Global variables should be avoided for several reasons, but the primary reason is because they increase your program’s complexity immensely. For example, say you were examining a program and you wanted to know what a variable named g_nValue was used for. Because g_nValue is a global, and globals can be used anywhere in the entire program, you’d have to examine every single line of every single file! In a computer program with hundreds of files and millions of lines of code, you can imagine how long this would take!Second, global variables are dangerous because their values can be changed by any function that is called, and there is no easy way for the programmer to know that this will happen. Consider the following program:// declare global variableint g_nMode = 1; void doSomething(){ g_nMode = 2;} int main(){ g_nMode = 1; doSomething(); // Programmer expects g_nMode to be 1 // But doSomething changed it to 2! if (g_nMode == 1) cout << "No threat detected." << endl; else cout << "Launching nuclear missiles..." << endl; return 0;}Note that the programmer set g_nMode to 1, and then called doSomething(). Unless the programmer had explicit knowledge that doSomething() was going to change the value of g_nMode, he or she was probably not expecting doSomething() to change the value! Consequently, the rest of main() doesn’t work like the programmer expects (and the world is obliterated).Global variables make every function call potentially dangerous, and the programmer has no easy way of knowing which ones are dangerous and which ones aren’t! Local variables are much safer because other functions can not affect them directly. Consequently, global variables should not be used unless there is a very good reason! C++
Topic: Variable
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