What is the difference between external variables and global variables in C++?

In C++, external variables and global variables refer to variables declared outside of a function, and their differences are as follows:

1. Scope: The scope of global variables is the entire program, and they can be accessed and used anywhere in the program. The scope of external variables is from the declaration until the end of the program file, and they cannot be directly accessed in other files.
2. Visibility: Global variables can be accessed and used by any function in the program. External variables can only be accessed directly by functions in the file where they are defined, and cannot be accessed directly by functions in other files.
3. Storage location: Global variables are stored in the static storage area, allocated at program startup and released at program end. External variables are stored in the stack, allocated when a function is called and released when the function returns.
4. Lifetime: The lifetime of global variables lasts throughout the execution of the entire program, even after a function call ends. External variables have a lifetime that is valid within their file and will be destroyed once the function call ends.
5. Global variables can be initialized when declared, and will automatically be initialized to 0 if not explicitly initialized. External variables cannot be initialized when defined, but can only be initialized outside of functions.

It is important to note that global variables and external variables have global visibility, so it is advisable to avoid excessive use of them in program design in order to prevent naming conflicts and decrease the readability and maintainability of the program. It is recommended to use local variables and parameters to limit the scope of variables, thus enhancing the robustness and reusability of the program.