Globals can make a program hard to reason about.

For example, if you have a function, and if the behavior of that function
depends on its parameters that's one thing. But if its behavior also depends
on global variables which are themselves changed in far-flung locations in
your application then it becomes much more difficult to analyze that
function and predict its behavior.

Some here might tell you never to use them. That's not bad advice.

I tend to use them sparingly for truly static quantities, truly global to an
application, which once set, don't ever change and don't require
synchronized access. But then, I'm an old hacker. :-)

For example, in a windowed application, I'll often keep the handle to the
application's main window in a global variable. Of course, in a function
that I intend to share across projects, the window handle will be a
parameter.

Regards,
Will