What are "first-class" objects?

Question

In short, it means there are no restrictions on the object's use. It's the same as any other object.

A first class object is an entity that can be dynamically created, destroyed, passed to a function, returned as a value, and have all the rights as other variables in the programming language have.

Depending on the language, this can imply:

• being expressible as an anonymous literal value
• being storable in variables
• being storable in data structures
• having an intrinsic identity (independent of any given name)
• being comparable for equality with other entities
• being passable as a parameter to a procedure/function
• being returnable as the result of a procedure/function
• being constructible at runtime
• being printable
• being readable
• being transmissible among distributed processes
• being storable outside running processes

Source.

In C++ functions themselves are not first class objects, however:

• You can override the '()' operator making it possible to have an object function, which is first class.
• Function pointers are first class.
• boost bind, lambda and function do offer first class functions

In C++, classes are not first class objects but instances of those classes are. In Python both the classes and the objects are first class objects. (See this answer for more details about classes as objects).

Here is an example of Javascript first class functions:

// f: function that takes a number and returns a number
// deltaX: small positive number
// returns a function that is an approximate derivative of f
function makeDerivative( f, deltaX )
{
    var deriv = function(x)
    { 
       return ( f(x + deltaX) - f(x) )/ deltaX;
    }
    return deriv;
}
var cos = makeDerivative( Math.sin, 0.000001);
// cos(0)     ~> 1
// cos(pi/2)  ~> 0

Source.

Entities that are not first class objects are referred to as second-class objects. Functions in C++ are second class because they can't be dynamically created.

Regarding the edit:

EDIT. When one says "everything is an object" (like in Python), does he indeed mean that "everything is first-class"?

The term object can be used loosely and doesn't imply being first class. And it would probably make more sense to call the whole concept 'first class entities'. But in Python they do aim to make everything first class. I believe the intent of the person who made your statement meant first class.

"When one says "everything is an object" (like in Python), does he indeed mean that "everything is first-class"?"

Yes.

Everything in Python is a proper object. Even things that are "primitive types" in other languages.

You find that an object like 2 actually has a fairly rich and sophisticated interface.

>>> dir(2)
['__abs__', '__add__', '__and__', '__class__', '__cmp__', '__coerce__', '__delattr__', '__div__', '__divmod__', '__doc__', '__float__', '__floordiv__', '__getattribute__', '__getnewargs__', '__hash__', '__hex__', '__index__', '__init__', '__int__', '__invert__', '__long__', '__lshift__', '__mod__', '__mul__', '__neg__', '__new__', '__nonzero__', '__oct__', '__or__', '__pos__', '__pow__', '__radd__', '__rand__', '__rdiv__', '__rdivmod__', '__reduce__', '__reduce_ex__', '__repr__', '__rfloordiv__', '__rlshift__', '__rmod__', '__rmul__', '__ror__', '__rpow__', '__rrshift__', '__rshift__', '__rsub__', '__rtruediv__', '__rxor__', '__setattr__', '__str__', '__sub__', '__truediv__', '__xor__']

Because everything's a first-class object in Python, there are relatively few obscure special cases.

In Java, for example, there are primitive types (int, bool, double, char) that aren't proper objects. That's why Java has to introduce Integer, Boolean, Double, and Character as first-class types. This can be hard to teach to beginners -- it isn't obvious why both a primitive type and a class have to exist side-by-side.

It also means that an object's class is -- itself -- an object. This is different from C++, where the classes don't always have a distinct existence at run-time.

The type of 2 is the type 'int' object, which has methods, attributes, and a type.

>>> type(2)
<class 'int'>

The type of a built-in type like int is the type 'type' object. This has methods and attributes, also.

>>> type(type(2))
<class 'type'>

“First class” means you can operate on them in the usual manner. Most of the times, this just means you can pass these first-class citizens as arguments to functions, or return them from functions.

This is self-evident for objects but not always so evident for functions, or even classes:

void f(int n) { return n * 2; }

void g(Action<int> a, int n) { return a(n); }

// Now call g and pass f:

g(f, 10); // = 20

This is an example in C# where functions actually aren't first-class objects. The above code therefore uses a small workaround (namely a generic delegate called Action<>) to pass a function as an argument. Other languages, such as Ruby or Python, allow treating even classes and code blocks as normal variables (or in the case of Ruby, constants).

From a slide in Structure and Interpretation of Computer Programs, lecture 2A (1986), which in turns quotes Christopher Stracey:

The rights and privileges of first-class citizens:

• To be named by variables.
• To be passed as arguments to procedures.
• To be returned as values of procedures.
• To be incorporated into data structures