Just as in C, C++ has four storage classes:

1. automatic (keyword auto),
2. register,
3. static, and .
4. external (keyword extern)

While the use of these storage classes remains largely the same as in C, it's worth taking a closer look at how C++ uses external and static declarations in multifile programs.

The ability to compile files separately is important when writing large programs. If we write a large program that is compiled as two separate files, then the use of an extern variable in one file tells the compiler to expect that the variable will be defined in the other file.

Static declarations allow a local variable to retain its previous value when the block is re-entered.
Another, more subtle use of static is in connection with external declarations, with which it provides a privacy mechanism that is very important for program modularity. By privacy, we mean visibility or scope restrictions on otherwise accessible variables or functions.
This use restricts the scope of the function. Static functions are visible only within the file in which they are defined. Unlike ordinary functions, which can be accessed from other files, a static function is available throughout its own file, but in no other. Again, this facility is useful in developing private modules of function definitions.