Example: Struct Basics

This ProcScript example demonstrates how multiple references access the same Struct, and that a Struct is deleted when there is no longer a struct variable that refers to it or any of its parents.

entry STRUCT_BASICS
variables
  struct vStruct, vPerson  ; declare struct variables
endvariables

; --- Build up a Struct ---
; Create a Struct
  vStruct = $newstruct         

; Create Struct members:       
  vStruct->group = $newstruct  
  vStruct->group->person = $newstruct
  vStruct->group->person->firstname = "John"
  vStruct->group->person->email = "john@home.com"

; Display the Struct in the message frame:
  call printHeader("STRUCT_BASICS")   ; display entry header in the message frame
  putmess "Struct referred to by vStruct: "
  putmess vStruct->$dbgstring  

; --- Update the Struct using a different variable ---
  vPerson = vStruct->group->person 

  putmess "Struct referred to by vPerson: "
  putmess vPerson->$dbgstring  

  vPerson->email = "john.smith@home.com" 

  putmess "Struct referred to by vStruct: "
  putmess vStruct->$dbgstring  
  putmess "Note: 'email' changed with vPerson is also changed in vStruct: "

; --- Delete a Struct ---
  ; Display the parent of vPerson 
  putmess "The parent of vPerson is: [%%(vPerson->$parent->$name)%%%]"

  ; Assign an empty Struct to vStruct, and display the Structs 
  vStruct = $newstruct         
  putmess "After assigning a new value to vStruct, vStruct points to empty Struct:"
  putmess vStruct->$dbgstring
  putmess "vPerson is still valid: [%%(vPerson->firstname)%%%]"
  putmess "but the parent of vPerson is now invalid: [%%(vPerson->$parent->$name)%%%]"
  putmess "resulting in a ProcScript error: %%$procerror: %%$item("DESCRIPTION", %\
    $procerrorcontext)"
  
end ; entry STRUCT_BASICS

The code builds up the Struct, updates the Struct using a different struct variable, then deletes a Struct.

Create a new empty Struct and assigns it to the struct variable. However, this is not strictly required. The Struct is created on the fly when a member is assigned to the variable.
Create the members of a Struct. This must be done explicity; members are not created on the fly.
Display a representation of the Struct in the message frame using the $dbgString (or $dbgstringplain) Struct function.
```
Struct referred to by vStruct: 
[]
  [group]
    [person]
      [firstname] = "John"
      [email] = "john@home.com"
```
Get a reference to the person node and assign it to the struct variable vPerson.

Display a representation of the person Struct in the message frame:

Struct referred to by vPerson: 
 [person]
  [firstname] = "John"
  [email] = "john@home.com"

Change the value of the email member of the person node refered to by vPerson.

Display a representation of the main Struct (vStruct) in the message frame:

Member 'email' changed with vPerson is also changed in vStruct: 
[]
  [group]
    [person]
      [firstname] = "John"
      [email] = "john.smith@home.com"

Show that the parent Struct of the person member can be accessed using vPerson.
Note: When using the -> operator in string substitutions, the expression must be enclosed within parentheses.
```
The parent of vPerson is: [group]
```
Assign a new member to the vStruct variable. This deletes the Struct that it referred to before. The person Struct remains but has lost its outer context (its direct parent and above), so an attempt to get its parent results in an error. After assigning a new value to vStruct:
vStruct points to empty Struct: [] = ""
but the parent of vPerson is now invalid: []
resulting in a ProcScript error: -84: Not a valid Handle or Struct specified