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I have seen many times statements like- "Please make this feature a first class citizen in so and so language/platform". For example, it is said about enums in C#/.net. So, when is a feature considered a "First class citizen" in a programming language/platform?

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Fantastic question. – Mark Canlas Jan 25 '11 at 5:33
@Mark - True, but does it not belong on SO? – bjarkef Jan 25 '11 at 7:32
I've always seen the term used rather subjectively, and never thought to apply any kind of rigorous definition to it. (I don't see the benefit of such a definition, even if you could find one most people could agree on.) – Fred Nurk Jan 25 '11 at 10:24
-1: Since this is already a wikipedia topic, it doesn't seem all that relevant here. Perhaps if you quoted the wikipedia entry to ask for clarification on the specific points that confused you, it would be a better question. – S.Lott Jan 25 '11 at 10:56
@S.Lott I didn't know about the wikipedia entry. And I have seen questions having wikipedia entry. Because the asker didn't know about entry. – Gulshan Jan 25 '11 at 11:24
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5 Answers

up vote 14 down vote accepted

Definition

An object is first-class when it:

  • can be stored in variables and data structures
  • can be passed as a parameter to a subroutine
  • can be returned as the result of a subroutine
  • can be constructed at runtime
  • has intrinsic identity (independent of any given name)

The term "object" is used loosely here, not necessarily referring to objects in object-oriented programming. The simplest scalar data types, such as integer and floating-point numbers, are nearly always first-class.

http://en.wikipedia.org/wiki/First_class_object

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So, what makes enums second class object in .net/C#? – Gulshan Jan 25 '11 at 8:17
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@Gulshan - you could argue the lack of intrinsic identity - C# enums are basically just syntactic sugar (i.e. a "given name") for an integer value. Compare with Java, where the enums are objects in their own right. – mikera Jan 25 '11 at 8:24
@mikera, in .NET enums are values in their own right. Java just does not have any values, only objects, that's the only difference. – SK-logic Jan 25 '11 at 11:08
@mikera: Though that prevents Java's enums from having some nice properties such as being able to represent bit fields with them. While their implementation is probably more first-class-y most of their APIs still has plenty of integer (or string) constants and many uses of those cannot easily be replaced with enums. – Joey Jan 25 '11 at 13:37
I don't think enums can be constructed at runtime in .Net, can they? I thought they were always constants. – travis Jan 25 '11 at 15:54
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up vote 12 down vote

The notion of "first-class citizen" or "first-class element" in a programming language was introduced by British computer scientist Christopher Strachey in the 1960s in the context of first-class functions. The most famous formulation of this principle is probably in Structure and Interpretation of Computer Programs by Gerald Jay Sussman and Harry Abelson:

  • They may be named by variables.
  • They may be passed as arguments to procedures.
  • They may be returned as the results of procedures.
  • They may be included in data structures.

Basically, it means that you can do with this programming language element everything that you can do with all other elements in the programming language.

It's all about "equal rights": you can do all of the above, with, say, integers, so why should any other thing be different?

The definition above is a bit restrictive in the sense that it only really talks about the aspect of first-classness as related to being objects of the program. A more general definition would be that a thing is first-class if you can do everything with it you can also do with other things of similar kind.

For example, Java operators and Java methods are of similar kind. You can define new methods, you can (somewhat) freely choose the names of your own methods, you can override methods, you can overload methods. James Gosling can do all of that with operators, too, but you and I can't. I mean, contrary to popular belief, Java does support operator overloading: for example, the + operator is overloaded for byte, short, int, long, float, double and String, and IIRC in Java 7 also for BigInteger and BigDecimal (and probably a couple I forgot), it's just that you don't have any influence over it. That clearly makes operators second-class according to this second definition. Note that methods still aren't first-class objects according to the first definition, though. (Does that make operators third-class?)

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+1 for history. – Ben Jan 25 '11 at 16:45
up vote 6 down vote

Usually this refers to a construct that is passable as a parameter, can be defined as a return type from a function or can be assigned a value. Normally you need to be able to construct them at runtime. For example an instance of a class would be a first class citizen in c++ or java, but a function in C would not be.

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What makes a class a first class citizen in c++ ? – bjarkef Jan 25 '11 at 7:33
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@bjarkef: Sounds like that was already answered by its matching the description offered in the preceding sentences. – Jonathan Hobbs Jan 25 '11 at 7:47
@Jonathan: Yes, sorry, I misread the "construct them at runtime". Yes you can construct an instance of a class at runtime (an object), but not the class itself. That is what confused me. – bjarkef Jan 25 '11 at 7:53
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Passing by parameter is still not enough. In C/C++ I would still consider functions as second class citizens. They can be passed as parameters, returned as results placed inside other objects. But they can not be manipulated without the help of other constructs (like std::bind is required to bind parameters to a function). – Loki Astari Jan 25 '11 at 8:13
@Martin I never said that functions were first class citizens in C/C++. – Pemdas Jan 25 '11 at 15:21
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up vote 1 down vote

I would say a feature is a first class citizen if it is implemented solely by the language.
i.e. it does not require multiple language features or a standard library to implement that feature.

Example:

In C/C++ I do not consider functions to be a first class citizen (others may).
This is because there are ways to manipulate functions that are nut supported directly by the language but require the use of other language features. Binding parameters to a function is not directly supported and you must build a functor to implement this feature.

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Wouldn't that make bound functions (or "closures") not be first-class, while functions themselves are? How does 0x's support for closures factor in to your analysis? – Fred Nurk Jan 25 '11 at 10:19
@Fred Nurk: It all depends on the language. In some languages closures are first class systems. In others not. I am not familiar enough with C++0x yet to make an explicit comment. – Loki Astari Jan 25 '11 at 16:18
Let's say the language is either C or C++ (but not 0x), as in you answer. Wouldn't your definition of "first-class" make bound functions (or "closures") not be first-class, while functions themselves are? – Fred Nurk Jan 26 '11 at 1:03
@Fred Nurk: If you limit the only thing you can do with a function is make them a closure, then sure. But to me that's like saying if you platform supports integers addition only by importing a library. Then integers are first class citizens but addition of integers is not considered. In my view closure is an operation that can be performed on a function that effectively returns a new function (but it depends how you define it). But closure and binding are only two operations how many others are we excluding from the discussion (I am not sure that was a question). – Loki Astari Jan 26 '11 at 1:16
@Martin: I must not be explaining myself clearly. Given "a feature is a first class citizen if it is implemented solely by the language", then functions in both C and C++ are implemented solely by the language and would thus be first-class. Bound functions (which can also be called "closures") are what you're talking about with binding parameters, etc., but that's a different feature. – Fred Nurk Jan 26 '11 at 1:19
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up vote 0 down vote

To add an example to the answers already provided:

In WCF/C# you currently have to mark a class object with a service contract attribute to have it operate as a service. There is no such thing as: 

public **service** MyService (in relation public **class** MyClass).

A class is a first class citizen in c#, where a service is not.

Hope this helps

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