Constructor is automatically called when object is created. Our objective is to provide some insights into virtual construction, creating objects dynamically based on some input. A constructor that initializes an object using values of another object passed to it as parameter, is called copy constructor. The following is an excerpt from scott meyers new book, effective c. You may be wondering why a pure virtual function requires a function body. And, supplying a default constructor in a derived class makes it easier to use if classes are subsequently derived from it. Since you defined a constructor yourself, the default one is no longer available. A constructor for a local object can be identified if a function is called with a this. There is always at least one constructor in every class. Remove the virtual keyword from the function declaration. In this case when a pointer of the base class is defined in a main function and derived class objects address is passed on to the base class pointer, then calling the overridden function will invoke the derived class member. The compiler must be aware of the class type to create the object. If the users consumer the main in our case needs derived2 functionality, user.
Supplying a default constructor in our base classes allows for the most straightforward class design. The constructor cant be virtual as the constructor is a code which is responsible for creating an instance of a class and it cant be delegated to any other object by virtual keyword means. A copy constructor is a member function which initializes an object using another object of the same class. If you dont tell c s constructor which constructor to call, itll try calling one that takes no parameters, which now no longer exists.
The main function is consuming base hierarchy functionality via user class the user class constructor is creating derived1 object, always. Lets start by going through an example that does not use virtual destructors, and then we will go through an example that does use virtual destructors. In the above sample, assume that the hierarchy base, derived1 and derived2 are part of library code. Constructor function is a special function that is a member of the class and has same name as that of the class.
Virtual destructors programmer and software interview. A special type of syntax is used for constructor chaining as follows. Note that a pointer to the virtual functions table is added at the beginning of the layout. A class or struct may have multiple constructors that take different arguments. Yes, it is possible to have pure virtual destructor. For example, as already noted earlier, if you do not declare a copy constructor, the compiler will always generate one for you. The virtual constructor idiom is a technique for delegating the act of copying the object to the derived class through the use of virtual functions. To demonstrate, we declare a virtual member function of base on line 8 typically named clone that each of the derived classes will implement to return a copy of themselves. A constructor will have exact same name as the class and it does not have any return type at all, not even void. A constructors job is to transform raw memory into a welldefined object. Constructors are never virtual, only destructors can be virtual. The constructor has the same name as the class and it doesnt return any type, while the destructors name.
But the compiler cannot possibly know what derived class to. It is a good practice to declare the destructor after the end of using constructor. Virtual tables are used to map the virtual methods at runtime. The only difference between virtual and pure virtual destructor is, that pure virtual destructor will make its base class abstract, hence you cannot create object of that class. Example to see how constructor and destructor are called. The purpose of virtual functions is to allow a caller to invoke a function from a base class pointer. The base object is shared between all objects in the inheritance tree and it is only constructed once. A class constructor is a special member function of a class that is executed whenever we create new objects of that class a constructor will have exact same name as the class and it does not have any return type at all, not even void. Whenever a class or struct is created, its constructor is called. When a virtual function is called directly or indirectly from a constructor or from a destructor, including during the construction or destruction of the classs nonstatic data members, and the object to which the call applies is the object call it x under construction or destruction, the function called is the final overrider in the. It is used to call a function based on the type of object pointed by the pointer variable instead of the type of the pointer variable. The pattern world demonstrates different ways to implement the above concept. The virtual constructor helps you create your plans, estimate a construction project, and calculate a list of materials for a deck, shed, garage, cottage, expansion and house.
On the other hand a virtual destructor is allowed and should be used when ever there are virtual methods in the base class. Then the method cant be overridden in a morederived class, so it is clear which method were calling. Deleting a derived class object using a pointer to a base class that has a nonvirtual destructor results in undefined behavior. An abstract class contains at least one pure virtual function. Function overriding virtual functions abstract class and pure virtual functions virtual destructors. Motorbikemotorbike will be invoked because youre calling new motorbike. All the derived class destructors are made virtual in spite of having the same name as the base class destructor. Since constructors are intended to instantiate and create the object there can be no virtual constructor.
Mar 23, 2014 even when the code is executing the base class constructor and has not yet arrived at the derived class constructor, if the base class uses the object reference this, the type of that object is that of the derived class, and a call to a virtual method which is overriden in the derived class will execute the override rather than the base class. You can get the effect of a virtual constructor by a virtual clone member function for copy constructing, or a virtual create member function for the default constructor. In case the object in the hierarchy is destroyed explicitly by using delete operator to the base class pointer to a derived object, the appropriate destructor will be invoked. An objects constructor is automatically called whenever the object is created statically or dynamically. This ensures that a class inheriting from a virtual base class can be sure the virtual base class is safe to use inside the inheriting classs constructor. There can be multiple constructors of the same class, provided they have different signatures. Constructor and destructor constructor it is a member function having same name as its class and which is used to initialize the objects of that class type with a legel initial value. Q how many vptr is created internally for a base class and a derived class. The misuse of virtual destructors can lead to memory leaks and bad side effects rahul september 27, 20 flag reply. Constructors are very different from other member functions so the concept of a virtual constructor doesnt make any sense. The class user is utility class trying to make use of the hierarchy. Virtual functions employ late binding by allocating memory space during execution time and not during compilation time. For example, using an object pool or region based memory requires using strategy 2. Without spe ci c mechanisms to handle virtual inheritance, current solutions.
You declare a pure virtual function by using a pure specifier 0 in the declaration of a virtual member function in the class declaration. Constructor and destructor information technology and. Only nonstatic member functions excluding all forms of constructors can be virtual. Virtual tables are created at the time the constructor of the class is invoked. Also, pure virtual destructors must be defined, which is against the pure virtual behaviour. Before that there is no object to dispatch a virtual call to a constructor, and after that you cannot call a constructor again on this object at least not without using very lowlevel language features. The only place you would need a virtual constructor is if youre calling new on a base object and trying to create a derived object. For example, following program results in undefined behavior.
An abstract class is a class that is designed to be specifically used as a base class. This creates what is called a virtual base class, which means there is only one base object. If a class has n virtual methods, there would be one and only one virtual table in memory where the mapping information of all the n virtual methods would be defined. The vtable for virtual function mechanism is not intialised properly during the constructor invocation. 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. Constructor is a special member function of a class that initializes the object of the class. The last line of the transaction constructor calls the virtual function logtransaction, but this is where the. The virtual method table vtable for each object is chosen when constructing an object, so no object has a virtual method table before the constructor is called. A class constructor is a special member function of a class that is executed whenever we create new objects of that class. Once you see the difference, you will understand why virtual destructors are needed.
The effect of a virtual constructor by a create member function for creation and a clone member function for copy construction as shown below. Virtual destructor an overview sciencedirect topics. In other words, what type of object to be created is a. The virtual mechanism works only when we have a base class pointer to a derived class object. This is valid code, but there is no need for a virtual constructor here. Because valid use cases exist that involve calling nonpure virtual functions from the constructor of a class, it is permissible to call the virtual function with an explicitly qualified id. Constructors are responsible for object initialization and memory allocation of its class.
To demonstrate, we declare a virtual member function of base on line 8 typically named clone that each of the derived classes will implement to return a. My test environment for this was a win32 console application with stdafx. A constructor s job is to transform raw memory into a welldefined object. Destructor names are same as the class name but they are preceded by a tilde. Whenever upcasting is done, destructors of the base class must be made virtual for proper destrucstion of the object when the program exits. Also, there are some potential design issues with the above code. It doesnt make sense to have a virtual constructor.
When you refer to a derived class object using a pointer or a reference to the base class, you can call a virtual function for that object and execute the derived classs version of the function. To correct this situation, the base class should be defined with a virtual destructor. To share a base class, simply insert the virtual keyword in the inheritance list of the derived class. Virtual functions are used to implement polymorphic behaviour. Whenever we define one or more nondefault constructors with parameters for a class, a default constructor without parameters should also be explicitly defined as the compiler will not provide a default constructor in this case. The qualified id signifies to code maintainers that the expected behavior is for the class under construction or destruction to be the final overrider for the function call. Note that for the sake of brevity, preprocessor directives have been omitted. It acts like virtual constructor, also referred as factory method in pattern terminology. If there is any chance that consumers of the api will want more control over memory management, strategy 2. Constructor name is same as class name and it doesnt have a. The answer of which approach to use depends on the memory management strategy that users of your api will employ.
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