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Classes

# Class Overview

Classes define types of Qore objects. Classes can define members and methods, which are attributes of the class and functions that operate only on the objects of that class, respectively. Furthermore access to class members and methods can be restricted with the `private` keyword, and classes can be subclassed to support polymorphism.

Note
  Each Qore type has a "pseudo-class" associated with it (the default is <value>); methods from the data type's "pseudo-class" can be run on any value of that type; see <value> and "Pseudo Class for Type" headings in Basic Data Types for more information.

In-Line Class Declaration Syntax
  `[public] [final] class [`namespace_path`::...]`class_identifier`] [inherits [private|public] [`namespace_path`::...]`parent_class_identifier`[, ...]] {`
  `[[private]` member_name`[, ...];]`
  `[private|public {`
  `[[`member_type`]` member_name `[=` initialization_expression`];]`
  `[static [`static_member_type`]` static_member_name `[=` initialization_expression`];]`
  `[const` constant_name `=` initialization_expression`;]`
  `}]`
  `[[final] [private|public] [deprecated] constructor([`[param_type] param_name `[=` default_initialization_expression`], ...]) [:` parent_class_name`(`args...`), ...] {`
  `}]`
  `[copy(`object` arg_name) {`
  `}]`
  `[destructor() {`
  `}]`
  `[[static|final] [synchronized] [private|public] [deprecated] [`return_type`]` method_name`([`[param_type] param_name `[=` default_initialization_expression`], ...]) {`
  `}]`
  `[[abstract] [private|public] [`return_type`]` method_name`([`[param_type] param_name `[=` default_initialization_expression`], ...]);]`
  `...`
  `}`

Note
  To define a class interface, define a class with abstract methods.

When defining a user module, class declarations can be preceded by public, which means that the class will be available (imported) in the Program object importing the module. When a class is declared public outside of a user module, it means that the class can be inherited in any child Program objects created in the same scope. See public for more information. Note that classes can also be imported singly by using the Program::importClass() method.

In-Line Class Declaration Example
  class MyClass inherits MyBaseClass {
  public {
  string pub_attr = "hello";
  const pub_const = "foo";
  static pub_static_var = "bar";
  }
  constructor(string arg) : MyBaseClass(arg + 1) {
  }
  softstring myMethod(softint i) {
  return i + 2;
  }
  }

Note
  No semicolon (`";"`) is required to terminate a class declaration (in fact using a semicolon would raise a parse exception).

Out-Of-Line Class Declaration Syntax
  Alternatively class declarations and method declarations can be defined out of line as follows:
  `[public] class [`namespace_path`::...]`class_identifier`[inherits [private|public] [`namespace_path`::...]`parent_class_identifier`[, ...]];`
  `[[private|public] [deprecated] [`namespace_path`::...]`class_identifier`::constructor([`[param_type] param_name `[=` default_initialization_expression`], ...]) [:` parent_class_name`(`args...`), ...] {`
  `}]`
  `[[static|final] [synchronized] [private|public] [deprecated] [`return_type`] [`namespace_path`::...]`class_identifier`::`method_name`([`[param_type] param_name `[=` default_initialization_expression`], ...]) {`
  `}]` `[abstract [private|public] [`return_type`] [`namespace_path`::...]`class_identifier`::`method_name`([`[param_type] param_name `[=` default_initialization_expression`], ...]);`
  As with inline class definitions, when defining a user module, class declarations can be preceded by public, which means that the class will be available (imported) in the Program object importing the module. When a class is declared public outside of a user module, it means that the class can be inherited in any child Program objects created in the same scope. See public for more information. Note that classes can also be imported singly by using the Program::importClass() method.

Out-Of-Line Class Declaration Example
  class MyNamespace::MyClass inherits MyBaseClass;
  MyNamespace::MyClass::constructor(string arg = "temp") : MyBaseClass(arg + ".txt") {
  }
  softstring MyNamespace::MyClass::myMethod(softint i) {
  return i + 2;
  }

Note
  Because method definitions are allowed out-of-line, this means that builtin classes may be extended with new user methods, however user constructor, destructor, and copy methods cannot be added to builtin classes; to customize the behavior of these methods for builtin classes, subclass the class instead.
  When parse option %allow-bare-refs or %new-style is enabled, no `"$"` or `"$."` prefixes can be used with variable or method or member names. Class members, class constants, and static class variables can only be declared in an in-line class definition (the first example above). If a class has at least one public member declared (or inherits a class with at least one public member declared), then only those members declared as public can be accessed from outside the class, and from within the class only members explicitly declared can be accessed as well (unless the class also defines a memberGate() method). In this way typographical errors in member names can be caught (at parse time if types are declared).
  In a class hierarchy, base class constructor methods can be explicitly specified using a special syntax unique to subclass constructor methods. Please see Class Inheritance for more information.
  It's possible to write purely object-oriented scripts/programs in Qore by defining an application class and using the `"-x"` or `"--exec-class"` command-line arguments to tell Qore to instantiate the class instead of doing normal top-level execution (in fact, the `"--exec-class"` argument disallows the use of top-level statements entirely). For more information, please see qore Executable Command-Line Processing and Parse Directives.

Final Classes
  Classes declared `final` cannot be subclassed, and, by extension, no methods of a `final` class can be reimplemented as well.
  Currently the Qore standard class library (delivered with Qore) has no `final` classes.

# Class Methods

Public Methods
  All class methods are public by default, but methods can also be explicitly declared with the `public` keyword as well. Public methods have no class protection and can be called from any code context.

Private Methods
  Methods declared with the `private` keyword can only be called by other member functions of the same class or of derived classes. Any attempt to call these methods from outside the class hierarchy will result in a run-time exception.

Synchronized Methods
  Methods declared with the `synchronized` keyword will only run in one thread at a time.
  For more information, see the synchronized keyword.

Static Methods
  Methods declared with the `static` keyword are like regular functions that are attached to the class. These methods are not associated with a particular object's state and therefore are not allowed to refer to object members or call non-static methods. Also, no reference to the special `self` variable is allowed within static methods.
  Static methods may be declared `private` or `public` or `synchronized` like non-static methods; static methods can also access private members of a class (through an object of the class for non-static members).
  Static method calls take a special syntax as documented in Static Method Calls.

Abstract Methods
  Methods declared with the `abstract` keyword define methods interfaces that must be defined in child classes for the class to be instantiated. Methods defining the declared interfaces in child classes must define exactly the same parameters in order for the abstract method to match, but the return type of the concrete method in the child class has to simply be compatible with the return type of the abstract method in the parent class.
  Classes with undefined abstract methods cannot be instantiated. Abstract methods cannot have a method body; an abstract method's declaration must be terminated with a semicolon (`";"`).
  The `abstract` keyword can only be used in abstract method declaration; the use of this keyword with a method declaration with a method body will cause a parse exception to be thrown.

Constructors, Destructors, and Other Special Methods
  All class methods are optional, but some methods have a special meaning.
  Special Methods

  Name | Description

  `constructor(`[params...]`) {}`

  Called when objects are created when instantiated by a variable declaration with a class type and constructor arguments or explicitly with the new operator. User code may not explicitly call `constructor()` methods directly. In a class tree, `constructor()` methods are called for base classes first in left-to-right, depth-first declaration order.
  `constructor()` methods may be overloaded and also private constructors may be defined. Private constructors can only be called from within the class.

  `copy() {}`

  When a user explicitly calls a copy method, Qore will generate a new object with references to the same members as the source object. Then, if there are any base classes, base class `copy()` methods are called in the same order as the `constructor()` methods. If a `copy()` method is defined, it will be run in the new object with a reference to the old object passed as the first argument. Any other arguments passed to the `copy()` method are ignored.
  `copy()` methods cannot be overloaded and cannot be private.

  `destructor() {}` | Called when objects are implicitly collected (see Garbage Collection and Resource Management) or are explicitly deleted. User code may not explicitly call `destructor()` methods. In a class tree, `destructor()` methods are called for base classes in the opposite order in which the constructors are called. `destructor()` methods cannot be overloaded and cannot be private.
  any `memberGate(`string `$`member_param_name`) {}` | If this method is implemented in the class, it is called when read access is attempted to private member or members that do not exist in the current object; the return value of this method is returned as the value of the member. `memberGate()` methods cannot be overloaded and are not inherited by subclasses.
  any `methodGate(`string `$`method_param_name`, ...) {}` | If this method is implemented in the class, it is called when methods are called on the object that do not exist in the current object and the return value of this method is returned as the value of the method call. `methodGate()` methods cannot be overloaded and are not inherited by subclasses.

  `memberNotification(`string `$`member_param_name`) {}`

  If this method is implemented in the class, it is called when an object member is updated outside the class with the member name as the argument. Note that this method is called after the member has been updated and without locking; the call is not atomic respective to other threads that also may update the same member simultaneously.
  `memberNotification()` methods cannot be overloaded and are not inherited by subclasses.

Final Methods
  Methods declared `final` cannot be reimplemented in a subclass. Methods can be declared `final` to prevent a subclass from altering the behavior of a method that is critical to the class's functionality.
  None of the following special methods can be declared `final`:
  - constructor(): declare the class final instead
  - destructor(): declare the class final instead
  - copy(): declare the class final instead
  - memberGate(): these methods are not inherited so they may not be declared `final`
  - memberNotification(): these methods are not inherited so they may not be declared `final`
  - methodGate(): these methods are not inherited so they may not be declared `final`
  Currently the Qore standard class library has no `final` methods.

Class Constants
  Class constants, like non-class constants, allow programmers to refer to values with Qore identifiers rather than using the value or the expression that generates the value.
  See Class Overview for a description of the syntax required to declare a class constant.
  Like other attributes of classes, class constants may be declared `private` or `public`. The following is an example of a class constant definition:
  class Test {
  public {
  const Version = "1.0";
  }
  private {
  const Limit = 100;
  }
  }

Note
  Class constants cannot have the same names as static class variables.

Static Class Variables
  Static class variables are like global variables that belong to a class. They are not associated with any particular object.
  See Class Overview for a description of the syntax required to declare a static class variable.
  Like other attributes of classes, static class variables may be declared `private` or `public`. The following is an example of a static class variable definition:
  class Test {
  public {
  static string lastFile = "none";
  }
  private {
  static int numProcessed = 0;
  }
  }

Note
  Static class variables cannot have the same name as class constants.

# Class Members

Public Member Declarations
  If a class has at least one public member declared (or inherits a class with at least one public member declared), then only those members declared as public can be accessed from outside the class, and from within the class only members explicitly declared can be accessed as well (unless the class also defines a memberGate() method). In this way typographical errors in member names can be caught (at parse time if types are declared).

Private Members
  Members declared `private` can only be accessed within the class hierarchy; trying to access private members form outside the class hierarchy will result in either a parse or runtime exception, depending on when the illegal acceess is caught.

Class Member References
  When defining a class, members of instantiated objects are referred to with a special syntax as follows:
  `$.`member_name
  Furthermore, the automatic variable `self` is instantiated in every non-static method, representing the current object (similar to `this` in C++ or Java). Therefore if you need to access hash members which are not valid Qore identifiers, then enclose the member name in double quotes after the dot operator as follows:
  self."&member-name"

memberGate() Method
  If the class implements a `memberGate()` method, then whenever a non-existent member of the class is accessed (read), this method will be called with the name of the member as the sole argument, so that the class can create the member (or react in some other way) on demand. This method is also called when methods of the same class try to access (read) non-existent methods, but is not called from within the `memberGate()` method itself.

Note
  `memberGate()` methods are not inherited; they must be explicitly implemented in each class

memberNotification() Methods
  To monitor writes to the object, the class can implement a `memberNotification()` method, which is called whenever an object member is modified from outside class member code. In this case, the `memberNotification()` method is called with the name of the member that was updated so that an object can automatically react to changes to its members (writes to members) from outside the class. This method is not called when members are updated from within class member code.

Note
  `memberNotification()` methods are not inherited; they must be explicitly implemented in each class

Member Initialization
  Members that have intialization expressions in the class definition are initialized before the constructor is executed (but after any base class constructors have run). An exception raised in a member initialization expression will cause the constructor to fail and for the object to be deleted immediately.

Note
  The automatic `argv` local variable is instantiated as usual in all class methods where there are more arguments than variables declared in the method declaration.

# Object Method Calls

In-Class Method Call Syntax
  Within a class method definition, calls to methods in the same class hierarchy (of the current class or a base class) can be made as follows:
  `[[`namespace_path`::...]`parent_class_name`::]$.`method_name`([`args`, ...])`

In-Class Method Call Example
  # to call a specific method in a base class
  Thread::Mutex::lock();
  # to call lock() in the current (or lower base) class
  lock();

In-Class Method Calls with allow-bare-refs
  When the %allow-bare-refs or %new-style parse directive is set, then object methods are called without the `"$."` prefix as in the following example:
  # to call a specific method in a base class
  Thread::Mutex::lock();
  # to call lock() in the current (or lower base) class
  lock();
  Calls to object methods can be made outside the class by using the above syntax as well. If the object's class is not known at parse time, then the call is resolved at run-time, and if a call is attempted to a private function outside the defining class, then a run-time `METHOD-IS-PRIVATE` (if the method is private) or `BASE-CLASS-IS-PRIVATE` (if the method resolves to a privately-inherited base class) exception is raised.

methodGate() methods
  If the class implements a `methodGate()` method, then whenever a non-existent method of the class is called, the methodGate() method will be called with the name of the member as the first argument (prepended to the other arguments to the non-existant method), so that the class simulate or redirect the method call. This method is also called when methods of the same class try to call non-existent methods, but is not called from within the `methodGate()` method itself.

Note
  `methodGate()` methods are not inherited; they must be explicitly implemented in each class

# Class Inheritance

Class inheritance is a powerful concept for easily extending and reusing object-oriented code, but is also subject to some limitations. This section explains how class inheritance works in Qore.

Classes inherit the methods of a parent class by using the `inherits` keyword as specified above. Multiple inheritance is supported; a single Qore class can inherit one or more classes. When a class is inherited by another class, it is called a base class or parent class. Private inheritance is specified by including the keyword `private` before the inherited class's name. When a class is privately inherited, it means that the inherited class's public declarations (members, constants, methods, etc) are treated as private in the context of accesses outside the class.

Inheritance is `public` by default, to inherit a class privately, use the `private` keyword before the class name or class path to inherit as follows:

class ChildClass inherits private ParentClass {
}

It is not legal to directly inherit the same class directly more than once; that is; it is not legal to list the same class more than once after the `inherits` keyword. However, it is possible that a base class could appear more than once in the inheritance tree if that class is inherited separately by two or more classes in the tree.

In this case, the base class will actually only be inherited once in the subclass, even though it appears in the inheritance tree more than once. This must be taken into consideration when designing class hierarchies, particularly if base class constructor parameters for that class are explicitly provided in a different way by the inheriting classes.

Note
  Class members only exist once for each object; therefore if classes in an inheritance tree have different uses for members with the same name, then a class hierarchy built of such classes will probably not function properly.

Subclasses can give explicit arguments to their base class constructors using a special syntax (only available to subclass constructors) similar to the C++ syntax for the same purpose as follows:
`[[private|public] [deprecated] constructor([`[param_type] `$`param_name `[=` default_initialization_expression`], ...]) [:` parent_class_name`(`args...`), ...] {`
`}]`

Here is a concrete example of giving arguments to an inherited base class:

class XmlRpcClient inherits Qore::HTTPClient {
# calls the base class HTTPClient constructor, overrides the "protocols" key to "xmlrpc"
constructor(hash opts = hash()) : Qore::HTTPClient(opts + ( "protocols" : "xmlrpc" )) {
}
}

Because base class constructors are executed before subclass constructors, the only local variables in the constructor that can be referenced are those declared in the subclass constructor declaration (if any). What this means is that if you declare local variables in the expressions giving base class arguments, these local variables are not accessible from the constructor body.

Note
  Base classes that give explicit arguments to their base class constructors can be overridden by subclasses by simply listing the base class in the base class constructor list and providing new arguments.

## Private Inheritance

Classes inherited using the `private` keyword encapsulate the functionality (members, constants, methods, etc) of the parent class privately in the child class; that is, any access to the privately-inherited parent class's functionality from outside the class will result in either a parse-time or runtime exception, depending on when the error is caught.

## Public Inheritance

Classes are inherited publically by default; public inheritance means that the parent class's functionality (members, constants, methods, etc) have the same visibility in the child class as they do in the parent class. For example, a public method in the parent class will also be public in the child class if the parent class is inherited publically. Respectively, private methods of publically-inherited parent classes will still be private in child classes.