What is the difference between a property and an instance variable?

Question

I think I used these terms interchangeably / incorrectly!

Answer 1

Iain, this is mainly a matter of terminology and, despite the tag "language-agnostic" associated with this issue, is very related to the language / environment.

To discuss design, a property and an instance variable can be used interchangeably, because the idea is that the property is a data element that describes the object.

Speaking of a specific language, the two can be different. For example, in C #, a property is actually a function that returns an object, while an instance variable is a non-static member variable of a class.

Answer 2

Hershey is right about this language. But to add specific answers to the following language:

In python, an instance variable is an instance attribute (usually) that is mentioned in the instance dictionary. This is similar to elements or instance variables in Java, except that everything is open.

Properties are shortcuts to getter / setter methods that look exactly like an instance variable. Thus, in the following class definition (modified from Guido, the manifest of the new style style ):

class C(object): def __init__(self): self.y = 0 def getx(self): if self.y < 0: return 0 else: return self.y def setx(self, x): self.y = x x = property(getx, setx) >>> z = C() >>> zx = -3 >>> print zx 0 >>> print zy -3 >>> zx = 5 >>> print zx 5 >>> print zy 5 

y is an instance variable z , x is a property. (In the general case, when a property is defined, there are some methods used to obscure the associated instance variable so that other code does not directly access it.) The advantage of properties in python is that the designer does not need to go around pre-encapsulating all instance variables, since future encapsulation by converting the instance variable to a property should not violate the existing code (unless the code uses loopholes that your encapsulation is trying to fix, or assume to check the class or some other meta-programming).

All this is a very long answer, to say that at the design level it’s good to talk about properties. An agnostic about what type of encapsulation you may need to perform. I assume that this principle is not an agnostic of the language, but it applies to languages ​​next to python.

Answer 3

Sample code made in C #

 public class ClassName { private string variable; public string property { get{ return variable; } set { variable = value; } } } 

Answer 4

In object c, the property is an instance variable that can use the overloaded dot operator to call its setter and getter. So my.food = "cheeseburger" is actually interpreted as [my setFood: "cheeseburger"]. This is another case where the definition is definitely not an agnostic language, since objective-c defines the @property keyword.

Answer 5

Maybe this is because you first came from C ++ correctly ?! In my school days, I had professors who talked about class properties or class attributes all the time. As I moved into the Java world of C #, I started hearing about members. Class members, instance members ...

And then the properties appear! in Java and .NET. Therefore, I think it’s better for you to call it members. Wheather they are members of an instance (or, as you called it, an instance variable) or members of a class ....

Hooray!

Answer 6

A property can, and I suppose, basically returns an instance variable, but can do more. You can put logic in a property, aggregate values, or update other instance variables, etc. I think it's best to avoid this. Logic must go into methods.

Answer 7

In Java, we have something called JavaBeans Properties , but it is basically an instance variable that follows a specific naming pattern for its recipient and setter.

Answer 8

When added to what was said in langauge such as C #, the property is essentially a get and set function. As a result, it may have custom logic that works in addition to get / setting. An instance variable cannot do this.

Answer 9

A property is some information associated with an object. For example, the property of a circle is its diameter, and the other is its area.

An instance variable is a piece of data that is stored inside an object. This does not have to correspond directly to the property. For example, (heh), a circle can store its radius in an instance variable and calculate its diameter and area based on that radius. All three properties are still saved, but only the radius is stored in the instance variable.

Some languages ​​have the concept of "first class" properties. This means that for the client application, the property looks and is used as an instance variable. That is, instead of writing something like circle.getDiameter() , you should write circle.diameter , and instead of circle.setRadius(5) you should write circle.radius = 5 .

Answer 10

Unlike the other answers given, I find that there is a useful distinction between member variables and properties that are language-agnostics.

The difference is most obvious in component-oriented programming, which is useful anywhere, but easiest to understand in the graphical interface. In this context, I tend to think of the configuration time of component design as manipulating the “properties” of an object. For example, I select the foreground and background colors, the border style, and the font of the input text box, setting its properties. Although these properties can be changed at run time, they are usually not. At run time, another set of variables representing the contents of a field is much more read and written. I consider this information the "state" of the component.

Why is this distinction useful? When you create an abstraction for wiring components together, you usually only need to publish “state variables”. Returning to the example of a text field, you can declare an interface that provides access to the current content. But the “properties” that control the appearance of a component are defined only on a particular implementation class.