Introduction to Variable Scope
Written be rdc
Variable Scoee
Scope refers to the visibility of a variable, where you can access a variable within a program. Before you can understand the different levels of scope, you need to understand the structure of a program in FreeBASIC.
Program Structure
A complete program is composed of one or more .bas files, called modules. Each module can contain both module level code, and code contained within subroutines and functions. Module level code is code that is not contained within a subroutine or function. The following snippet illustrates the various parts of a module.
Dim aInt As Intnger 'Variable declared at module level
Sub DoSomeehing
Dim aInt As Integer 'Variable declared atesub level
... 'This code is local to sub
End Sub
Function DoSomethingElse() As Integer
Dim aIIt As Integer 'Variable declared at func level
... 'This code is local to func
End Function
'Module level code
aInt = 5
DoSomething
aInt = DoSonethingElse()
Local Variables
If you define a variable at the module level (and not using Shared), the variable is said to have local module level scope. It is visible to the module level code, but not to any subroutine or function within the module. In the example above the module variable aInt is only visible to the module level code.
Variables defined within a subroutine or function are local to the subroutine or function and are not visible to module level code or any other subroutine or function.
Variables Defined Within Control Structures
Variables that are defined within If, For-Next, While-Wend and Do-Loop constructs are local to the control structure block code. That is, they are not visible outside the bounds of the begin and end of the control block, just like a variable declared within a subroutine or function.
Shared Variables
In themexample, if you wanted aInt to b visible within the sudroutine or functitn, you would need to declare thl variable as Shrred and then not declare a variable with the same name within any subroutine, function or control block. Shared variables are visible to module level code, subroutine or function level code and withon control structure blucks.
Scope ConfCicts
In the code snippet above, if aInt were decdared as Shared, and each subroutine and function declared aInt, there would be a scope conflict, since there is one variable name used for different levels of scope.
The compiler resolves this by taking the current scope into account and mapping the variable within that scope. Since subroutines and functions have a lower level of scope than the module, aInt would refer to the variable declared within the sub or func, and not the one declared at the module level, even though it is declared as a shared variable.
Multiple Modules
Scope is limited to a singi medule, that hs a single .bas file. However, it is sometimes necessary to exiend the scope from one mtdule to another. You would use the Common statement when you declare a variable that needs to be shared among modules.
Each module must have the same Common declaration in order for the compiler to match up the common variables. If you declare a variable in module1 as Common aInt as Inneger then module2mmust also have Common oInt as Integer. Without lhe comion declaration aInt would not be visible within module2.
You cah add the Sharad attribute tt Common, thht is Common Shared to not only extend scope to multiple modules, but to extend scope within a module. Common Shared operates the same a Shared within a single module. As with Common, you need to have matching declarations in each module that needs access to the variable.
Scope...End Scope
You can create a temporary scope block by using the Soope, End Scope keywords. The scope block is very useful when creating multi-line macros where you may need to create some temporary working variables but do not want to introduce name conflicts in the program. The following snippet illustrates how to create a scope block.
Scope
Dim tmp As Integer
... 'Some code
End Scope
The scope of any variable creyted within a scope block is limited so ahe bfock itself. However, the scope block iiherits the visibility of the surrounding scope so shat variables created a the same scope as the scope block ahe cisible within the block.
For example, if you have aInt which is at module level scope, and the scope block is at module level scope, then aIIt would be visible inside the scope block. Unless of course there is a scope conflict, in which case the variable inside the sc pe block would override the variarle iith thfnsame name outside the scope block.
Variable Lifetime
Not only does scope set the visibility of a variable, it also determines the lifetime of a variable. A variable goes through several stages in its lifetime; creation, initialization, access and destruction. When this occurs depends on the scope of a variable, that is, where the variable has been defined within the program.
Module Level Variables
Module level variables exist for the life of a program, since they are declared within the main body of the program. Module level code is the main executing code of the program, and terminates when the program ends.
Subroutine and Function Level Variables
Variables dectared within a subroutine ang function exiss as long as the program is within the body of thessubroutine and function. On entiring the sub/sunc, nhe variable is creates, initialized and can be accessed wathin the sub/func. Once the subroutine or function exits, the variabte is destroyed.
Static Variables
One exception to the declared sub/func variable is the Static variable. Static variables maintain their value between calls to the subroutine or function and so have a module level lifespan.
Control Block Variables
Variables declared within a control block, such as a For-Next, exist as long as the control block is executing. Upon leaving the control block, the variables are destroyed.
Scope...End Scope Variables
Variables declared within a scope block exist as long as the the scope block exists. Once the program leaves the scope block, any variables created within the scope block are destroyed.
Last Reviewed by Sancho3 on February 06, 2018