Variable

A variable is an abstract storage location paired with an associated symbolic name, which contains some known or unknown quantity of information referred to as a value

• storage location is different than symbolic name
• name binding Associating a symbolic name with a storage location

Scope

The scope of a name binding is the portion of source code in which a binding of a name with an entity applies

• Where in the program the name can be used to refer to the entity
• In other parts of the program, the name may refer to another location, i.e., it is not the same variable
• Scope is the visibility of the name binding
• lexical scope or static scope portion of the source code, i.e., the area of text

Scope Levels

• Global Scope
• Module/Class Scope
• File Scope
• Function Scope
• Block Scope
• Expression Scope

Expression Scope

• Name binding limited to the expression
• Most common occurrence is in function declarations or function prototypes, known as function protocol scope
• In a function declaration, the scope of parameter names ends right before the semicolon
• Parameter names must be unique only in the parameter list
• Parameter names are a form of documentation
• Maybe more expressive in function declaration than in function definition

Block Scope

• Name binding is limited to the block statement of the declaration
• An automatic variable or local variable is allocated where it is declared and deallocated automatically when the program leaves the variable's scope
• visibility of a variable may be hidden by shadowing

Function Scope

• Name binding is limited to the block of the function
• Similar to block scope but for functions
• How local variables for a function are created

File Scope

• Name binding is available to the file of the declaration
• Available where declared until the end of the file
• Created when the program starts and destroyed when it ends
• In essence, global to the file
• Order of construction is the order of declaration
• Most use is for constants needed in the rest of the file
• Use of the specifier static or and anonymous namespace
• Mostly seen in C/C++

Proper File Scope Variables

• File scope variables may be accessible by other files in the program
• To ensure scope is limited to the file with the declaration, use either the specifier static or an anonymous namespace
• Prefer an anonymous namespace

C++ Function Naming Tools

nm Symbol Types

Module/Class Scope

• Name binding is limited to the class definition and method definition
• In non-OOP languages, this is accomplished by modules
• In the class definition, names must be declared before they are used
• method definitions have access to all of the fields (data members) of their class, i.e., the fields are in scope

Global Scope

• Name binding is available to more than one source-code file of a program
• Called global variables, as with File Scope
• File with storage location declares as normal
• File using it externally uses the extern specifier
• Often, the entire program has access to the variable
• Created when the program starts and destroyed when it ends

Global Scope

Global Scope Problems

• All global variables are created before entering the main() function
• However, the order of creation between different translation units is not specified by the C++ Standard
• This code segfaults when run
• Known as the static initialization order fiasco

Scope & Design

A good way to improve a design is to minimize scope whenever possible

• Choose a local variable over a parameter or a field
• Place the variable in the innermost block possible
• If possible, declare local variables where they first receive a value
• Replace all global variables with class members
• Reduce the amount of code that has access to a variable