Default constructor in C++

Introduction

A default constructor helps create an object with default values automatically.

When you want to create an object without giving initial values.

When you want to set default values for object properties.

When you want to create multiple objects quickly with the same starting state.

Syntax

C++

class ClassName {
public:
    ClassName() {
        // code to set default values
    }
};

The default constructor has no parameters.

If you don't write one, C++ creates a default constructor automatically if no other constructors exist.

Examples

This default constructor sets speed to 0 and color to red when a Car object is created.

C++

class Car {
public:
    Car() {
        speed = 0;
        color = "red";
    }
private:
    int speed;
    std::string color;
};

This uses the compiler-generated default constructor without custom code.

C++

class Box {
public:
    Box() = default;
};

Sample Program

This program creates a Person object using the default constructor. It sets name to "Unknown" and age to 0, then prints these values.

C++

#include <iostream>
#include <string>

class Person {
public:
    Person() {
        name = "Unknown";
        age = 0;
    }
    void display() {
        std::cout << "Name: " << name << ", Age: " << age << std::endl;
    }
private:
    std::string name;
    int age;
};

int main() {
    Person p;
    p.display();
    return 0;
}

Important Notes

If you define any constructor with parameters, C++ will not create a default constructor automatically.

You can use = default; to ask the compiler to create a default constructor for you.

Summary

A default constructor creates objects with default values.

It has no parameters and can be written by you or generated by the compiler.

Use it to make object creation simple and consistent.

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