Default constructor in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a default constructor for class Car.

C++

class Car {
public:
    [1]() {}
};

Drag options to blanks, or click blank then click option'

Avoid

B~Car

CCar

Dint

Attempts:

3 left

💡 Hint

Common Mistakes

Using a return type like void or int for the constructor.

Using the destructor name (~Car) instead of the constructor.

The default constructor has the same name as the class and no return type.

2fill in blank

medium

Complete the code to create an object car1 using the default constructor.

C++

Car [1];

Drag options to blanks, or click blank then click option'

Acar1()

Bcar1

CCar

DCar()

Attempts:

3 left

💡 Hint

Common Mistakes

Adding parentheses after the object name like car1().

Trying to call the class name instead of the object name.

To create an object with the default constructor, just declare the object name.

3fill in blank

hard

Fix the error in the default constructor declaration.

C++

class Bike {
public:
    [1]() {}
};

Drag options to blanks, or click blank then click option'

Avoid

B~Bike

Cint

DBike

Attempts:

3 left

💡 Hint

Common Mistakes

Adding a return type to the constructor.

Using the destructor name instead of the constructor.

Constructors should not have a return type like void.

4fill in blank

hard

Fill both blanks to define a default constructor that sets wheels to 4.

C++

class Vehicle {
public:
    int wheels;
    [1]() {
        wheels [2] 4;
    }
};

Drag options to blanks, or click blank then click option'

AVehicle

Bwheels

C=

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '=' for assignment.

Using a different name than the class for the constructor.

The constructor name matches the class name, and the assignment operator '=' sets wheels to 4.

5fill in blank

hard

Fill all three blanks to create a default constructor that initializes brand and sets speed to 0.

C++

class Bike {
public:
    std::string [1];
    int [2];
    [3]() {
        brand = "Yamaha";
        speed = 0;
    }
};

Drag options to blanks, or click blank then click option'

Abrand

Bspeed

CBike

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'void' as a return type for the constructor.

Mixing up variable names or constructor name.

Member variables brand and speed are declared, and the constructor named Bike initializes them.

Practice

1. What is a default constructor in C++?

easy

A. A constructor that requires at least one argument.

B. A constructor with no parameters that initializes objects with default values.

C. A function that deletes objects automatically.

D. A special function that runs only when the program ends.

Solution

1. Step 1: Understand constructor basics

   A constructor is a special function to create objects.

2. Step 2: Identify default constructor meaning

   A default constructor has no parameters and sets default values.

3. Final Answer:

   A constructor with no parameters that initializes objects with default values. -> Option B

4. Quick Check:

   Default constructor = no parameters, default values [OK]

Hint: Default constructor has zero parameters [OK]

Common Mistakes:

• Thinking default constructor requires arguments
• Confusing destructor with constructor
• Assuming default constructor runs at program end

2. Which of the following is the correct syntax for a default constructor in C++?

easy

A. class MyClass { MyClass() {} };

B. class MyClass { MyClass(int x) {} };

C. class MyClass { void MyClass() {} };

D. class MyClass { MyClass() int {} };

Solution

1. Step 1: Check constructor syntax

   Constructor has the same name as the class and no return type.

2. Step 2: Identify default constructor syntax

   Default constructor has empty parentheses and no parameters.

3. Final Answer:

   class MyClass { MyClass() {} }; -> Option A

4. Quick Check:

   Constructor syntax = class name + () + no return type [OK]

Hint: Constructor name = class name, no return type, empty () [OK]

Common Mistakes:

• Adding void return type to constructor
• Using parameters in default constructor
• Incorrect placement of braces

3. What will be the output of this code?

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

medium

A. Compilation error

B. 0

C. 5

D. Random value

Solution

1. Step 1: Understand the default constructor

   The constructor sets length to 5 when object b is created.

2. Step 2: Check output statement

   Printing b.length outputs the value set by constructor, which is 5.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   Constructor sets length=5, output = 5 [OK]

Hint: Default constructor sets values before output [OK]

Common Mistakes:

• Assuming uninitialized int defaults to 0
• Expecting compilation error due to missing initialization
• Thinking output is random without constructor

4. Identify the error in this class definition:

class Sample {
public:
  int x;
  Sample() {
    x = 10
  }
};

medium

A. Class missing closing brace

B. Constructor name does not match class name

C. Missing return type in constructor

D. Missing semicolon after x = 10

Solution

1. Step 1: Check constructor body syntax

   Each statement inside constructor must end with a semicolon.

2. Step 2: Locate missing semicolon

   The line 'x = 10' lacks a semicolon at the end.

3. Final Answer:

   Missing semicolon after x = 10 -> Option D

4. Quick Check:

   Statements need semicolons [OK]

Hint: Check for missing semicolons inside constructor [OK]

Common Mistakes:

• Adding return type to constructor
• Ignoring missing semicolon errors
• Confusing constructor name with return type

5. Given this class:

class Point {
public:
  int x, y;
  Point() { x = 0; y = 0; }
  Point(int a, int b) { x = a; y = b; }
};

// In main:
Point p1;
Point p2(3, 4);
std::cout << p1.x << "," << p1.y << " " << p2.x << "," << p2.y;

What is the output and why?

hard

A. 0,0 3,4 because p1 uses default constructor, p2 uses parameterized constructor.

B. 3,4 0,0 because constructors are swapped.

C. Compilation error due to multiple constructors.

D. 0,0 0,0 because default constructor overrides parameterized one.

Solution

1. Step 1: Understand constructor calls

   p1 is created with no arguments, so default constructor sets x=0, y=0.

2. Step 2: Understand parameterized constructor

   p2 is created with (3,4), so x=3, y=4 from parameterized constructor.

3. Step 3: Check output statement

   Output prints p1's coordinates then p2's coordinates separated by space.

4. Final Answer:

   0,0 3,4 because p1 uses default constructor, p2 uses parameterized constructor. -> Option A

5. Quick Check:

   Default and parameterized constructors work together [OK]

Hint: Default constructor for no args, parameterized for args [OK]

Common Mistakes:

• Assuming default constructor overrides parameterized
• Expecting compilation error with multiple constructors
• Mixing up which constructor is called