Type introspection

In computing, type introspection is the ability of a program to examine the type or properties of an object

at runtime.

Some programming languages possess this capability.

Introspection should not be confused with reflection, which goes a step further and is the ability for a program to manipulate the metadata, properties, and functions of an object at runtime. Some programming languages also possess that capability (e.g.,

Java,

Python,

Julia,

and

Go).

Examples

C++

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

The expression can be used to determine whether a particular object is of a particular derived class. For instance:

Person* p = dynamic_cast<Person*>(obj);

if (p) {

p->walk();

}

</syntaxhighlight>

The operator retrieves a object describing the most derived type of an object:

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

serializePerson(obj);

}

</syntaxhighlight>

C#

In C# introspection can be done using the keyword. For instance:

if (obj is Person)

{

// Do whatever you want

}

</syntaxhighlight>

Objective-C

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

For example, say we have an and an class inheriting from .

Now, in the 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();

}

</syntaxhighlight>

Now, when is called with a generic object (an ), the function will behave correctly depending on the type of the generic object.

Object Pascal

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;

</syntaxhighlight>

Java

The simplest example of type introspection in Java is the operator. The 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();

}

</syntaxhighlight>

The 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), and can be used:

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

</syntaxhighlight>

PHP

In PHP introspection can be done using operator. For instance:

if ($obj instanceof Person) {

// Do whatever you want

}

</syntaxhighlight>

Perl

Introspection can be achieved using the and 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();

</syntaxhighlight>

using:

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

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

</syntaxhighlight>

Meta-Object Protocol

Much more powerful introspection in Perl can be achieved using the Moose object system and the meta-object protocol; for example, you can check if a given object does a role :

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

do something ...

}

</syntaxhighlight>

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";

}

</syntaxhighlight>

Python

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

class Foo:

def __init__(self, val: int) -> None:

self.x = val

def get_x(self) -> int:

return self.x

</syntaxhighlight>

print(dir(Foo(5)))

prints ['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__firstlineno__', '__format__', '__ge__', '__getattribute__', '__getstate__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__static_attributes__', '__str__', '__subclasshook__', '__weakref__', 'get_x', 'x']

</syntaxhighlight>

Also, the built-in functions and can be used to determine what an object is while can determine what an object does. For example:

a: Foo = Foo(10)

b: Bar = Bar(11)

print(type(a))

prints <type 'Foo'>

isinstance(a, Foo)

prints True

print(isinstance(a, type(a)))

prints True

print(isinstance(a, type(b)))

prints False

print(hasattr(a, 'bar'))

prints True

</syntaxhighlight>

Further, Python type hints can be accessed at runtime and can be used by libraries such as Pydantic for serialization and validation, and FastAPI for dependency injection and API schema generation.

Ruby

Type introspection is a core feature of Ruby. In Ruby, the Object class (ancestor of every class) provides and 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

=> #<A:0x2e44b78>

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

=> #<B:0x2e431b0>

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

</syntaxhighlight>

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

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

</syntaxhighlight>

ActionScript

In ActionScript (as3), the function <code>flash.utils.getQualifiedClassName</code> 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(getQualifiedClassName("I'm a String")); // "String"

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

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

</syntaxhighlight>

Alternatively, the operator <code>is</code> 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

</syntaxhighlight>

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 Sprite() is Sprite); // true

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

trace(new Sprite() is String); // false

</syntaxhighlight>

Meta-type introspection

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 <code>flash.utils.describeType</code> 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 Sprite()); // "flash.display.Sprite"

var classRef: Class = getDefinitionByName(className); // Class reference to flash.displaySprite

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

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

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

</syntaxhighlight>

References

External links

Introspection on Rosetta Code

Examples

C++

C#

Objective-C

Object Pascal

Java

PHP

Perl

Meta-Object Protocol

Python

Ruby

ActionScript

Meta-type introspection

See also

References

External links