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From: "Lothan" <lothan AT newsguy DOT com>
To: "Cygwin" <cygwin AT cygwin DOT com>
Subject: RE: misdefined macro _T in winnt.h
Date: Mon, 5 Feb 2001 22:08:28 -0800
Message-ID: <EJEMICJMAGCJAPFPGPLGCEGFCHAA.lothan@newsguy.com>
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I suppose we might consider this the definitive answer. I pasted in this
source code:

	#define __DIR "dir"
	#define _UNICODE

	#include <stddef.h>
	#include <tchar.h>
	main()
	{
		size_t len = wcslen(_T(__DIR)) + wcslen(_T("dir"));
		size_t len2 = wcslen(_TEXT(__DIR)) + wcslen(_TEXT("dir"));
		exit(len);
	}

I ran it through the preprocessor for both Borland C++ 5.0 and Microsoft
Visual C++ 6.0 and got identical output from both preprocessors.

	main()
	{
		size_t len = wcslen(L"dir") + wcslen(L"dir");
		size_t len2 = wcslen(L"dir") + wcslen(L"dir");
		exit(len);
	}

I get this output from gcc:

	main()
	{
	        size_t len = wcslen(L__DIR  ) + wcslen(L"dir"  );
	        size_t len2 = wcslen(L__DIR  ) + wcslen(L"dir"  );
	        exit(len);
	}

In my opinion, the output from Borland C++ and MSVC is correct. The output
from gcc is dead wrong. Not surprisingly, the declarations are also
identical for Borland C++ and MSVC:

	#define _T(x) __T(x)
	#define _TEXT(x) __T(x)
	#define __T(x) L ## x

The relevant declarations from gcc are:

	#define _TEXT(x) L ## x
	#define _T(x) L ## x

If you replace the erroneous definitions in tchar.h with the correct
definitions from above, it parses correctly.

> > >   #define _T(x) L ## x
> > > and if you pass a macro FOO as an argument to this macro you get LFOO
> > > returned and not the value of FOO appended to L.  If I change this to
> > >   #define _T(x) L(x)
> > > then I get returned L("bar") where "bar" is the value of FOO.  This
> > > allows the program to compile but does L"bar" == L("bar")?
> >
> > No. I'm certain of that, more or less ...
> >
>
> 1) If that is true then what is the purpose of function L()?
> 2) Can someone who has MSVC++ tell me what the _T(__dir) expands to
> using the above example?
> 3) Is it legal to pass a MACRO as an argument to the _T() macro?
>
> Earnie.
>
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