Are global variables bad?
在C / C ++中，全局变量和我的教授认为的一样糟糕吗？
The problem with global variables is that since every function has access to these, it becomes increasingly hard to figure out which functions actually read and write these variables.
To understand how the application works, you pretty much have to take into account every function which modifies the global state. That can be done, but as the application grows it will get harder to the point of being virtually impossible (or at least a complete waste of time).
If you don't rely on global variables, you can pass state around between different functions as needed. That way you stand a much better chance of understanding what each function does, as you don't need to take the global state into account.
The important thing is to remember the overall goal: clarity
The "no global variables" rule is there because most of the time, global variables make the meaning of code less clear.
However, like many rules, people remember the rule, and not what the rule was intended to do.
I've seen programs that seem to double the size of the code by passing an enormous number of parameters around simply to avoid the evil of global variables. In the end, using globals would have made the program clearer to those reading it. By mindlessly adhering to the word of the rule, the original programmer had failed the intent of the rule.
So, yes, globals are often bad. But if you feel that in the end, the intent of the programmer is made clearer by the use of global variables, then go ahead. However, remember the drop in clarity that automatically ensues when you force someone to access a second piece of code (the globals) to understand how the first piece works.
My professor used to say something like: using global variables are okay if you use them correctly. I don't think I ever got good at using them correctly, so I rarely used them at all.
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