Ctors & Dtors Basics

Part of the Object Oriented Programming section of Coddy's C++ journey — lesson 10 of 104.

A constructor is a special method that runs automatically when an object is created. A destructor runs when the object is destroyed.

Default constructor

class Book {
private:
    std::string title;
    int pages;

public:
    Book() {
        this->title = "Unknown";
        this->pages = 0;
    }
};

Parameterized constructor

Book(std::string title, int pages) {
    this->title = title;
    this->pages = pages;
}

Constructor overloading — multiple constructors

class Book {
private:
    std::string title;
    int pages;

public:
    Book() {
        this->title = "Unknown";
        this->pages = 0;
    }
    
    Book(std::string title) {
        this->title = title;
        this->pages = 0;
    }
    
    Book(std::string title, int pages) {
        this->title = title;
        this->pages = pages;
    }
};

Destructor — runs when object is destroyed

class Book {
public:
    ~Book() {
        std::cout << "Book destroyed" << std::endl;
    }
};

Creating objects with different constructors

Book book1;                        // Default constructor
Book book2("Harry Potter");         // One parameter
Book book3("1984", 328);            // Two parameters

Constructors have the same name as the class with no return type. Destructors use ~ClassName() and are called automatically in reverse order of creation when objects go out of scope.

Challenge

Medium

Create a Product class with three overloaded constructors and a destructor:

3-parameter constructor: name, price, stock
2-parameter constructor: name, price (stock = 0)
Default constructor: name = "Unknown", price = 0, stock = 0
Destructor: prints "Destroying: <name>"

Cheat sheet

A constructor is a special method that runs automatically when an object is created. A destructor runs when the object is destroyed.

Default constructor — initializes with default values:

class Book {
private:
    std::string title;
    int pages;

public:
    Book() {
        this->title = "Unknown";
        this->pages = 0;
    }
};

Parameterized constructor — accepts parameters:

Book(std::string title, int pages) {
    this->title = title;
    this->pages = pages;
}

Constructor overloading — multiple constructors with different parameters:

class Book {
private:
    std::string title;
    int pages;

public:
    Book() {
        this->title = "Unknown";
        this->pages = 0;
    }
    
    Book(std::string title) {
        this->title = title;
        this->pages = 0;
    }
    
    Book(std::string title, int pages) {
        this->title = title;
        this->pages = pages;
    }
};

Destructor — runs when object is destroyed, uses ~ClassName():

class Book {
public:
    ~Book() {
        std::cout << "Book destroyed" << std::endl;
    }
};

Creating objects with different constructors:

Book book1;                        // Default constructor
Book book2("Harry Potter");         // One parameter
Book book3("1984", 328);            // Two parameters

Constructors have the same name as the class with no return type. Destructors are called automatically in reverse order of creation when objects go out of scope.

Try it yourself

#include <iostream>
#include "Product.h"

int main() {
    std::string name;
    int price, stock;
    std::getline(std::cin, name);
    std::cin >> price >> stock;
    
    Product product1(name, price, stock);
    Product product2("Keyboard", 79);
    Product product3;
    
    std::cout << "Product 1: " << product1.getName() << " - $" << product1.getPrice() << " (Stock: " << product1.getStock() << ")" << std::endl;
    std::cout << "Product 2: " << product2.getName() << " - $" << product2.getPrice() << " (Stock: " << product2.getStock() << ")" << std::endl;
    std::cout << "Product 3: " << product3.getName() << " - $" << product3.getPrice() << " (Stock: " << product3.getStock() << ")" << std::endl;
    return 0;
}