Scope

Scope and Persistence of Variables / Constants

Variable / Constant Scope

In OVM variables (and constants) do not have to share a single global scope as they do in Ozterm. This allows variable names to be reused where convenient.

Consider the following example.
@integer fred = 10           ! define a global variable fred and set to 10
@comment("fred = ",fred)     ! display the value of the global fred

@begin                       ! start a new scope
    @string fred = "Hello"   ! define a local variable fred
    @comment("fred = ",fred) ! display the value of the local fred
@end                         ! end of the new scope

@comment("fred = ",fred)     ! display the value of the global fred
The first definition of fred has file or global scope (I.e. fred is available for use throughout the file). The second definition has local scope, and is not accessible after the @end command is reached.

Note. All session variables (variable substitution variables) share global scope, and are also available between files opened within the same session.

Persistance of Variables

All variables / parameters in OVM are static (ie Only a single copy of a variable exists throughout program execution). This means that the variable’s value is not lost when it goes out of scope - ie execution has left the command block the variable was defined in. This is unlike most 3GL’s where a variable’s value is lost when it goes out of scope.

Consider the following example.
@procedure proc1()
@begin
    @integer fred              ! initially fred's value will be 0
    @set fred = fred + 1       ! increment fred's value by 1
    @comment("fred = ",fred)   ! display fred's value
@end

! Now call the procedure proc1 3 times
@proc1                         ! will display fred = 1
@proc1                         ! will display fred = 2
@proc1                         ! will display fred = 3
In this example it should become clear that the previous value of fred has been remembered between calls to the procedure proc1.

A side effect of variable persistance is that recursion (where a function or procedure calls itself) will probably not work as desired as the recursive calls will interfere with each other. This also applies to the parameters of user defined procedures and functions.

Libraries

Contents

Working with OVM Streams

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