Many books on programming often avoid using global variable because it is considered harmful. Why is that? Does global variable use lesser instruction because it is treated as static by compiler, so it have more performance than locals ( which is treated as stack dynamic ).



Precision is not truth!

A Global Variable is accessible by all functions in the program. This means that it may be changed anywhere in the program by mistake. If you have a large program (especially one that several porgrammers are working on) global variables can cause a lot of problems.

Local variables are visible only within the function in which they are created and cease to exist once the program returns from the function.

It is much safer to pass variables into functions as parameters. The parameters passed in have the same scope as local variables within the function. This way a much tighter control over the use of variables exists and mistakes are fewer. It is also much easier to debug a program if you have a tight control over which functions use any particular variable.

Alan

Is'nt there a slight performance gain as well, using local variables? More effecient use of memory or something like that.

Local variables are allocated on stack memory (the stack frame for the function)only when the function starts and the memory is automatically released when the function returns. This means that there are not large chunks of memory allocated that may not be used at the time.

Global variables are allocated memory in the Data Segment when the program loads and stay allocated for the whole life of the program. This may be OK if you have variables that are used by many functions and if it makes sense to have them as Globals. In general, global variables should be avoided if it is reasonable to do so.

You may also allocate Global (outside the main() function)Dynamic Variables on the Heap memory using the "new" function and the memory must be released by using the "delete" function when the program terminates. Dynamic Variables may also be allocated from within any function but they must be released before leaving the function otherwise they memory stays allocated but you may not access it because it is no longer in scope. This is a "memory leak".

Alan