Difference between revisions of "Documentation/DevGuide/ProUNO/Basic/Mapping of Simple Types"

Revision as of 11:07, 22 October 2009

In general, the OpenOffice.org Basic type system is not rigid. Unlike C++ and Java, OpenOffice.org Basic does not require the declaration of variables, unless the Option Explicit command is used that forces the declaration. To declare variables, the Dim command is used. Also, a OpenOffice.org Basic type can be optionally specified through the Dim command. The general syntax is:

  Dim VarName [As Type][, VarName [As Type]]...

All variables declared without a specific type have the type Variant. Variables of type Variant can be assigned values of arbitrary Basic types. Undeclared variables are Variant unless type postfixes are used with their names. Postfixes can be used in Dim commands as well. The following table contains a complete list of types supported by Basic and their corresponding postfixes:

Consider the following Dim examples.

  Dim a, b ' Type of a and b is Variant
  Dim c as Variant ' Type of c is Variant
 
  Dim d as Integer ' Type of d is Integer (16 bit!)
 
  ' The type only refers to the preceding variable
  Dim e, f as Double ' ATTENTION! Type of e is Variant!
  ' Only the type of f is Double
 
  Dim g as String ' Type of g is String
 
  Dim h as Date ' Type of h is Date
 
  ' Usage of Postfixes
  Dim i% ' is the same as
  Dim i as Integer
 
  Dim d# ' is the same as
  Dim d as Double
 
  Dim s$ ' is the same as
  Dim s as String

The correlation below is used to map types from UNO to Basic and vice versa.

The simple UNO type type is mapped to the com.sun.star.reflection.XIdlClass interface to retrieve type specific information. For further details, refer to UNO Reflection API.

When UNO methods or properties are accessed, and the target UNO type is known, Basic automatically chooses the appropriate types:

  ' The UNO object oExample1 has a property "Count" of type short
  a% = 42
  oExample1.Count = a% ' a% has the right type (Integer)
 
  pi = 3,141593
  oExample1.Count = pi ' pi will be converted to short, so Count will become 3
 
  s$ = "111"
  oExample1.Count = s$ ' s$ will be converted to short, so Count will become 111

Occasionally, OpenOffice.org Basic does not know the required target type, especially if a parameter of an interface method or a property has the type any. In this situation, OpenOffice.org Basic mechanically converts the OpenOffice.org Basic type into the UNO type shown in the table above, although a different type may be expected. The only mechanism provided by OpenOffice.org Basic is an automatic downcast of numeric values:

Long and Integer values are always converted to the shortest possible integer type:

• to byte if -128 <= Value <= 127
• to short if -32768 <= Value <= 32767

The Single/Double values are converted to integers in the same manner if they have no decimal places.

This mechanism is used, because some internal C++ tools used to implement UNO functionality in OpenOffice.org provide an automatic upcast but no downcast. Therefore, it can be successful to pass a byte value to an interface expecting a long value, but not vice versa.

In the following example, oNameCont is an object that supports com.sun.star.container.XNameContainer and contains elements of type short. Assume FirstValue is a valid entry.

  a% = 42
  oNameCount.replaceByName( "FirstValue", a% ) ' Ok, a% is downcasted to type byte
 
  b% = 123456
  oNameCount.replaceByName( "FirstValue", b% ) ' Fails, b% is outside the short range

The method call fails, therefore the implementation should throw the appropriate exception that is converted to a OpenOffice.org Basic error by the OpenOffice.org Basic RTL. It may happen that an implementation also accepts unsuitable types and does not throw an exception. Ensure that the values used are suitable for their UNO target by using numeric values that do not exceed the target range or converting them to the correct Basic type before applying them to UNO.

Always use the type Variant to declare variables for UNO Basic objects, not the type Object. The OpenOffice.org Basic type Object is tailored for pure OpenOffice.org Basic objects and not for UNO OpenOffice.org Basic objects. The Variant variables are best for UNO Basic objects to avoid problems that can result from the OpenOffice.org Basic specific behavior of the type Object:

  Dim oService1 ' Ok
  oService1 = CreateUnoService( "com.sun.star.anywhere.Something" )
 
  Dim oService2 as Object ' NOT recommended
  oService2 = CreateUnoService( "com.sun.star.anywhere.SomethingElse" )