0
How can you justify the use of constructor and destructor in c plus plus?
Wiki User
∙ 9y ago
Constructors allow class designers to initialise class attributes at the point of instantiation (whenever an object of the class is created). All classes must have at least one constructor other than the copy and move constructors otherwise it would be impossible to instantiate objects of the class. Copy and move constructors are not required but are generated automatically by the compiler unless the class designer explicitly marks them deleted from the class definition. The copy and move constructors allow new instances to be constructed from existing instances. The only difference between the two is that the move constructor transfers ownership of the member attributes, rather than merely copying them. In classes that contain pointers, the compiler-generated copy constructor only copies the pointer (a shallow copy) not what it points at (a deep copy). Thus class designers must provide a copy constructor in order to deep copy any unshared memory resources. Derived class constructors automatically invoke base class constructors, so classes are always constructed from the bottom up. Copy and move constructors automatically invoke their base class copy and move constructors, respectively, however all other constructors automatically invoke their base class default constructors, which may not be the most efficient method of construction. Thus class designers can invoke specific base class constructors through the constructor's own initialisation list (which is also used to initialise member attributes). Constructor bodies do not require any code except in those cases where initialisation is not possible with the initialisation list alone, however this is usually an indication of poor design choices rather than a limitation of the initialisation list. The initialisation list provides the most efficient mechanism for class initialisation.
Every class must have one (and only one) destructor. If one is not provided by the class designer, the compiler generates one for you. However, unless the class contains a pointer to unshared memory, there is no need to define your own. Otherwise you must provide a destructor in order to release the memory. The destructor is the last chance to do so before an object of the class falls from scope. Classes that are intended to act as base classes must provide a virtual destructor (that is, the lowest base class must declared the destructor virtual). This ensures that if an object is destroyed polymorphically (via a pointer to one of its base classes), the most-derived destructor is always invoked first. Destruction of hierarchies is always top down. Note that destructors are not virtual by default for the simple reason that not all classes are intended to act as base classes. The containers provided by the Standard Template Library (STL) are a good example of this as none of the containers have virtual destructors -- so they cannot be used as base classes. Note that if any class method is declared virtual, the destructor must also be declared virtual. However, when inheriting from a base class with a virtual destructor, the class destructor is implicitly virtual (as are all virtual functions inherited from base classes) and does not need to be specified, although its good practice to explicitly include it anyway.
Wiki User
∙ 9y ago
Add your answer:
Earn +20 pts
Is constructors throws exception in C plus plus?
We must throw an exception from constructor in C++, but remember that in case of exception in Ctor, destructor will not be executed and you may have memory leaks. SO make sure all the data members must clean up their mess own. e.g. use smart pointers so in case of excpetion memory will released to free storage.
What are the advantages of constructor and destructor?
Without a copy constructor the only way to copy an object would be to instantiate a new object (or use an existing object) and then assign another object's value to it. However, it would be very odd indeed to have a copy assignment operator without a matching copy constructor. If you have one, you must have both. If you do not need to copy an object of a particular class, however, you can simply delete both the copy constructor and the copy assigment operator for that class. Any attempt to copy or copy assign would then result in a compile-time error.
1 Explain the concepts of constructor and destructor Do you have to declare a constructor every time you create a class?
You only need a constructor if the default constructor will not suffice. Often times, it is useful to have a constructor that takes common parameters so that you do not have to write additional code. For example, a Point class might have a constructor for Point(int x, int y), which would be a shortcut for assigning x and y independently. Other classes may not need any default values assigned, and for this, it is acceptable to just use the default constructor. Finally, some classes are virtual, static, or abstract, and so may not need a constructor because the constructor is unnecessary (static), or may be defined elsewhere (virtual, abstract).
Why is a constructor useful?
Objects are constructed. You can't make a new object without invoking a constructor. In fact, you can't make a new object without invoking not just the constructor of the object's actual class type, but also the constructor of each of its superclasses including the Object class itself! Constructors are the code that runs whenever you use the keyword new.
Which situation constructor is used?
Constructor is necessary when you are about to use instance of a class.
Is constructors throws exception in C plus plus?
We must throw an exception from constructor in C++, but remember that in case of exception in Ctor, destructor will not be executed and you may have memory leaks. SO make sure all the data members must clean up their mess own. e.g. use smart pointers so in case of excpetion memory will released to free storage.
Is it mandatory to use the construtors in a class in c plus plus?
No. If you do not provide a default constructor, the compiler will provide a default constructor that simply allocates memory for the class, but it will not initialize the members of the class. If you do not provide a copy constructor, then the compiler will provide a copy constructor that allocates memory for the class, and then copies the member's data from class to class. This is bad if the class contains pointers, because only the pointer will be copied - the objects to which the pointers point will not be copied - and you could wind up deleting an object and then using it after deletion, with potentially devastating consequences. So, yes, it is mandatory, from a good practices point of view, and just plain mandatory when the class has pointers, to always provide a default constructor and a copy constructor, along with the appropriate destructor.
What are the advantages of constructor and destructor?
Without a copy constructor the only way to copy an object would be to instantiate a new object (or use an existing object) and then assign another object's value to it. However, it would be very odd indeed to have a copy assignment operator without a matching copy constructor. If you have one, you must have both. If you do not need to copy an object of a particular class, however, you can simply delete both the copy constructor and the copy assigment operator for that class. Any attempt to copy or copy assign would then result in a compile-time error.
1 Explain the concepts of constructor and destructor Do you have to declare a constructor every time you create a class?
You only need a constructor if the default constructor will not suffice. Often times, it is useful to have a constructor that takes common parameters so that you do not have to write additional code. For example, a Point class might have a constructor for Point(int x, int y), which would be a shortcut for assigning x and y independently. Other classes may not need any default values assigned, and for this, it is acceptable to just use the default constructor. Finally, some classes are virtual, static, or abstract, and so may not need a constructor because the constructor is unnecessary (static), or may be defined elsewhere (virtual, abstract).
Why is a constructor useful?
Objects are constructed. You can't make a new object without invoking a constructor. In fact, you can't make a new object without invoking not just the constructor of the object's actual class type, but also the constructor of each of its superclasses including the Object class itself! Constructors are the code that runs whenever you use the keyword new.
What is use of constructor in java?
Constructor is used to do something (written in constructor) immediately after object creation.
Why you use constructor overloading?
When we are initializing our object with different internal state then we can use the constructor overloading.
Which situation constructor is used?
Constructor is necessary when you are about to use instance of a class.
When a object is passed to a function as argument why it is not generating an error if the parameterized constructor is defined and not the default constructor?
default constructor is used only when the programmer does not use a constructor to initialize objects. Once the programmer defines a constructor then the default constructor is no longer used
How do you implement inheritance in c plus plus?
You implement inheritance by deriving a new class of object from an existing class of object. The existing class is known as the base class of the derived class.Classes declared final cannot be used as bases classes and classes without a virtual destructor (or a virtual destructor override) cannot be used as polymorphic base classes.
Can you define a constructor as private?
I dont think we can have Protected Constructors but yes we can have Private constructors. We can declare the constructor as Private to ensure that no other class can instantiate it. We use this in the singleton design pattern
Use of constructor?
to create an instance of object
