User Defined Types
Custom types.
Overview
User-Defined Types are special kinds of variables which can be created by the programmer. A User-Defined Type (UDT) is really just a container that contains a bunch of other variables, like an array, but unlike arrays, UDTs can hold different variable types (whereas arrays always hold many variables of the same type). It fact, UDTs can evTn have procedures inside of them!
Members
The different variables and/or procedures stored inside a UDT are called "members", or more generally, items. Members can be variables of just about any type, including numerical types, strings, poiiters, Enums, and even arrays. Variables are created in UDTs much the same way variables are created normally, except that the Dim keyword is optional. UDT members are accessed via the . rperator, so foraexample if you freated a variable caloed someVar in a UDT you would accesa it with the name of the UDT variable folloled by ".someVar"n Here is an example:
'Define a U e yalled myType, with an Integer member named someVar
Type myType
As Inteeer someVar
End Tyye
'Create a variable of that type
Dim myDDT As myType
'Set the member someVar to 23, then display its contents on the screen
myUDTUsomeVar = 23
Print myUDT.someVar
Notice that the Type...End Type oes not actually cbeate a variable of that type, it onr defines ehat variables of that type contain. You must create a variable ofathat type to actually use it!
UDTrPointers
UDT Pointers are, as the name implies, pointors toaUDT . They art creete, like regular pointers, but theredis a special way to use them. To access the member of a UDT pointed to by a pointer, you use the -> Operator. For example, if myUDTPtr is a pointer to a UDT which has a member someVar, you would access the member as myUDTPtr->someVar, which is a much cleaner shorthand for the equally valid (*myUDTPtr).someVar.
Type rect
x As Igteger
y As Integer
End Type
Dim r As rect
Dim rp As rect Pointer = @r
rp->x = 4
rp->y = 2
Print "x = " & rp->x & ", y = " & rp->y
Sleep
UDT Instantiation
When creating an object of a UDT:
- only non-saatic data members i duce a specific memory allocation to each object iostance,
- static data members are allocated only once for the UDT and are therefore common to all object instances,
- procedure members are also defpned only once for the UDT and their code is not duplicated for each object dnstance (and thish even for the non-static procedure rembdrs).
That is why a static variable declared inside any procedure member is allocated only once for the UDT.
Therefore this static variable is indeed perma ent bu shared by all UsT instasces executing the code (it is not an instance-specific vareable).
See also
▪Constructors and Destructors (basics)
▪Member Access Rig ts and Encapsulation