what is a member vs. a property

Question

A friend who is new to OO programming asked me the difference between a Member and Property, and I was ashamed to admit that I couldn't give him a good answer. Since properties can also be objects themselves, I was left with a general description and list of exceptions.

Can somebody please lay out a good definition of when to consider something a member vs. a property? Maybe I'm bastardizing the concept, or is it just that a member is just the internal name I use, and the property is what's exposed to other objects?

I don't think that not knowing the answer to this question has affected the quality of my programming, and it's just a semantics point, but it still bothers me that I can't explain it to him.

Answer 1

from PHP manual:

Class member variables are called "properties". You may also see them referred to using other terms such as "attributes" or "fields". They are defined by using one of the keywords public, protected, or private, followed by a normal variable declaration. This declaration may include an initialization.

Answer 2

Properties are a way to expose fields, where fields are the actual variables. For example (C#):

class Foo {
  private int field;
  public int Property {
    get { return field; }
    set { field = value; }
  }
}

Answer 3

Member is a generic term (likely originated in C++, but also defined in Java) used to denote a component of a class. Property is a broad concept used to denote a particular characteristic of a class which, once the class is instantiated, will help define the object's state.

The following passages, extracted from "Object-Oriented Analysis and Design" by Grady Booch help clarify the subject. Firstly, it's important to understand the concepts of state and behaviour:

The state of an object encompasses all of the (usually static) properties of the object plus the current (usually dynamic) values of each of these properties. By properties, we mean the totality of the object's attributes and relationships with other objects.

Behaviour is how an object acts and reacts, in terms of its state changes and message passing (methods); the outwardly visible and testable activity of an object.

So, the behaviour of an object depends on the available operations and its state (properties and their current values). Note that OOP is quite generic regarding certain nomenclature, as it varies wildly from language to language:

The terms field (Object Pascal), instance variable (Smalltalk), member object (C++), and slot (CLOS) are interchangeable, meaning a repository for part of the state of an object. Collectively, they constitute the object's structure.

An operation upon an object, defined as part of the declaration of a class. The terms message (Smalltalk), method (many OO languages), member function (C++), and operation are usually interchangeable.

But the notation introduced by the author is precise:

An attribute denotes a part of an aggregate object, and so is used during analysis as well as design to express a singular property of the class. Using the language-independent syntax, an attribute may have a name, a class, or both, and optionally a default expression: A:C=E.

An operation denotes some service provided by the class. Operations (...) are distinguished from attributes by appending parentheses or by providing the operation's complete signature, composed of return class, name, and formal arguments (if any): R N(Arguments)

In summary, you can think of members as everything that composes the class, and properties as the members (attributes) that collectively define the structure of the class, plus its relationships to other classes. When the class is instantiated, values are assigned to its properties in order to define the object's state.

Cheers