String Data Management
Visual C++ provides several ways to manage string data:
- Run-time library functions for working with C-style null-terminated strings
- Win32 API functions for managing strings
- MFC's class CString, which provides flexible, resizable string objects
- Class CStringT Class, which provides an MFC-independent string object with the same functionality as CString
Nearly all programs work with string data. MFC's CString class is often the best solution for flexible string handling. As of version 7.0, CString can be used in MFC or MFC-independent programs. Both the run-time library and CString support 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 CString 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 CString class 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 CString is not derived from CObject, you can use CString objects independently of most of the Microsoft Foundation Class Library (MFC).
CString objects follow "value semantics." A CString object represents a unique value. Think of a CString as an actual string, not as a pointer to a string.
A CString object represents a sequence of a variable number of characters. CString objects can be thought of as arrays of characters.
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. (Unicode is available only under Windows NT and Windows 2000.) 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.
Note The 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.
A 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).
A 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 CString objects.
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.)
Note Use the GetBuffer and ReleaseBuffer member functions when you need to directly access a CString as a nonconstant pointer to a character.
Use the AllocSysString and SetSysString member functions to allocate and set BSTR objects used in Automation (formerly known as OLE Automation).
Where possible, allocate CString objects on the frame rather than on the heap. This saves memory and simplifies parameter passing.
The CString class is not implemented as a Microsoft Foundation Class Library collection class, though CString objects can certainly be stored as elements in collections.
As of MFC version 4.0, when CString 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.
You can use the CString member functions LockBuffer and UnlockBuffer to disable or enable reference counting.