Constructor and Destructor of instances in c++ Polymorphism

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I have an example of me trying to create a derived class "Dog" from base class "Animal" but I want to create instances of Animal class and use its members (I'm fairly new to polymorphism in c++). The result produces 2 lines of "An animal is created!" and 1 line of "A dog is created!" while I'm expecting 1 line of each.

class Animal
{

private:

    int numberOfLegs;
    const char* name;
public:

    int getNumberOfLegs();
    const char* getName();
    virtual void ToString()
    {
        std::cout << "This animal has " << this->getNumberOfLegs();
        std::cout << "leg and its name is " << getName() << std::endl;
    }
    Animal(int, const char*);
    ~Animal();
};

int Animal::getNumberOfLegs()
{

    return this->numberOfLegs;
}

const char* Animal::getName()
{

    return this->name;
}

Animal::Animal(int legs, const char* name)
{

    this->numberOfLegs = legs;
    this->name = name;
    std::cout << "An animal is created!" << std::endl;
}

Animal::~Animal()
{

    std::cout << "An animal is destroyed!" << std::endl;
}

class Dog : public Animal
{
private:

    const char* speech;
public:
    void ToString()
    {

        std::cout << "This dog has " << this->getNumberOfLegs();
        std::cout << " legs and its name is " << this->getName();
        std::cout << " and it says " << this->speech << std::endl;
    }

    Dog(int, const char*, const char*);

    ~Dog();
};

Dog::Dog(int legs, const char* name, const char* speech) : Animal (legs, name)
{

    this->speech = speech;
    std::cout << "A dog is created!" << std::endl;
}

Dog::~Dog()
{

    std::cout << "A dog is destroyed!" << std::endl;
}

int main()
{

        Animal anAnimal(3, "Godzilla");
        Dog myDoge(4, "CuteDoge", "woof");
        char c;
        std::cin.get(c);
    }
}

In your hierarchy, Dog inherits from Animal. This means that when a Dog is constructed Animal, its parent, is constructed first. That's why when you create a Dog you get both 'Animal is created' and 'Dog is created'.

To avoid other issues, you must follow the rule of 3: you have a non-trivial destructor, so you should also define copy constructor and assignment operator. Otherwise you will have a double-free and/or use freed memory.

Also, since you may have non-trivial children to Animal, you must make the destructor virtual. Otherwise,

std::unique_ptr<Animal> ptr = std::make_unique<Dog>();

without a virtual destructor in Animal, the Dog destructor will never be called.

Difference Between Constructor and Destructor in C++ , Difference Between Constructor and Destructor in C++. 01-10-2019. Constructor: A constructor is a Example/Implementation of Constructor and Destructor:. The destructor implements the statements to be executed during the garbage collection process. A destructor is a function with the same name as the name of the class but starting with the character ~. Example: class Complex {public Complex() {// constructor} ~Complex() {// Destructor}} Remember that a destructor can't have any modifiers like

As Dog is an Animal, and in your constructor you might see:

Dog::Dog(int legs, const char* name, const char* speech) : Animal(legs, name)
                                                           ^^^^^^^^^^^^^^^^^^

calls base class constructor.

So

Dog myDoge(4, "CuteDoge", "woof");

would call both:

std::cout << "An animal is created!" << std::endl;
// ...
std::cout << "A dog is created!" << std::endl;

C++ Class Constructor and Destructor, C++ Class Constructor and Destructor - A class constructor is a special member function Following example explains the concept of constructor − If for a class C, you have multiple fields X, Y, Z, etc., to be initialized, then use can use same� Constructor Destructor; 1. Constructor helps to initialize the object of a class. Whereas destructor is used to destroy the instances. 2. It is declared as Classname( arguments if any ){Constructor’s Body }. Whereas it is declared as ~ ClassName( no arguments ){ };. 3. Constructor can either accept the arguments or not. While it can’t have

This line:

Dog::Dog(int legs, const char* name, const char* speech) : Animal (legs, name)

Is the first line of the Dog constructor - objects must also construct all its Base classes (base-class) as part of construction - and here you are calling the Animal constructor with the arguments of legs and name

Constructors and Destructors in C++, Overview of constructors and destructors, including constructor/destructor order in C++ and virtual destructors. Constructors and Destructors in C++. By Andrei Every time an instance of a class is created the constructor method is called. Copy Constructor: A copy constructor is a member function which initializes an object using another object of the same class. Detailed article on Copy Constructor . Whenever we define one or more non-default constructors( with parameters ) for a class, a default constructor( without parameters ) should also be explicitly defined as the compiler

Constructor and Destructor of instances in c++ Polymorphism, In your hierarchy, Dog inherits from Animal. This means that when a Dog is constructed Animal, its parent, is constructed first. That's why when� Constructor and destructor are the member functions with the same name as their class. The former type constructor helps in initializing an object. Conversely, a destructor is different from the constructor which deletes the created constructor when it is of no use.

Constructor and Destructor in C++, Creating Constructor and Destructor in C++? (Examples). 1. Constructor Example . Let's take the same example of class Box to declare the constructor. Let's assign � Destructor is a special class function which destroys the object as soon as the scope of object ends. The destructor is called automatically by the compiler when the object goes out of scope. The syntax for destructor is same as that for the constructor, the class name is used for the name of destructor, with a tilde ~ sign as prefix to it.

How to Use C++ Class Constructors and Destructors with Examples, Constructors and destructors are fundamental to the concept of classes in C++. Both constructor and destructor are more or less like normal� Using Constructors (C# Programming Guide) 07/20/2015; 4 minutes to read +12; In this article. When a class or struct is created, its constructor is called. Constructors have the same name as the class or struct, and they usually initialize the data members of the new object.

Comments
  • The rule of 3: you have a non-trivial destructor, so you need copy constructor and assignment operator. Also, the destructor in such a case must be virtual