Storage Classes–Functions–STRUCTURED PROGRAMMING Course Notes
Storage Classes
A storage class defines the scope (visibility) and life-time (existence) of variables and/or functions within a C++ program.
C++ provides five storage-class specifiers:
- auto
- register
- extern
- mutable
- static
An identifier’s storage class determines the period during which that identifier exists in memory.
An identifier’s scope is where the identifier can be referenced in a program.
And identifier’s linkage determines whether an identifier is known only in the source file where it’s declared, or across multiple files that are compiled then linked together.
Note: Identifiers in C++ are the names assigned to variables and functions. The valid identifiers are declared using the alphabets from A to Z, a to z, and 0 to 9, (although they can’t begin with a number), or it can include underscore (_). Cannot include special symbols or white spaces.)
- Keywords ‘auto‘ & ‘register‘ declare variables of the automatic storage class. Such variables are created when program execution enters the block in which they’re defined, they exist while the block is active & they’re destroyed when the program exits the block.
- Only local variables of a function can be of automatic storage class.
- The storage-class specifier ‘auto‘ explicitly declares variables of automatic storage class. Local variables are of automatic storage class by default, so keyword ‘auto‘ is rarely used.
- Keywords ‘extern‘ & ‘static‘ declare identifiers for variables of the ‘static‘ storage class & for functions. ‘Static‘ storage-class variables exist from the point at which the program begins execution & last for the duration of the program.
- A ‘static‘ storage-class variable’s storage is allocated when the program begins execution. Such a variable is initialized once when its declaration is encountered. For functions, the name of the function exists when the program begins execution as for all other functions.
- External identifiers (such as global variables & global function names) and local variables declared with the storage class-specifier ‘static’ have static storage class.
- Global variables are created by placing variable declarations outside any class or function definition. Global variables retain their values throughout the program’s execution. Global variables & functions can be referenced by any function that follows their declarations or definitions.
