Introduction to Global Variables

Global variables are accessible from any part of the program, but they are often discouraged due to potential issues. The focus is usually on non-const global variables.

The Problem with (non-const) Global Variables

• The biggest issue is their ability to be changed by any function leading to unpredictable program state.
• Example: Changing a global variable without the programmer's knowledge can lead to unexpected outcomes.

int g_mode;

void doSomething() {
    g_mode = 2;
}

int main() {
    g_mode = 1;
    doSomething();
    
    if (g_mode == 1) {
        // Programmer expects g_mode to be 1, but it's changed to 2
        std::cout << "No threat detected.\n";
    } else {
        std::cout << "Launching nuclear missiles...\n";
    }
    
    return 0;
}

Drawbacks of (non-const) Global Variables

• Debugging challenges: Locating all places where a global variable is modified can be difficult.
• Reduced modularity: Global variables make the program less modular and flexible.
• Difficulty in understanding: The usage of global variables may require examining the entire program.

Best Practice: Avoid (non-const) Global Variables:

• The advice is to use local variables instead of global variables whenever possible.
• Global variables should only be used if there's a single instance of what the variable represents.