Adding functions to UDT

You can enhance the UDT with functions as well - this approach is appropriate anytime you need to perform the data manipulation of the UDT itself.

Imagine our Point2D example - you will most probably need to initialize x and y straight away, and you will most probably need to be able to get string representation of the variable.

Let's use this example to demonstrate the use of UDT functions.

The first fact you should know is that UDT functions do not take the UDT variable as input explicitly.

The data is always reachable via pre-defined ME variable, accessible in each UDT function.

There are two kinds of UDT functions:

• user defined functions
• functions with special meaning, currently _create and _destroy

User defined function within TYPE definition

You can define a function directly inside the type / end type block.

The same rules apply as for any other function, except the advantage of having ME as a way to reference UDT elements.

TYPE Point2D
  x AS SINGLE
  y AS SINGLE
  
  FUNCTION setXY(x AS SINGLE, y AS SINGLE)
    ME.x = x
    ME.y = y
  END FUNCTION
  
  FUNCTION toString() AS STRING
    RETURN strFormat$("[{1}, {2}]", ME.x, ME.y)
  END FUNCTION
END TYPE

DIM point AS Point2D

point.setXY(1, 2)
msgBox point.toString()

The main advantage of this type of definition is that the type / end type contains everything the UDT can do.

User defined function outside TYPE definition

You can define the function body outside the type / end type block as well.

In such a case, please register the function via <functionName> AS FUNCTION within the type / end type first. This registration does not increase the memory footprint of the UDT, but it creates a logical link between UDT and function.

UDT function defined outside the type / end type block follows the same rules as any other function, except:

• having ME as a way to reference UDT elements
• need to prefix the function name with name of type and dot

TYPE Point2D
  x AS SINGLE
  y AS SINGLE

  setXY    AS FUNCTION
  toString AS FUNCTION  
END TYPE

FUNCTION Point2D.setXY(x AS SINGLE, y AS SINGLE)
  ME.x = x
  ME.y = y
END FUNCTION

FUNCTION Point2D.toString() AS STRING
  RETURN strFormat$("[{1}, {2}]", ME.x, ME.y)
END FUNCTION

DIM point AS Point2D

point.setXY(1, 2)
msgBox point.toString()

The main advantage of this type of definition is that you can spread the function across multiple source code files.

Functions with special purpose

ThinBASIC reserves the functions starting with underscore for special purpose.

There are currently two such functions provided:

• _create
• _destroy