Are raw C++ pointers first class objects?

Question

According to Wikipedia:

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)

Somebody had once told me that raw pointers are not first class objects while smart pointers like std::auto_ptr are. But to me, a raw pointer (to an object or to a function) in C++ does seem to me to satisfy the conditions stated above to qualify as a first class object. Am I missing something?

Answer 1

The definition in Wikipedia of "first-class" is incorrect. In a programming language, an entity is considered to be "first-class" if it does not require any special treatment by the compiler and can be interpreted, understood, used, etc. just like any other object in the language. The reason pointers in C++ are not first class objects is that *p for a pointer p is not interpreted as simply invoking an overloaded operator* like it would be for any other object; instead, the compiler has to treat p specially based on the fact that it is a pointer type. When passing a pointer or a reference you cannot simply pass any value object (and some of those value objects happen to be pointers), but you are actually saying that it is a different type (a pointer type or a reference type, in place of a value type), and its interpretation / usage is subject to its type.

A good example of a language where all objects are first-class is Python. In Python, everything has some sort of type associated with it, it can be treated as an object, it can have members, functions, etc. As an extreme example, even functions in Python are objects that simply contain code and happen to be callable; in fact, if you try to call the function using the __call__ member function (which is akin to overloading operator() in C++, and which other non-function objects can provide), an ordinary function will return a function wrapper as a result (so that you don't have to treat functions as a special case). You can even add members to functions like you would to other objects. Here is an example of such a thing:

>>> def f(x):
...     return x;
... 
>>> func = f;
>>> print f.__class__

>>> print f(5)
5
>>> print f.__call__(5)
5
>>> f.myOwnMember = "Hello world!"
>>> print f.myOwnMember
Hello world!

I apologize for the Python in a C++ post, but it is hard to explain the concept without the contrast.