;
    std::cout << b.length << std::endl;
    return 0;
}

A 0

B 5

C Compilation error

D Garbage value

💡 Hint

The default constructor sets length to a fixed value.

The default constructor sets length to 5, so printing b.length outputs 5.

❓ Predict Output

intermediate

Default constructor with member initializer list

What will this program print?

C++

#include <iostream>
class Point {
    int x, y;
public:
    Point() : x(0), y(0) {}
    void print() { std::cout << x << "," << y << std::endl; }
};
int main() {
    Point p;
    p.print();
    return 0;
}

A 0,1

B Garbage,Garbage

C Compilation error

D 0,0

💡 Hint

The constructor uses an initializer list to set values.

The default constructor sets x and y to 0 using the initializer list, so output is '0,0'.

🔧 Debug

advanced

Why does this default constructor cause a compilation error?

Identify the reason this code does not compile.

C++

#include <iostream>
class Sample {
    int a;
public:
    Sample() a = 10; {}
};
int main() {
    Sample s;
    return 0;
}

A Variable 'a' is not declared

B Missing semicolon after class definition

C Syntax error: assignment outside constructor body

D Constructor missing return type

💡 Hint

Look at how the constructor tries to assign a value.

The assignment 'a = 10;' is incorrectly placed outside the constructor body, causing a syntax error.

❓ Predict Output

advanced

Output when default constructor is deleted

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

C++

#include <iostream>
class Test {
public:
    Test() = delete;
};
int main() {
    Test t;
    return 0;
}

A Compilation error: use of deleted function 'Test::Test()'

B Program prints 0

C Runtime error

D Program runs but does nothing

💡 Hint

Deleting the default constructor disables object creation without arguments.

The default constructor is deleted, so creating 'Test t;' causes a compilation error.

🧠 Conceptual

expert

Effect of default constructor on class member initialization

Consider this class with no user-defined constructor: class Data { int x; int y; }; What is the value of x and y after creating an object Data d;?

A Both x and y contain garbage values

B Both x and y are initialized to zero

C Compilation error due to missing constructor

D Both x and y are initialized to -1

💡 Hint

Without a constructor, members are not initialized automatically.

Without a user-defined constructor, members are uninitialized and contain garbage values.