Why people dislike global variables so much while I find them so convenient?

[Ola Fosheim Grøstad]

On Tuesday, 25 January 2022 at 09:53:25 UTC, rempas wrote:
> So can someone make examples about how global variables can 
> mess me up. I know that probably everyone here has more 
> personal experience than me so I really want to learn why 
> global variables are considered so harmful.

Harder to debug. Harder to reason about as your program grows in 
size. Harder to scale your program to higher degree of 
concurrency. But there is a place and room for everything.

If you use globals you should keep them local to one object-file 
and limit access to accessor functions to make it easier to debug 
your code.

If you only use globals because you want to have fewer parameters 
then the common approach is to instead use a context-object and 
pass that as single parameter.

Unfortunately, D made the mistake of making thread-local the 
default, and claimed this was a great improvement. Thread local 
globals are basically something you only want to use in runtime 
and framework "kernel" like code. That gets you an additional 
layer of issues, including performance.

If you use globals, make sure you either mark them appropriately 
so that you don't get thread local globals.