What are the rules for using global variables in C++?

Global variables in C++ are variables defined outside of functions, and can be used anywhere in the program. Here are some rules regarding the use of global variables in C++.

1. Global variables can be accessed throughout the whole program after they are declared.
2. Global variables exist throughout the entire execution of the program, with memory allocated at the start of the program and released only when the program ends.
3. foreign
4. external
5. Global variables can be accessed within a function by directly referencing them or using the scope resolution operator (::).
6. Global variables should be used with caution as they can be modified anywhere in the program. Excessive use of global variables can lower the readability and maintainability of the code.

It is important to note that global variables may have thread safety issues in multi-threaded or concurrent programming. Therefore, it is recommended to avoid using global variables in a multi-threaded environment or to utilize synchronization mechanisms to protect the access of global variables.