From: Jano Svitok <jan.svitok@...>
Date: 2007-08-08T20:51:04+09:00
Subject: Re: non-constant strings

On 8/8/07, Dmitry Bilunov <kmeaw@kmeaw.com> wrote:
> dohzya wrote:
> > Hi,
> >
> > 2007/8/7, Dmitry Bilunov <kmeaw@kmeaw.com>:
> >>   # Creates an instance. Guaranties, that a command is safe.
> >>   end
> >>
> >>
> >>
> > You want to freeze the object @command, not the class String, don't
> > you ?
> > So you don't need to use an Immutable_String class, just add
> > @command.freeze in your code
>
> It will make impossible changing @command inside instance methods (which
> can be guaranteed to be safe). If there is a way to unfreeze a frozen
> object, then this method could be called externally and one can gain
> write access to an internal instance variable.

Not exactly.

@command is just a pointer to a string, incidentally pointing to same
string as command pointer points to.

So you have three things: 1. the string "safe_command" 2. variable
(=pointer) command, 3. member variable (=pointer) @command.

Now, if you issue @command.freeze, you are not freezing the pointer
@command, but the string it points to, i.e. "safe_command". So if you
do @command.freeze, it is the same as doing "safe_pointer".freeze.

After @command.freeze, you cannot do command.replace - because command
points to a frozen string. However, you can still do command = "abc",
i.e. make command point to another string.

Finally after @command.freeze, you can't do @command.replace either.
But you can do @command = "adsfg" and thus make the pointer point to
another string.

In fact, Hash#[]= dups and freezes String keys, just for the same reason.

Jano