Parameterized constructor in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a parameterized constructor in C++?

A parameterized constructor is a special constructor that takes arguments to initialize an object with specific values when it is created.

beginner

How does a parameterized constructor differ from a default constructor?

A default constructor takes no arguments and initializes objects with default values, while a parameterized constructor requires arguments to set initial values.

beginner

Consider this code snippet:<br><pre>class Car {<br>  public:<br>    std::string brand;<br>    Car(std::string b) { brand = b; }<br>};<br>Car myCar("Toyota");<br>What does this code do?

It creates an object myCar of class Car and initializes its brand to "Toyota" using the parameterized constructor.

intermediate

Can a class have multiple parameterized constructors?

Yes, a class can have multiple parameterized constructors with different parameters. This is called constructor overloading.

beginner

Why use a parameterized constructor instead of setting values after creating an object?

Using a parameterized constructor ensures the object is fully initialized right when it is created, reducing errors and making code cleaner.

What is the main purpose of a parameterized constructor?

A To initialize an object with specific values when created

B To delete an object

C To copy an object

D To create a default object without values

A parameterized constructor initializes an object with values passed as arguments.

Which of the following is a valid parameterized constructor declaration in C++?

A void Car(std::string brand);

B Car() { brand = ""; }

C Car(std::string brand) { this->brand = brand; }

D Car brand(std::string);

Option C correctly defines a constructor with a parameter to initialize the brand.

What happens if you do not define any constructor in a class?

A The compiler provides a default constructor

B The program will not compile

C Objects cannot be created

D Only parameterized constructors are created

If no constructor is defined, the compiler automatically provides a default constructor.

Can a parameterized constructor be called without arguments?

A Only if the class has no other constructors

B Yes, always

C Only if default values are provided

D No, it requires arguments

A parameterized constructor requires arguments unless default values are specified.

What is constructor overloading?

A Using constructors in inheritance

B Having multiple constructors with different parameters

C Calling a constructor multiple times

D Deleting constructors

Constructor overloading means defining multiple constructors with different parameter lists.

Explain what a parameterized constructor is and why it is useful.

Think about how you give an object its initial details when you create it.

Describe how constructor overloading works with parameterized constructors.

Imagine having different ways to create an object depending on what information you have.

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