c++ - When are global variables actually considered good/recommended practice?

Question

I've been reading a lot about why global variables are bad and why they should not be used. And yet most of the commonly used programming languages support globals in some way.

So my question is what is the reason global variables are still needed, do they offer some unique and irreplaceable advantage that cannot be implemented alternatively? Are there any benefits to global addressing compared to user specified custom indirection to retrieve an object out of its local scope?

As far as I understand, in modern programming languages, global addressing comes with the same performance penalty as calculating every offset from a memory address, whether it is an offset from the beginning of the "global" user memory or an offset from a this or any other pointer. So in terms of performance, the user can fake globals in the narrow cases they are needed using common pointer indirection without losing performance to real global variables. So what else? Are global variables really needed?

Answer 1

Global variables aren't generally bad because of their performance, they're bad because in significantly sized programs, they make it hard to encapsulate everything - there's information "leakage" which can often make it very difficult to figure out what's going on.

Basically the scope of your variables should be only what's required for your code to both work and be relatively easy to understand, and no more. Having global variables in a program which prints out the twelve-times tables is manageable, having them in a multi-million line accounting program is not so good.