ArcGIS Engine Developer Guide

VISUAL C++
}
return TRUE;
Boolean types
Historically, ANSI C did not have a Boolean data type and used int value instead,
where 0 represents false and nonzero represents true. However, the bool data
type has now become part of ANSI C++. COM APIs are language independent
and define a different Boolean type, VARIANT_BOOL. In addition, Win32 API
uses a different bool type. It is important to use the correct type at the appropriate
time. The following table summarizes their usage:
Type True Value False Value Where Defined When to Use
bool true (1) false (0)
Defined by
compiler
BOOL (int) TRUE (1) FALSE (0) Windows Data
Type (defined in
windef.h)
VARIANT_BOOL
(16bit short)
VARIANT_
TRUE (-1)
VARIANT_
FALSE (0)
COM boolean
values (wtypes.h)
This is an intrinsic compiler type so there is
more potential for the compiler to optimize
use. This type can also be promoted to
an int value. Expressions (e.g., i!=0) return
a type of Bool. Typically used for class
member variables and local variables.
Used with Windows API functions, often as
a return value to indicate success or failure.
Used in COM APIs for boolean values. Also
used within VARIANT types, if the VARIANT
type is VT_BOOL, then the VARIANT value
(boolVal) is populated with a
VARIANT_BOOL. Take care to convert a
bool class member variable to the correct
VARIANT_BOOL value. Often the
conditional test "hook - colon" operator is
used. For example where bRes is defined
as a bool, then to set a result type:
*pVal = bRes ? VARIANT_TRUE :
VARIANT_FALSE;
String types
Considering that strings (sequences of text characters) are a simple concept, they
have unfortunately become a complex and confusing topic in C++. The two main
reasons for this confusion are the lack of C++ support for variable length strings
combined with the requirement to support ANSI and Unicode character sets
within the same code. As ArcGIS is only available on Unicode platforms, it may
simplify development to remove the ANSI requirements.
The C++ convention for strings is an array of characters terminated with a 0.
This is not always good for performance when calculating lengths of large strings.
To support variable length strings, the character arrays can be dynamically allocated
and released on the heap, typically using malloc and free or new and delete.
Consequently, a number of wrapper classes provide this support; CString defined
in MFC and WTL is the most widely used. In addition, for COM usage the BSTR
type is defined and the ATL wrapper class CComBSTR is available.
To allow for international character sets, Microsoft Windows migrated from an
8-bit ANSI character string (8-bit character) representation (found on
Windows 95, Windows 98, and Windows Me platforms) to a 16-bit Unicode
character string (16-bit unsigned short). Unicode is synonymous with wide characters
(wchar_t). In COM APIs, OLECHAR is the type used and is defined to be
wchar_t on Windows. Windows operating systems, such as Windows NT,
Windows 2000, and Windows XP, natively support Unicode characters. To allow
the same C++ code to be compiled for ANSI and Unicode platforms, compiler
Chapter 4 • Developer environments • 105