VC 字符串相关宏 _T、TEXT,_TEXT、L 的作用

一、 在字符串前加一个L作用:

   如 L"我的字符串"    表示将ANSI字符串转换成unicode的字符串，就是每个字符占用两个字节。

 strlen("asd")   =   3;  

 strlen(L"asd")   =   6;

 

二、 _T宏可以把一个引号引起来的字符串，根据你的环境设置，使得编译器会根据编译目标环境选择合适的（Unicode还是ANSI）字符处理方式

   如果你定义了UNICODE，那么_T宏会把字符串前面加一个L。这时 _T("ABCD") 相当于 L"ABCD" ，这是宽字符串。

   如果没有定义，那么_T宏不会在字符串前面加那个L，_T("ABCD") 就等价于 "ABCD"

 

三、TEXT,_TEXT 和_T 一样的

 

如下面三语句：  

 TCHAR   szStr1[]   =   TEXT("str1");  

 char   szStr2[]   =   "str2";  

 WCHAR   szStr3[]   =   L("str3");  

 那么第一句话在定义了UNICODE时会解释为第三句话，没有定义时就等于第二句话。 

 但二句话无论是否定义了UNICODE都是生成一个ANSI字符串，而第三句话总是生成UNICODE字符串。  

 为了程序的可移植性，建议都用第一种表示方法。  

 但在某些情况下，某个字符必须为ANSI或UNICODE，那就用后两种方法。

 

 

Many C++ Windows programmers get confused over what bizarre identifiers like TCHAR, LPCTSTR are. Here, in brief, I would try to clear out the fog.

In general, a character can be 1 byte or 2 bytes. Lets say 1-byte character is ANSI, using which English characters are represented. And lets say 2-byte character is Unicode, which can represent ALL languages in the world.

VC++ support char and wchar_t as native datatypes for ANSI and Unicode characters respectively.

What if you want your C/C++ program to be Character-mode independent?
That means, instead of replacing:

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with

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You can simply code it:

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Thus, when your project is being compiled as Unicode, the TCHAR would translate to wchar_t. If it is being compiled as ANSI/MBCS, it would translated to char. Likewise, instead of using strcpy, strlen, strcat (including the secure versions suffixed with _s); or wcscpy, wcslen, wcscat (including secure), you can simply use _tcscpy, _tcslen, _tcscat functions.

When you need to express hard-coded string, you can use:

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The non-prefixed string is ANSI string, the L prefixed string is Unicode, and string specified in _T or TEXT would be either, depending on compilation.


String classes, like MFC/ATL's CString implement two version using macro. There are two classes named CStringA for ANSI, CStringW for Unicode. When you use CString (which is a macro/typedef), it translates to either of two classes.

Okay. The TCHAR type-definition was for a single character. You can definitely declare an array of TCHAR.
What if you want to express a character-pointer, or a const-character-pointer - Which one of the following?

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After reading about TCHAR stuff, you'd definitely select the last one as your choice. But here is better alternative. Before that, note that TCHAR.H header file declares only TCHAR datatype and for the following stuff, you need to include Windows.h (defined in WinNT.h).

NOTE: If your project implicitly or explicitly includes Windows.h, you need not to include TCHAR.H

• char* replacement: LPSTR
• const char* replacement: LPCSTR
• WCHAR* replacement: LPWSTR
• const WCHAR* replacement: LPCWSTR (C before W, since const is before WCHAR)
• TCHAR* replacement: LPTSTR
• const TCHAR* replacement: LPCTSTR

Now, I hope, you understand the following signatures :

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Continuing. You must have seen some functions/methods asking you to pass number of characters, or returning the number of characters. Well, like GetCurrentDirectory, you need to pass number of characters, and not number of bytes. For example::

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On the other side, if you need to allocate number or characters, you must allocate proper number of bytes. In C++, you can simply use new:

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But if you use memory allocation functions like malloc, LocalAlloc, GlobalAlloc etc; you must specify the number of bytes!

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Typecasting the return value is required, as you know. The expression in malloc's argument ensures that it allocates desired number of bytes - and makes up room for desired number of characters.

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