Global variables (again)

Question

I keep hearing that global variables should never be used, but I have a tendency to dismiss \"never\" rules as hot-headed. Ar

Answer 1

Like any other design decision, using global variables has a cost. It saves you having to pass variables unnecessarily, and allows you to share state among running functions. But it also has the potential to make your code hard to follow, and to reuse.

Some applications, like embedded systems, use global variables regularly. For them, the added speed of not having to pass the variable or even a pointer into the activation record, and the simplicity, makes it a good decision [arguably]. But their code suffers for it; it is often hard to follow execution and developing systems with increasing complexity becomes more and more difficult.

In a large system, consisting of heterogeneous components, using globals may become a nightmare to maintain. At some point you may need a different screen buffer with different properties, or the screen buffer may not be available until it's initialized meaning you'll have to wrap every call to it with a check if it's null, or you'll need to write multithreaded code, and the global will require a lock.

In short, you are free to use global vars while your application is small enough to manage. When it starts to grow, they will become a liability, and will either require refactoring to remove, or will cripple the programs growth (in terms of capability or stability). The admonition not to use them stems from years of hard-learned lessons, not programmer "hot-headedness".