ArcGIS Engine Developer Guide

DEVELOPING WITH A RCOBJECTS
Language
Base types
Extended
types
IDL Microsoft C++ Visual Basic Java
boolean unsigned char unsupported
char
byte unsigned char unsupported
char
small char unsupported
char
short short Integer
short
long long Long
int
hyper __int64 unsupported
long
float float Single
float
double double Double
double
char unsigned char unsupported
char
wchar_t wchar_t Integer
short
enum enum Enum
int
Interface Pointer Interface Pointer Interface Ref.
Interface Ref.
VARIANT VARIANT Variant
ms.com.Variant
BSTR BSTR String
java.lang.String
VARIANT_BOOL short (-1/0) Boolean
[true/false]
Note the extended data types at the bottom of the table: VARIANT, BSTR, and
VARIANT_BOOL. Although it is possible to pass strings using data types such as
char and wchar_t, these are not supported in languages such as Visual Basic. Visual
Basic uses BSTRs as its text data type. A BSTR is a length-prefixed wide character
array in which the pointer to the array points to the text contained within it
and not the length prefix. Visual C++ maps VARIANT_BOOL values onto 0
and –1 for the False and True values, respectively. This is different from the
normal mapping of 0 and 1. Hence, when writing C++ code, be sure to use the
correct macros—VARIANT_FALSE and VARIANT_TRUE—not False and
True.
USING COMPONENT CATEGORIES
Component categories are used extensively in ArcObjects so developers can
extend the system without requiring any changes to the ArcObjects code that will
work with the new functionality.
ArcObjects uses component categories in two ways. The first requires classes to
be registered in the respective component category at all times—for example,
ESRI Mx Extensions. Classes, if present in that component category, have an
object that implements the IExtension interface and is instantiated when the
ArcMap application is started. If the class is removed from the component category,
the extension will not load, even if the map document (.mxd file) is referencing
that extension.
The second use is when the application framework uses the component category
to locate classes and display them to a user to allow some user customization to
occur. Unlike the first method, the application remembers (inside its map document)
the objects being used and will subsequently load them from the map
document. An example of this is the commands used within ArcMap. ArcMap
reads the ESRI Mx Commands category when the Customization dialog box is
displayed to the user. This is the only time the category is read. Once the user
selects a command and adds it to a toolbar, the map document is used to determine
what commands should be instantiated. Later, when debugging in Visual
Basic is covered in ‘The Visual Basic 6 development environment’ section of this
chapter, you’ll see the importance of this.
Chapter 4 • Developer environments • 77