_T( ) macro changes for UNICODE character data

问题

I have UNICODE application where in we use _T(x) which is defined as follows.

#if defined(_UNICODE)
#define _T(x) L ##x
#else
#define _T(x) x
#endif

I understand that L gets defined to wchar_t, which will be 4 bytes on any platform. Please correct me if I am wrong. My requirement is that I need L to be 2 bytes. So as compiler hack I started using -fshort-wchar gcc flag. But now I need my application to be moved to zSeries where I don't get to see the effect of -fshort-wchar flag in that platform.

In order for me to be able to port my application on zSeries, I need to modify _T( ) macro in such a way that even after using L ##x and without using -fshort-wchar flag, I need to get 2byte wide character data.Can some one tell me how I can change the definition of L so that I can define L to be 2 bytes always in my application.

回答1:

You can't - not without c++0x support. c++0x defines the following ways of declaring string literals:

"string of char characters in some implementation defined encoding" - char
u8"String of utf8 chars" - char
u"string of utf16 chars" - char16_t
U"string of utf32 chars" - char32_t
L"string of wchar_t in some implementation defined encoding" - wchar_t

Until c++0x is widely supported, the only way to encode a utf-16 string in a cross platform way is to break it up into bits:

// make a char16_t type to stand in until msvc/gcc/etc supports
// c++0x utf string literals
#ifndef CHAR16_T_DEFINED
#define CHAR16_T_DEFINED
typedef unsigned short char16_t;
#endif

const char16_t strABC[] = { 'a', 'b', 'c', '\0' };
// the same declaration would work for a type that changes from 8 to 16 bits:

#ifdef _UNICODE
typedef char16_t TCHAR;
#else
typedef char TCHAR;
#endif
const TCHAR strABC2[] = { 'a', 'b', 'b', '\0' };

The _T macro can only deliver the goods on platforms where wchar_t's are 16bits wide. And, the alternative is still not truly cross-platform: The coding of char and wchar_t is implementation defined so 'a' does not necessarily encode the unicode codepoint for 'a' (0x61). Thus, to be strictly accurate, this is the only way of writing the string:

const TCHAR strABC[] = { '\x61', '\x62', '\x63', '\0' };

Which is just horrible.

回答2:

Ah! The wonders of portability :-)

If you have a C99 compiler for all your platforms, use int_least16_t, uint_least16_t, ... from <stdint.h>. Most platforms also define int16_t but it's not required to exist (if the platform is capable of using exactly 16 bits at a time, the typedef int16_t must be defined).

Now wrap all the strings in arrays of uint_least16_t and make sure your code does not expect values of uint_least16_t to wrap at 65535 ...

来源：https://stackoverflow.com/questions/4133144/t-macro-changes-for-unicode-character-data

标签

c++

string

gcc

widechar