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.
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.
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
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
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
module2.bas
Output:
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
module4.bas
Output:
See examples above.
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.
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
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
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
'$lang: "qb"
Declare Sub Print_Values()
Common m1 As Integer
Common m2 As Integer
' This is executed after all other modules
m1 = 1
Print "Module1"
Print "m1 = "; m1 ' m1 = 1 as set in this module
Print "m2 = "; m2 ' m2 = 2 as set in module2
Print_Values
'' fbc -lang qb module1.bas module2.bas
'$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
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
'' 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
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.
See also