Simple Variable Lifetime vs Scope
Lifetime of Simple Variable, created from declaration keyword for static memory allocation, relative to its Scopes.
Preamble:
- The Lifetime of a variable is the time period in which the variable has valid memory (the Scope referring to the program part where its name is visible). The value of a variable may change during its lifetime, but it retains some consistent value.
- The simple variables considered are the predefined variables including the raw pointers, and the fixed-length strings/arrays, but excluding the variable-length strings/arrays. Instances of simple UDT (without any dynamic data allocated) are also considered.
- The declaration keywords for static memory allocation are: 'Dim', 'Static', 'Var'.
For such variables allocated in static way as defined above, the lifetime generally matches the surrounding scope, otherwise it can be greater than this one.
Declaration syntax for a lifetime matching the surrounding scope
For such variables declared anywhere, as follows (or similar syntax):
they always have a lifetime matching their surrounding scope (global scope, or scope block, or compound statement block, or procedure scope).
With (1) or (2) syntax, the local variable is always allocated on the program stack at the time of its declarations, and is automatically deallocated when going out its scope.
With (3) or (4) or (5) syntax, the global variable is always allocated in the .BSS or .DATA section of the executable (its scopes and lifetimes begins at program creation and ends with program termination).
or
orWith (1) or (2) syntax, the local variable is always allocated on the program stack at the time of its declarations, and is automatically deallocated when going out its scope.
With (3) or (4) or (5) syntax, the global variable is always allocated in the .BSS or .DATA section of the executable (its scopes and lifetimes begins at program creation and ends with program termination).
Declaration syntax for a lifetime that may be greater than the surrounding scope
For such variables declared anywhere, as follows (or similar syntax):
they always have a lifetime equal to the program duration, so greater than their surrounding scope if there are declared in any local scope block (matching their surrounding scope if there are declared in the global scope).
The static variable is always allocated in the .BSS or .DATA section of the executable (its lifetimes begins at program creation and ends with program termination).
Interest of declaring such static variables in a compound instruction block or in a procedure scope:
they always have a lifetime equal to the program duration, so greater than their surrounding scope if there are declared in any local scope block (matching their surrounding scope if there are declared in the global scope).
The static variable is always allocated in the .BSS or .DATA section of the executable (its lifetimes begins at program creation and ends with program termination).
Interest of declaring such static variables in a compound instruction block or in a procedure scope:
- As for 'Dim', the 'Static' keyword is used in a compound statement block or in a procedure scope to declare variables whose scope stops at the end of the compound statement block or the procedure.
- However, unlike 'Dim', the lifetime differs because the variables declared with the 'Static' keyword retain their value between the successive loops of the compound instruction block or the successive calls to the procedure.
- In summary, a declared static variable has a local scope, but its lifetime is comparable to that of a global scope variable.
- So, static variables with the same name can be declared in several different compound statement blocks and in different procedure scopes. Each of these variables therefore remains independent and retains its own value in its own local scope.
- However, unlike 'Dim', the lifetime differs because the variables declared with the 'Static' keyword retain their value between the successive loops of the compound instruction block or the successive calls to the procedure.
- In summary, a declared static variable has a local scope, but its lifetime is comparable to that of a global scope variable.
- So, static variables with the same name can be declared in several different compound statement blocks and in different procedure scopes. Each of these variables therefore remains independent and retains its own value in its own local scope.
Example
Lifetime's comparison between different variables declared in a local scope:
local variable vs static variable, both declared in a procedure scope
See also
local variable vs static variable, both declared in a procedure scope
Dim Shared As ZString Ptr pzl ' global variable to memorize the local Zstring address
Dim Shared As ZString Ptr pzs ' global variable to memorize the static Zstring address
Declare Sub prntSubString (ByVal p As ZString Ptr, ByVal size As Integer)
Sub s () ' beginning of procedure scope
Dim As ZString * 15 zl = "local variable" ' declare/initialize a local Zstring
pzl = @zl ' memorize the local Zstring address
Static As ZString * 16 zs = "static variable" ' declare/initialize a static Zstring
pzs = @zs ' memorize the static Zstring address
Print " From inside the procedure scope:"
prntSubString(pzl, 14) ' display address/content of the local zstring
prntSubString(pzs, 15) ' display address/content of the static zstring
End Sub ' end of procedure scope
Print "Lifetimes comparison between local/static variables declared in a local scope:"
s() ' call the procedure
Print " From outside the procedure scope:"
prntSubString(pzl, 14) ' display address/content of the local zstring after going out its scope
prntSubString(pzs, 15) ' display address/content of the static zstring after going out its scope
Sleep
Sub prntSubString (ByVal p As ZString Ptr, ByVal size As Integer)
Print , "&h" & Hex(p, SizeOf(Any Ptr) * 2),
Print """";
For I As Integer = 0 To size - 1
Dim As UByte u = (*p)[I]
If u < Asc(" ") Then
Print " ";
Else
Print Chr(u);
End If
Next I
Print """"
End Sub
Output example:Dim Shared As ZString Ptr pzs ' global variable to memorize the static Zstring address
Declare Sub prntSubString (ByVal p As ZString Ptr, ByVal size As Integer)
Sub s () ' beginning of procedure scope
Dim As ZString * 15 zl = "local variable" ' declare/initialize a local Zstring
pzl = @zl ' memorize the local Zstring address
Static As ZString * 16 zs = "static variable" ' declare/initialize a static Zstring
pzs = @zs ' memorize the static Zstring address
Print " From inside the procedure scope:"
prntSubString(pzl, 14) ' display address/content of the local zstring
prntSubString(pzs, 15) ' display address/content of the static zstring
End Sub ' end of procedure scope
Print "Lifetimes comparison between local/static variables declared in a local scope:"
s() ' call the procedure
Print " From outside the procedure scope:"
prntSubString(pzl, 14) ' display address/content of the local zstring after going out its scope
prntSubString(pzs, 15) ' display address/content of the static zstring after going out its scope
Sleep
Sub prntSubString (ByVal p As ZString Ptr, ByVal size As Integer)
Print , "&h" & Hex(p, SizeOf(Any Ptr) * 2),
Print """";
For I As Integer = 0 To size - 1
Dim As UByte u = (*p)[I]
If u < Asc(" ") Then
Print " ";
Else
Print Chr(u);
End If
Next I
Print """"
End Sub
Lifetimes comparison between local/static variables declared in a local scope: From inside the procedure scope: &h0019FE74 "local variable" &h00407004 "static variable" From outside the procedure scope: &h0019FE74 " p@ Çp@ ¿■ Y " &h00407004 "static variable"
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