String Data Management
Visual C++ provides several ways to manage string data:
String Manipulation for working with C-style null-terminated strings
Win32 API functions for managing strings
MFC's class CStringT Class, which provides flexible, resizable string objects
Class CStringT Class, which provides an MFC-independent string object with the same functionality as
Nearly all programs work with string data. MFC's
CStringclass is often the best solution for flexible string handling. Starting with version 7.0,
CStringcan be used in MFC or MFC-independent programs. Both the run-time library and
CStringsupport strings containing multibyte (wide) characters, as in Unicode or MBCS programming.
This article describes the general-purpose services that the class library provides related to string manipulation. Topics covered in this article include:
The CStringT Class class provides support for manipulating strings. It is intended to replace and extend the functionality normally provided by the C run-time library string package. The
CStringclass supplies member functions and operators for simplified string handling, similar to those found in Basic. The class also provides constructors and operators for constructing, assigning, and comparing CStrings and standard C++ string data types. Because
CStringis not derived from
CObject, you can use
CStringobjects independently of most of the Microsoft Foundation Class Library (MFC).
CStringobjects follow "value semantics." A
CStringobject represents a unique value. Think of a
CStringas an actual string, not as a pointer to a string.
CStringobject represents a sequence of a variable number of characters.
CStringobjects can be thought of as arrays of characters.
Unicode and MBCS Provide Portability
With MFC version 3.0 and later, MFC, including
CString, is enabled for both Unicode and multibyte character sets (MBCS). This support makes it easier for you to write portable applications that you can build for either Unicode or ANSI characters. To enable this portability, each character in a
CString object is of type TCHAR, which is defined as
wchar_t if you define the symbol _UNICODE when you build your application, or as
char if not. A
wchar_t character is 16 bits wide. MBCS is enabled if you build with the symbol _MBCS defined. MFC itself is built with either the _MBCS symbol (for the NAFX libraries) or the _UNICODE symbol (for the UAFX libraries) defined.
CString examples in this and the accompanying articles on strings show literal strings properly formatted for Unicode portability, using the _T macro, which translates the literal string to the form:
which the compiler treats as a Unicode string. For example, the following code:
CString strName = _T("Name");
is translated as a Unicode string if _UNICODE is defined or as an ANSI string if not. For more information, see the article Unicode and Multibyte Character Set (MBCS) Support.
CString object can store up to INT_MAX (2,147,483,647) characters. The TCHAR data type is used to get or set individual characters inside a
CString object. Unlike character arrays, the
CString class has a built-in memory allocation capability. This allows
CString objects to automatically grow as needed (that is, you do not have to worry about growing a
CString object to fit longer strings).
CStrings and const char Pointers
CString object also can act like a literal C-style string (an
PCXSTR, which is the same as const char\* if not under Unicode). The CSimpleStringT::operator PCXSTR conversion operator allows
CString objects to be freely substituted for character pointers in function calls. The CString( LPCWSTR
pszSrc ) constructor allows character pointers to be substituted for
No attempt is made to fold
CString objects. If you make two
CString objects containing
Chicago, for example, the characters in
Chicago are stored in two places. (This may not be true of future versions of MFC, so you should not depend on it.)
Where possible, allocate
CString objects on the frame rather than on the heap. This saves memory and simplifies parameter passing.
CString class is not implemented as a Microsoft Foundation Class Library collection class, though
CString objects can certainly be stored as elements in collections.
CString Reference Counting
As of MFC version 4.0, when CStringT Class objects are copied, MFC increments a reference count rather than copying the data. This makes passing parameters by value and returning
CString objects by value more efficient. These operations cause the copy constructor to be called, sometimes more than once. Incrementing a reference count reduces that overhead for these common operations and makes using
CString a more attractive option.
As each copy is destroyed, the reference count in the original object is decremented. The original
CString object is not destroyed until its reference count is reduced to zero.