A question on modification of a list via a function invocation

[Steve D'Aprano]

On Tue, 5 Sep 2017 11:37 pm, Gregory Ewing wrote:

> Dennis Lee Bieber wrote:
>> Pascal, probably Modula-2, Visual BASIC are closer to the C++ reference
>> semantics, in that the definition of a function declares how the
>> argument(s) are passed.
> 
> Well, sort of. In Pascal and Modula, and also VB I think,
> parameters are the only things that can be declared as having
> reference semantics, whereas references in C++ are first-class
> things that can be stored in any variable.

No, they aren't first-class. 

- It is not possible to refer to a reference after it is defined; any 
  occurrence of its name refers directly to the object it references.

- Since you cannot refer directly to a reference, but only the object
  it points to, you cannot have a reference to a reference.

- Containers of references are not allowed.

- Once a reference to an object is created, it cannot be changed to 
  reference another object ("to be reseated").

The last is only a limitation if you think of references as mutable pointers in
the C sense. But if you think of them as objects in the Python sense, that
makes them merely immutable.

But the inability to refer to the reference itself, the lack of references to
references, and the inability to have a container of references, makes them
second-class values -- or possibly not values at all.

(I don't know enough about C++ to distinguish between the last two opinions, but
I'm strongly leaning towards "not values at all".)



-- 
Steve
“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.