ArcGIS Engine Developer Guide

VISUAL C++
IXxxImpl
CComXxxThreadModel
CComObjectRootEx
CMyObject
IMyInt
CComObject
IMyInt2
The hierarchical layers of ATL
A more detailed discussion on Direct- To-COM
follows in the section ‘Direct-To-COM smart
types’.
provides a set of wizards that extends the Visual Studio development environment.
These wizards automate some of the tedious plumbing code that all ATL
projects must have. The wizards include, but are not limited to, the following:
• Application—Used to initialize an ATL C++ project.
• Object—Used to create COM objects. Both C++ and IDL code are generated,
along with the appropriate code to support the creation of the objects at
runtime.
• Property—Used to add properties to interfaces.
• Method—Used to add methods to interfaces; both the Property and Method
wizards require you to know some IDL syntax.
• Interface Implementation—Used to implement stub functions for existing
interfaces.
• Connection Point Implement—Used to implement outbound events’ interfaces.
Typically, these are accessed by a right-click on a project, class, or interface in
Visual Studio Workspace/Class view.
ATL provides base classes for implementing COM objects as well as implementations
for some of the common COM interfaces, including IUnknown, IDispatch,
and IClassFactory. There are also classes that provide support for ActiveX controls
and their containers.
ATL provides the required services for exposing ATL-based COM objects including
registration, server lifetime, and class objects.
These template classes build a hierarchy that sandwiches your class. These inheritances
are shown to the left. The CComxxxThreadModel class supports thread-safe
access to global, instance, and static data. The CComObjectRootEx class provides
the behavior for the IUnknown methods. The interfaces at the second level represent
the interfaces that the class will implement; these come in two varieties. The
IxxxImpl interface contains ATL-supplied interfaces that also include an implementation;
the other interfaces have pure virtual functions that must be fully
implemented within your class. The CComObject class inherits your class; this class
provides the implementation of the IUnknown methods along with the object
instantiation and lifetime control.
ATL and DTC
Along with smart types, covered later in this chapter, Direct-To-COM (DTC)
provides some useful compiler extensions you can use when creating ATL-based
objects. The functions __declspec and __uuidof are two such functions, but the
most useful is the #import command.
COM interfaces are defined in IDL, then compiled by the Microsoft IDL compiler
(MIDL.exe). This results in the creation of a type library and header files.
The project uses these files automatically when compiling software that references
these interfaces. This approach is limited in that, when working with
interfaces, you must have access to the IDL files. As a developer of ArcGIS, you
only have access to the ArcGIS type library information contained in .olb and
100 • ArcGIS Engine Developer Guide