Re: 'global' data

Sat, 23 May 2009 23:18:52 -0700 (PDT)
On May 23, 7:39 pm, Jeff Schwab <> wrote: wrote:

On May 23, 8:14 am, Jeff Schwab <> wrote:

Jerry Coffin wrote:

In article <c7dd39c0-c107-4e04-8d65->,
[ ... ]

Not sure if I agree with this. Not all 'global' is bad and as suc=


requires 'encapsulation in a class and accessed through a singleton'

Quite true. I seem to recall reading a study (sorry, can't remember
exactly where) that indicated globals weren't nearly as serious of a
problem as generally believed.
At the same time, my own take on things has generally been that globa=


themselves aren't necessarily all that bad, but they tend to result f=


code that isn't particularly great either. For some things like
configuration data, globals make sense -- but in a lot of casese, the
amount of communication that makes globals more attractive also
indicates that tasks haven't been broken up relatively poorly.

That's certainly true as far as it goes, but I think global and
namespace-scoped variables, even in implementation (.cc) files, are
inherently evil enough to be worth avoiding. Globals shared among
multiple TU have their own problems, e.g. indeterminate initialization
order, but even internally linked variables can cause severe design
problems. I would only make an exception for immutable variables wi=


trivial constructors, e.g. static constants of primitive types. I a=


occasionally use namespace-scoped variables in usenet posts, but only
for brevity.

The existence of a global (or module-static) variable hard-codes its o=


uniqueness, i.e. it is a pseudo-member variable of a singleton
pseudo-object. That in itself can be a problem. Somebody writes =


parser with some module-static state, and everything is hunky-dory unt=


the need arises to parse multiple, simultaneous input files; suddenly,
multiple "instances" of all the static variables are needed. A low-=


mid-level developer is particularly screwed in this case, because
refactoring takes a significant amount of time, during which managemen=


sees no visible progress.

I view global and static variables in roughly the same way that I view
public member variables: They often indicate an inflexible design (=


you said), and they're not hard to avoid.- Hide quoted text -

I have a global deque variable here called compressedfiles.

In a function called CalculateMarshallingSize, I compress
input files in order to figure out their sizes. I'm not
ready to marshall the compressed data yet, though, so I copy
the compressed file into the global. Then when I'm ready to
marshall the data I pop it off the deque. Technically it
wouldn't be hard to avoid the global by doing the
compression once in CalculateMarshallingSize and again in
Send. That doesn't seem the right answer to me though.
So I don't know of a way to avoid the global in this case.

For my money, the following would be a step in the right direction:

     typedef std::vector<char> compressed_file;
     typedef std::deque<compressed_file> compressed_file_deque;

     compressed_file_deque& compressed_files() {
         static compressed_file_deque deq;
         return deq;

You would also have to change existing, direct accesses of the variable,
to call the function instead. Then you can change the function
definition at will without altering the code that uses the deque,
piggy-backing levels of abstraction or adding hooks for aspect-oriented

For example, in the case I mentioned above, in which parallel instances
of the module become necessary, you can have compressed_files() return
thread-local deques, so that parallel calls do not interfere with each
other. Alternatively, if the threads need to share the deque, you can
insert a lock in the function to synchronize deque access.

Once you change the rest of the module to use function-call syntax, you
won't have to touch it again, even if you decide to go back to a
module-local variable:

     class compressed_file_deque_accessor
         compressed_file_deque m_deq;

         compressed_file_deque& operator()() {
             return m_deq;

     } compressed_files;

This is one type of module-local variable I'm OK with, and should have
mentioned earlier. It still gives you a place to insert hooks, e.g.
access counts, log messages, or thread locks, but allows flexible,
function-like syntax in client code.- Hide quoted text -

- Show quoted text -

Thanks. I guess it might be a step in the right direction
and I may use it.

Brian Wood
Ebenezer Enterpries

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