Type introspection

In Objective-C, for example, both the generic Object and NSObject (in Cocoa/OpenStep) provide the method {{code|isMemberOfClass:}} which returns true if the argument to the method is an instance of the specified class. The method {{code|isKindOfClass:}} analogously returns true if the argument inherits from the specified class.

For example, say we have an {{code|Apple}} and an {{code|Orange}} class inheriting from {{code|Fruit}}.

Now, in the {{code|eat}} method we can write

- (void)eat:(id)sth {

if ([sth isKindOfClass:[Fruit class]]) {

// we're actually eating a Fruit, so continue

if ([sth isMemberOfClass:[Apple class]]) {

eatApple(sth);

} else if ([sth isMemberOfClass:[Orange class]]) {

eatOrange(sth);

} else {

error();

}

} else {

error();

}

}

Now, when {{code|eat}} is called with a generic object (an {{code|id}}), the function will behave correctly depending on the type of the generic object.

C++ supports type introspection via the run-time type information (RTTI) typeid and dynamic cast keywords.

The {{code|dynamic_cast}} expression can be used to determine whether a particular object is of a particular derived class. For instance:

Person* p = dynamic_cast(obj);

if (p != nullptr) {

p->walk();

}

The {{code|typeid}} operator retrieves a {{code|std::type_info}} object describing the most derived type of an object:

if (typeid(Person) == typeid(*obj)) {

serialize_person( obj );

}

Type introspection has been a part of Object Pascal since the original release of Delphi, which uses RTTI heavily for visual form design. In Object Pascal, all classes descend from the base TObject class, which implements basic RTTI functionality. Every class's name can be referenced in code for RTTI purposes; the class name identifier is implemented as a pointer to the class's metadata, which can be declared and used as a variable of type TClass.

The language includes an is operator, to determine if an object is or descends from a given class, an as operator, providing a type-checked typecast, and several TObject methods. Deeper introspection (enumerating fields and methods) is traditionally only supported for objects declared in the $M+ (a pragma) state, typically TPersistent, and only for symbols defined in the published section. Delphi 2010 increased this to nearly all symbols.

procedure Form1.MyButtonOnClick(Sender: TObject);

var

aButton: TButton;

SenderClass: TClass;

begin

SenderClass := Sender.ClassType; //returns Sender's class pointer

if sender is TButton then

begin

aButton := sender as TButton;

EditBox.Text := aButton.Caption; //Property that the button has but generic objects don't

end

else begin

EditBox.Text := Sender.ClassName; //returns the name of Sender's class as a string

end;

end;

The simplest example of type introspection in Java is the {{code|instanceof}}[http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.20.2 Java Language Specification: instanceof] operator. The {{code|instanceof}} operator determines whether a particular object belongs to a particular class (or a subclass of that class, or a class that implements that interface). For instance:

if (obj instanceof Person p) {

p.walk();

}

The {{code|java.lang.Class}}[http://java.sun.com/javase/6/docs/api/java/lang/Class.html Java API: java.lang.Class] class is the basis of more advanced introspection.

For instance, if it is desirable to determine the actual class of an object (rather than whether it is a member of a particular class), {{code|Object.getClass()}} and {{code|Class.getName()}} can be used:

System.out.println(obj.getClass().getName());

In PHP introspection can be done using {{code|instanceof}} operator. For instance:

if ($obj instanceof Person) {

// Do whatever you want

}

Introspection can be achieved using the {{code|ref}} and {{code|isa}} functions in Perl.

We can introspect the following classes and their corresponding instances:

package Animal;

sub new {

my $class = shift;

return bless {}, $class;

}

package Dog;

use base 'Animal';

package main;

my $animal = Animal->new();

my $dog = Dog->new();

using:

print "This is an Animal.\n" if ref $animal eq 'Animal';

print "Dog is an Animal.\n" if $dog->isa('Animal');

Much more powerful introspection in Perl can be achieved using the Moose object system[https://metacpan.org/module/Moose::Manual::MOP Moose meta API documentation] and the {{code|Class::MOP}} meta-object protocol;[https://metacpan.org/module/Class::MOP Class::MOP - a meta-object protocol for Perl] for example, you can check if a given object does a role {{var|X}}:

if ($object->meta->does_role("X")) {

# do something ...

}

This is how you can list fully qualified names of all of the methods that can be invoked on the object, together with the classes in which they were defined:

for my $method ($object->meta->get_all_methods) {

print $method->fully_qualified_name, "\n";

}

The most common method of introspection in Python is using the {{code|dir}} function to detail the attributes of an object. For example:

class Foo:

def __init__(self, val):

self.x = val

def bar(self):

return self.x

>>> dir(Foo(5))

['__class__', '__delattr__', '__dict__', '__doc__', '__getattribute__',

'__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__',

'__repr__', '__setattr__', '__str__', '__weakref__', 'bar', 'x']

Also, the built-in functions {{code|type}} and {{code|isinstance}} can be used to determine what an object is while {{code|hasattr}} can determine what an object does. For example:

>>> a = Foo(10)

>>> b = Bar(11)

>>> type(a)

>>> isinstance(a, Foo)

True

>>> isinstance(a, type(a))

True

>>> isinstance(a, type(b))

False

>>> hasattr(a, 'bar')

True

Type introspection is a core feature of Ruby. In Ruby, the Object class (ancestor of every class) provides {{code|Object#instance_of?}} and {{code|Object#kind_of?}} methods for checking the instance's class. The latter returns true when the particular instance the message was sent to is an instance of a descendant of the class in question. For example, consider the following example code (you can immediately try this with the Interactive Ruby Shell):

$ irb

irb(main):001:0> A=Class.new

=> A

irb(main):002:0> B=Class.new A

=> B

irb(main):003:0> a=A.new

=> #

irb(main):004:0> b=B.new

=> #

irb(main):005:0> a.instance_of? A

=> true

irb(main):006:0> b.instance_of? A

=> false

irb(main):007:0> b.kind_of? A

=> true

In the example above, the {{code|Class}} class is used as any other class in Ruby. Two classes are created, {{code|A}} and {{code|B}}, the former is being a superclass of the latter, then one instance of each class is checked. The last expression gives true because {{code|A}} is a superclass of the class of {{code|b}}.

Further, you can directly ask for the class of any object, and "compare" them (code below assumes having executed the code above):

irb(main):008:0> A.instance_of? Class

=> true

irb(main):009:0> a.class

=> A

irb(main):010:0> a.class.class

=> Class

irb(main):011:0> A > B

=> true

irb(main):012:0> B <= A

=> true

In ActionScript (as3), the function [http://livedocs.adobe.com/flash/9.0/ActionScriptLangRefV3/flash/utils/package.html#getQualifiedClassName%28%29 flash.utils.getQualifiedClassName] can be used to retrieve the class/type name of an arbitrary object.

// all classes used in as3 must be imported explicitly

import flash.utils.getQualifiedClassName;

import flash.display.Sprite;

// trace is like System.out.println in Java or echo in PHP

trace(flash.utils.getQualifiedClassName("I'm a String")); // "String"

trace(flash.utils.getQualifiedClassName(1)); // "int", see dynamic casting for why not Number

trace(flash.utils.getQualifiedClassName(new flash.display.Sprite())); // "flash.display.Sprite"

Alternatively, the operator [http://livedocs.adobe.com/flash/9.0/ActionScriptLangRefV3/operators.html#is is] can be used to determine if an object is of a specific type:

// trace is like System.out.println in Java or echo in PHP

trace("I'm a String" is String); // true

trace(1 is String); // false

trace("I'm a String" is Number); // false

trace(1 is Number); // true

This second function can be used to test class inheritance parents as well:

import flash.display.DisplayObject;

import flash.display.Sprite; // extends DisplayObject

trace(new flash.display.Sprite() is flash.display.Sprite); // true

trace(new flash.display.Sprite() is flash.display.DisplayObject); // true, because Sprite extends DisplayObject

trace(new flash.display.Sprite() is String); // false

Like Perl, ActionScript can go further than getting the class name, but all the metadata, functions and other elements that make up an object using the [http://livedocs.adobe.com/flash/9.0/ActionScriptLangRefV3/flash/utils/package.html#describeType%28%29 flash.utils.describeType] function; this is used when implementing reflection in ActionScript.

import flash.utils.describeType;

import flash.utils.getDefinitionByName;

import flash.utils.getQualifiedClassName;

import flash.display.Sprite;

var className:String = getQualifiedClassName(new flash.display.Sprite()); // "flash.display.Sprite"

var classRef:Class = getDefinitionByName(className); // Class reference to flash.display{{Not a typo|.}}Sprite

// eg. 'new classRef()' same as 'new flash.display.Sprite()'

trace(describeType(classRef)); // return XML object describing type

// same as : trace(describeType(flash.display.Sprite));

• Reification (computer science)
• typeof

{{Reflist}}

• [http://rosettacode.org/wiki/Introspection Introspection] on [http://rosettacode.org/ Rosetta Code]

{{DEFAULTSORT:Type Introspection}}

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