CSC/ECE 517 Fall 2011/ch1 1h ps

Common Attribute/Member Syntax

Different object oriented programming languages include different ways of accessing the member variables and methods. This page discusses the various usages and their advantages and disadvantages.

Introduction

The purpose of Object Oriented Programming is to make programs model things the way people think or deal with the world i.e it's modeled around data rather than logic. Objects form the basis of OOP. Let us now look at objects, their manipulation and the ways to do so.

Objects - Attributes and Methods

Similar to real world objects, software objects also consist of state and related behavior. An object stores it's state in attributes and exposes its behavior through methods. Let's look at the pseudo code for a bicycle object. CLASS Bicycle ATTRIBUTES: speed: NUMBER gear: NUMBER pedal_cadence: NUMBER METHODS: ride(speed) changeGear(gear) changeCadence(pedal_cadence)

In the above pseudo code, the attributes define the object and the methods provide a way of achieving the desired functionality from that object. The methods use the attributes to manipulate the object and achieve the functionality. From this we can observe that this manipulation is the very essence of programming using objects. Therefore the access of attributes and way of defining method calls very important in OOP.

Method call / Instance Variable reference Syntax

A running debate in O-O languages is whether method calls and instance-variable references should have the same syntax. Different programming languages implement this in different ways. Let's look at them by examples.

Same Syntax

Languages like Ruby use the same syntax for method calls and variable references. There is no distinction between these. We can see this in the example below:

class Song
  def duration=(newDuration)
    @duration = newDuration
  end
end
aSong = Song.new("Bicylops", "Fleck", 260)
aSong.duration	»	260
aSong.duration = 257   # set attribute with updated value
aSong.duration	»	257

This makes it simpler for the programmer to code. Also, it presents uniformity in the code.

Differing Syntax

Languages like Java employ different syntax for method calls and variable references as seen in this example below:

class Song{
	int duration;
	Song(){}
	void setDuration(int dur){
		duration = dur;
	}
}

Song sng = new Song();
sng.setDuration(260);
System.out.println("Song duration = "+sng.duration);

As seen above, a method is always called using parenthesis, within which arguments may be included if required. However, instance variable is accessed directly.

Comparison of syntax between various Programming Languages

The usage syntax of methods, instance variables, getters and setters is in the table below

Programming Languages	Method call	Fields/Attributes	Getter / Setter	Comparison
ABAP (Advanced Business Application Programming)	CALL METHOD <meth> EXPORTING... <ii> =.<f i>... IMPORTING... <ei> =.<g i>... CHANGING ... <ci> =.<f i>... RECEIVING r = h EXCEPTIONS... <ei> = rc i...	data field type type	Same as other methods with the only difference that they start with SET or GET	Syntax is different for method calls and instance-variable reference
C++	<instance-variable>.method(parameters) [Eg: x.method()]	type field	Same as methods. Example below: int getHealth() const { return health; } void setHealth(int h) { health = h; }	Different syntax is used to call methods and instance-variable references
C#	<instance-variable>.method(parameters) [Eg: x.method()]	<instance-variable>.field [Eg: x.field]	They are just like methods. Example below: public string GetName() { return m_name; } public void SetName(string name) { m_name = name; }	Different syntax is used to call methods and instance-variable references
Java	<instance-variable>.method(parameters) [Eg: x.method()]	<instance-variable>.field [Eg: x.field]	They are nothing but method calls	Different syntax is used to call methods and instance-variable references
D	<instance-variable>.method(parameters) [Eg: x.method()]	<instance-variable>.field [Eg: x.field]	No need for getters and setters. It can be directly accessed. import std.stdio; class Car { float speed = 1.0; } void main(){ auto my_car = new Car(); my_car.speed = 2.5; writefln(my_car.speed); }	Syntax used for method calls and variable references is different
Objective C	output = [object methodWithOutput]	x->field	An example of getter is below #import <Cocoa/Cocoa.h> @interface Photo : NSObject { NSString* caption; NSString* photographer; } - (NSString*) caption; - (NSString*) photographer; @end An example of setter is below #import <Cocoa/Cocoa.h> @interface Photo : NSObject { NSString* caption; NSString* photographer; } - (NSString*) caption; - (NSString*) photographer; - (void) setCaption: (NSString*)input; - (void) setPhotographer: (NSString*)input; @end	Syntax used for method calls and variable references is different
Python	x.method(parameters)	x.field	Pythonic way of implementing getters and setters is to using public members and turning them into properties when needed	Syntax used for method calls and variable references is sometimes the same and different at times (when there are arguments)
Javascript	ObjectName.MethodName(Parameters);	ObjectName.Attribute	var lost = { loc : "Island", get location () { return this.loc; }, set location(val) { this.loc = val; } }; lost.location = "Another island";	Syntax is different for method calls and instance-variable reference
PHP	$ObjectName->MethodName(Parameters);	$ObjectName->Attribute	N/A. Attributes can be accessed directly.	Syntax is different for method calls and instance-variable reference
VB.NET	ObjectName.MethodName(Parameters); ObjectNmae.MethodName	ObjectName.Attribute	Public Property myProperty As String Get Return String.Empty End Get Private Set(ByVal value As String) somethingElse = value End Set End Property	Syntax may be same for method calls and instance-variable reference
Ruby	$ObjectName->MethodName(Parameters);	$ObjectName->Attribute	class Message attr_reader :message def message=(m) @message = m.dup end end class Message attr_writer :message def message @message.encode!("UTF-8") end end	Syntax is same for method calls and instance-variable reference
Lua	ObjectName.MethodName(Parameters);	ObjectName.Attribute	local index = getters and function(self, key) -- read from getter, else from fallback local func = getters[key] if func then return func(self) else return fallback[key] end end or fallback -- default to fast property reads through table local newindex = setters and function(self, key, value) -- write to setter, else to proxy local func = setters[key] if func then func(self, value) else rawset(self, key, value) end end	Syntax is different for method calls and instance-variable reference
Perl	$ObjectName->MethodName(Parameters);	$ObjectName->Attribute	BEGIN { my @attr = qw(author title number); no strict 'refs'; for my $a (@attr){ *{__PACKAGE__ . "::get_$a"} = sub { $_[0]->{$a} }; *{__PACKAGE__ . "::set_$a"} = sub { $_[0]->{$a} = $_[1] }; } }	Syntax is different for method calls and instance-variable reference

Conclusion

Using the same syntax for method calls and instance variable references have some advantages and disadvantages. One could argue that having the same syntax makes it simpler for the programmer. It makes the code lighter in terms of lines of code, thus avoiding bulky code. However, including distinct ways for using methods and variables enhances readability by enabling better understanding. This is possible because the manipulation is visible whereas if the syntax were the same(i.e. compact), the one reading has no idea about what is happening "under the hood".

References

1. Declaring and Calling Methods
2. Comparison of Programming Languages
3. Setters and Getters in ABAP
4. Getters and Setters
5. Objective C