Объект құруға арналған арнайы функциялар

Special function called to create an object

In class based, object oriented programming, a constructor (abbreviation: ctor) is a special type of function called to create an object. It prepares the new object for use, often accepting arguments that the constructor uses to set required member variables. A constructor resembles an instance method, but it differs from a method in that it has no explicit return type, it is not implicitly inherited and it usually has different rules for scope modifiers. Constructors often have the same name as the declaring class. They have the task of initializing the object's data members and of establishing the invariant of the class, failing if the invariant is invalid. A properly written constructor leaves the resulting object in a valid state. Immutable objects must be initialized in a constructor. Most languages allow overloading the constructor in that there can be more than one constructor for a class, with differing parameters. Some languages take consideration of some special types of constructors. Constructors, which concretely use a single class to create objects and return a new instance of the class, are abstracted by factories, which also create objects but can do so in various ways, using multiple classes or different allocation schemes such as an object pool.

Like C++, Java also supports "Copy Constructor". But, unlike C++, Java doesn't create a default copy constructor if you don't write your own. Copy constructors define the actions performed by the compiler when copying class objects. A Copy constructor has one formal parameter that is the type of the class (the parameter may be a reference to an object). It is used to create a copy of an existing object of the same class. Even though both classes are the same, it counts as a conversion constructor. While copy constructors are usually abbreviated copy ctor or cctor, they have nothing to do with class constructors used in NET using the same abbreviation.

Conversion constructors provide a means for a compiler to implicitly create an object belonging to one class based on an object of a different type. These constructors are usually invoked implicitly to convert arguments or operands to an appropriate type, but they may also be called explicitly.

In C++, move constructors take an Rvalue reference to an object of the class, and are used to implement ownership transfer of the parameter object's resources.

Java, C++, C#, ActionScript, PHP 4 and MATLAB have a naming convention in which constructors have the same name as the class with which they are associated. In PHP 5, a recommended name for a constructor is construct. For backwards compatibility, a method with the same name as the class will be called if construct method can not be found. Since PHP 5.3.3, this works only for non namespaced classes. In PHP 7, you should always name the constructor as construct. Methods with the same name as the class will trigger an E DEPRECATED level error. In Perl, constructors are, by convention, named "new" and have to do a fair amount of object creation. In Moose object system for Perl, constructors (named new) are automatically created and are extended by specifying a BUILD method. In Visual Basic NET, the constructor is called "New". In Python, the constructor is split over two methods, " new " and " init ". The new method is responsible for allocating memory for the instance, and receives the class as an argument (conventionally called "cls"). The init method (often called "the initialiser") is passed the newly created instance as an argument (conventionally called "self"). Object Pascal constructors are signified by the keyword "constructor" and can have user defined names (but are mostly called "Create"). In Objective C, the constructor method is split across two methods, "alloc" and "init" with the alloc method setting aside (allocating) memory for an instance of the class, and the init method handling the bulk of initializing the instance. A call to the method "new" invokes both the alloc and the init methods, for the class instance.

In Java, C#, and VB NET, the constructor creates reference type objects in a special memory structure called the
"heap". Value types (such as int, double, etc.) are created in a sequential structure called the "stack". VB NET and C# also allow the use of the new operator to create value type objects, but these value type objects are created on the stack regardless of whether the operator is used or not. In C++, objects are created on the stack when the constructor is invoked without the new operator, and created on the heap when the constructor is invoked with the new operator. Stack objects are deleted implicitly when they go out of scope, while heap objects must be deleted implicitly by a destructor or explicitly by using the delete operator.