Variable Scope

Visibility and access rules for variables and objects

A variable's scope refers to its visibility in a program. A variable is not visible (cannot be accessed) outside the scope in which it was declared. Where and how a variable is declared determines its scope.

In FreeBASIC, there are 4 categories of scope: local, shared, common and common shared. Each of these scopes has different visibility rules, which are detailed below.

Local Scope

Variables declared in the local scope are visible only in the most local instance of the IF, FOR, SCOPE, function, or module block in which they are declared.

Sub, Function, the main body, and each compound statement implicitly define a new local scope block.
Explicitly declared variables using Dim or ReDim take the scope of the local most block in which they are declared.
Implicit variables take the scope of the the local most Scope...End Scope block in which they are first used, otherwise take the scope of the Sub, Function, or main body in which they are used.

In the local scope, there is no visibility between module-level code and function level code. Furthermore, variables dimensioned within a block decision or loop statement will only be visible within the block in which they are dimensioned. Variables declared in the local scope of a module are not visible in any of the functions within that module. Similarly, local variables declared inside functions are not visible in the module-level code, nor any other function within the module.

Variables declared inside Scope blocks may only be declared of local scope, and are not visible outside the block. Scope blocks, however, inherit the surrounding scope, so local variables declared outside the Scope block will be visible inside (see example program).

You can declare a variable to be of local scope explicitly by using the Dim statement, or implicitly by simply introducing the variable (see Implicit Declarations). The example program local.bas demonstrates visibility rules for the local scope.

local.bas
'' visible only in this module Dim As Integer local_moduleLevel1 '' OK. Print local_moduleLevel1 Scope '' OK; SCOPE Blocks inherit outer scope Print local_moduleLevel1 '' visible only in this SCOPE Block Dim As Integer local_moduleLevel2 '' OK. Print local_moduleLevel2 End Scope '' Error; can't see inner-SCOPE vars '' print local_moduleLevel2 Function some_function( ) As Integer '' visible only in this function Dim As Integer local_functionLevel '' OK. Print local_functionLevel '' Error; can't see local module-level vars '' print local_moduleLevel1 '' Error; can't see local module-level vars '' print local_moduleLevel2 Function = 0 End Function '' print local_functionLevel '' Error; can't see function_level vars End 0

Shared Scope

Variables declared in the shared scope of a module are visible to both the module and all functions of that module.

Unlike the local scope, the shared scope makes module-level variables visible to functions of that module. In other words, the module shares its declarations with its functions.

Variables can only be declared to be of shared scope at the module-level. Ie., only modules can share variables. Neither functions nor Scope blocks can declare variables in the shared scope, thus variables declared there can only be local to that function or block.

You can declare a variable to be of shared scope by using the DIM statement with the Shared keyword. The example program shared_scope.bas demonstrates visibility rules for the shared scope.

shared.bas
'' visible throughout this module Dim Shared As Integer shared_moduleLevel1 '' OK. Print shared_moduleLevel1 Scope '' OK; can see outer-scope vars Print shared_moduleLevel1 '' Error; SCOPE-level vars cannot be shared '' dim shared as integer shared_ModuleLevel2 End Scope End 0 Function some_function( ) As Integer '' OK; can see shared module-level vars Print shared_moduleLevel1 '' Error; function-level vars cannot be shared '' dim shared as integer sharedFunctionLevel Function = 0 End Function

Common Scope

Variables declared in the common scope are visible to all modules.

Variables declared with Common are visible to other modules with a matching Common variable declaration. The variable name declared must match from between modules.

module1.bas
'' compile with: '' fbc -lang qb module1.bas module2.bas '$lang: "qb" Declare Sub Print_Values() Common m1 As Integer Common m2 As Integer ' This is executed after all other modules m1 = 1 Print "Module1" Print "m1 = "; m1 ' m1 = 1 as set in this module Print "m2 = "; m2 ' m2 = 2 as set in module2 Print_Values

module2.bas
Common m1 As Integer Common m2 As Integer m2 = 2 Print "Module2" ' This is executed first Print "m1 = "; m1 ' m1 = 0 (by default) Print "m2 = "; m2 ' m2 = 2 Sub Print_Values() Print "Module2.Print_Values" Print "m1 = "; m1 ' Implicit variable = 0 Print "m2 = "; m2 ' Implicit variable = 0 End Sub

Output:

  Module2
  m1 = 0
  m2 = 2
  Module1
  m1 = 1
  m2 = 2
  Module2.Print_Values
  m1 = 0
  m2 = 0

Common Shared Scope

Variables declared in the common shared scope are visible to all modules and all functions of those modules.

Variables declared with Common are visible to other modules with a matching Common variable declaration. The variable name declared must match from between modules. Within a module the Shared declaration modifier gives the variable module scope and makes the variable visible to all subs and functions.

module3.bas
'' compile with: '' fbc module3.bas module4.bas Declare Sub Print_Values() Common m1 As Integer Common m2 As Integer '' This is executed after all other modules m1 = 1 Print "Module3" Print "m1 = "; m1 '' m1 = 1 as set in this module Print "m2 = "; m2 '' m2 = 2 as set in module2 Print_Values

module4.bas
Common Shared m1 As Integer Common Shared m2 As Integer m2 = 2 Print "Module4" '' This is executed first Print "m1 = "; m1 '' m1 = 0 (by default) Print "m2 = "; m2 '' m2 = 2 Sub Print_Values() Print "Module4.Print_Values" Print "m1 = "; m1 '' m1 = 1 Print "m2 = "; m2 '' m2 = 2 End Sub

Output:

  Module4
  m1 = 0
  m2 = 2
  Module3
  m1 = 1
  m2 = 2
  Module4.Print_Values
  m1 = 1
  m2 = 2

Example

See examples above.