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

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of parameterized constructor initialization

What is the output of this C++ program using a parameterized constructor?

C++

#include <iostream>
using namespace std;

class Box {
  int length;
public:
  Box(int l) { length = l; }
  void show() { cout << length << endl; }
};

int main() {
  Box b(10);
  b.show();
  return 0;
}

A 0

B Compilation error

C Garbage value

D 10

💡 Hint

The constructor sets the length to the passed value.

The parameterized constructor sets length to 10, so show() prints 10.

❓ Predict Output

intermediate

Value of member after parameterized constructor call

What is the value of member 'width' after this code runs?

C++

#include <iostream>
using namespace std;

class Rectangle {
  int width;
public:
  Rectangle(int w) { width = w; }
  int getWidth() { return width; }
};

int main() {
  Rectangle r(15);
  cout << r.getWidth() << endl;
  return 0;
}

A Compilation error

B 0

C 15

D Undefined behavior

💡 Hint

The constructor sets width to the passed value.

The parameterized constructor assigns 15 to width, so getWidth() returns 15.

🔧 Debug

advanced

Identify the error in parameterized constructor usage

What error does this code produce when compiled?

C++

#include <iostream>
using namespace std;

class Circle {
  int radius;
public:
  Circle() { radius = 5; }
  Circle(int r) { radius = r; }
  void show() { cout << radius << endl; }
};

int main() {
  Circle c();
  c.show();
  return 0;
}

A Compilation error: 'c' is not an object

B Output: 5

C Output: 0

D Runtime error: segmentation fault

💡 Hint

Look at how 'c' is declared in main.

The line 'Circle c();' declares a function, not an object. So 'c.show()' causes a compilation error.

❓ Predict Output

advanced

Output of constructor with member initializer list

What is the output of this program?

C++

#include <iostream>
using namespace std;

class Point {
  int x, y;
public:
  Point(int a, int b) : x(a), y(b) {}
  void display() { cout << x << "," << y << endl; }
};

int main() {
  Point p(3, 4);
  p.display();
  return 0;
}

A 3,4

B 0,0

C Compilation error

D 4,3

💡 Hint

The member initializer list sets x and y to the passed values.

The constructor initializes x=3 and y=4, so display prints '3,4'.

🧠 Conceptual

expert

Behavior of parameterized constructor with default arguments

Consider this class with a parameterized constructor having default arguments. What will be the output of the program?

C++

#include <iostream>
using namespace std;

class Sample {
  int a, b;
public:
  Sample(int x = 1, int y = 2) { a = x; b = y; }
  void print() { cout << a << "," << b << endl; }
};

int main() {
  Sample s1;
  Sample s2(5);
  Sample s3(7, 8);
  s1.print();
  s2.print();
  s3.print();
  return 0;
}

A

0,0
5,2
7,8

B

1,2
5,2
7,8

C Compilation error due to ambiguous constructor calls

D

1,2
5,0
7,8

💡 Hint

Default arguments fill in missing parameters from right to left.

s1 uses defaults (1,2), s2 uses 5 and default 2, s3 uses 7 and 8.

Practice

1.

What is the main purpose of a parameterized constructor in C++?

easy

A. To initialize an object with specific values when it is created

B. To delete an object from memory

C. To copy one object to another

D. To create a function outside the class

Solution

1. Step 1: Understand constructor role

   A constructor sets up an object when it is created.

2. Step 2: Identify parameterized constructor use

   Parameterized constructors allow passing values to set object properties immediately.

3. Final Answer:

   To initialize an object with specific values when it is created -> Option A

4. Quick Check:

   Parameterized constructor = initialize with values [OK]

Hint: Think: constructor sets initial values when making an object [OK]

Common Mistakes:

• Confusing constructor with destructor
• Thinking constructors copy objects
• Believing constructors are separate functions

2.

Which of the following is the correct syntax for a parameterized constructor in C++?

class Box {
public:
    Box(int w, int h) { width = w; height = h; }
private:
    int width, height;
};

easy

A. Box(int w, int h) { width = w; height = h; }

B. void Box(int w, int h) { width = w; height = h; }

C. Box() { width = w; height = h; }

D. Box(int w, int h) -> void { width = w; height = h; }

Solution

1. Step 1: Check constructor name and return type

   Constructor must have the same name as the class and no return type.

2. Step 2: Verify parameter list and body

   Box(int w, int h) { width = w; height = h; } matches correct syntax: name matches class, parameters present, no return type.

3. Final Answer:

   Box(int w, int h) { width = w; height = h; } -> Option A

4. Quick Check:

   Constructor = class name + no return type [OK]

Hint: Constructor has no return type and matches class name [OK]

Common Mistakes:

• Adding void return type to constructor
• Using different name than class
• Missing parameters in parameterized constructor

3.

What will be the output of this code?

#include <iostream>
class Point {
public:
    int x, y;
    Point(int a, int b) { x = a; y = b; }
};

int main() {
    Point p(3, 4);
    std::cout << p.x << ", " << p.y << std::endl;
    return 0;
}

medium

A. Compilation error

B. 3, 4

C. 0, 0

D. Random values

Solution

1. Step 1: Understand constructor call

   The object p is created with values 3 and 4 passed to the constructor.

2. Step 2: Check member variable assignment and output

   The constructor sets x=3 and y=4, so printing p.x and p.y outputs "3, 4".

3. Final Answer:

   3, 4 -> Option B

4. Quick Check:

   Constructor sets values correctly = 3, 4 [OK]

Hint: Constructor sets values; output matches passed parameters [OK]

Common Mistakes:

• Assuming default values instead of passed ones
• Expecting compilation error due to missing default constructor
• Confusing output format

4.

Identify the error in this code snippet:

#include <iostream>
class Car {
public:
    int speed;
    Car(int s) { speed = s; }
};

int main() {
    Car c;
    std::cout << c.speed << std::endl;
    return 0;
}

medium

A. Variable speed is not declared

B. Incorrect syntax in constructor definition

C. Missing default constructor causes error when creating Car c;

D. Output will be garbage value

Solution

1. Step 1: Check object creation

   The line Car c; tries to create an object without parameters.

2. Step 2: Verify constructors available

   Only a parameterized constructor Car(int s) exists; no default constructor is defined.

3. Step 3: Understand compilation error

   Because no default constructor exists, Car c; causes a compilation error.

4. Final Answer:

   Missing default constructor causes error when creating Car c; -> Option C

5. Quick Check:

   No default constructor + no args object = error [OK]

Hint: If no default constructor, you must pass arguments when creating object [OK]

Common Mistakes:

• Assuming default constructor exists automatically
• Thinking variable declaration is missing
• Expecting garbage output instead of compile error

5.

Given this class, how can you create an object r with width 5 and height 10 using the parameterized constructor?

class Rectangle {
public:
    int width, height;
    Rectangle(int w, int h) { width = w; height = h; }
};

hard

A. Rectangle r = Rectangle{5};

B. Rectangle r();

C. Rectangle r = Rectangle();

D. Rectangle r = Rectangle(5, 10);

Solution

1. Step 1: Identify constructor parameters

   The constructor requires two integers: width and height.

2. Step 2: Match object creation syntax

   Rectangle r = Rectangle(5, 10); correctly calls the constructor with two arguments (5, 10) to create object r.

3. Step 3: Check other options

   Rectangle r(); is a function declaration, not object creation. Rectangle r = Rectangle(); calls default constructor which doesn't exist. Rectangle r = Rectangle{5}; passes only one argument, which is invalid.

4. Final Answer:

   Rectangle r = Rectangle(5, 10); -> Option D

5. Quick Check:

   Pass all required parameters to constructor [OK]

Hint: Call constructor with all parameters to create object [OK]

Common Mistakes:

• Using empty parentheses which declare a function
• Calling constructor without required arguments
• Passing wrong number of arguments