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From: Beman Dawes (bdawes_at_[hidden])
Date: 2002-03-12 15:17:47

At 06:01 PM 3/11/2002, dylan_nicholson wrote:

>--- In boost_at_y..., Beman Dawes <bdawes_at_a...> wrote:
>> At 08:50 PM 3/10/2002, dylan_nicholson wrote:
>> Many of those O/S properties are constants, not variables, so don't
>> the same set of problems. The existence of a file (and even
>whether it is
>> a file or directory, since that may change unexpectedly too) is in
>effect a
>> global variable; that's why error handling is so important in this
>> other I/O related libraries.
>Currently logged-in user is not constant under most modern OS's, nor
>is active window, nor is the environment, nor is current
>directory...etc. etc.
>And why is the existence (or indeed the contents) of a file an "ok"
>global variable to access/modify, but current directory not?

None of those are "OK". They are all dangerous in proportion to the
likelihood that they change unexpectedly.

> As it it I agree that 90%, maybe even
>95% of global variable uses are best avoided, and all global data
>should be used with extreme caution. But I honestly can't believe
>too many people have seriously thought about what *no global data at
>all* implies.

It isn't all global data. Constants are harmless. Global variables are
the problem.

>> And the global state issues are well-know; Boost just added a new
>> to help deal with that problem.
>Which library is that?

Daryle Walker's I/O state saving library.

But my point is, let's say my application
>uses several 'global' resources:
>* files in the current directory
>* standard output device
>* standard input device
>* standard error device
>* a log file
>* environment variables
>* an event dispatching table that keeps track of objects that recieve
>certain events
>* the current user id
>* configuration files specifying locations for various resources
>* current user preferences for the font/colour/size of GUI widgets
>etc. etc.
>Are you suggesting that *ALL* this information should be passed to
>every function that might ever need it (including obviously functions
>that don't need it directly, but make use of others that do).

Of course not. Provide a function that returns a constant value. Or save
and restore the state involved - that amounts to turning a variable into a
constant. Or write code that assumes the global variable changes value
between accesses. Sometimes it is harder, sometimes easier.

But the real issue isn't global variables in general, it is setting/getting
current working directory.

So far, the little bit of programming I've done using initial_directory()
rather than current_directory() has gone smoothly. That's the proof of the
pudding. Is the code better or worse because it doesn't set/get current
directory. So far, it seems easier to write, to read, and more
robust. It was easier because it didn't require keeping a mental image of
where in the directory tree "current" was. The variable names were more
indicative. But far more test cases are needed.


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