[boost] Re: Re: Re: Better Intel-Win32 support

From: Edward Diener (eddielee_at_[hidden])
Date: 2003-06-04 16:08:32

Randy Bowen wrote:
>> A disappointing aspect of this in regards to MSVC 7.0+ is
>> that there is no preprocessor macro ( as of 7.0, I haven't
>> checked 7.1 yet ) which MSVC defines for distinguishing
>> native C++ wide character from the previous typedef for
>> wchar_t.
>
> The MS-specific macro _NATIVE_WCHAR_T_DEFINED indicates the presence
> of
> the native type in MSVC 7.0+.  However, use of this can cause a number
> of problems with existing MS libraries (and especially anything that
> deals with COM).

The documentation is very confusing regarding this. First we have under
Predefined Macros:

"_WCHAR_T_DEFINED
and
_NATIVE_WCHAR_T_DEFINED

Defined when wchar_t is defined. Typically, wchar_t is defined when you use
/Zc:wchar_t or when typedef unsigned short wchar_t; is executed in code."

which implies that both the macros above are defined in both cases. Next we
have under the compiler switch "/Zc:wchar_t (wchar_t Is Native Type)":

"When /Zc:wchar_t is specified, _WCHAR_T_DEFINED and _NATIVE_WCHAR_T_DEFINED
symbols are defined; see Predefined Macros for more information."

which simply reiterates the first part of the previous topic.

Testing this out, both VC++ 7.0 and VC++ 7.1 yield the same result which
confirms what you have written above. When the /Zc:wchar_t switch is used,
both of the above macros are defined. When the /Zc:wchar_t is not used, only
the _WCHAR_T_DEFINED macro is defined.

So I was definitely wrong in thinking that there was no way to test for the
difference. Perhaps the previous documentation which I read didn't explain
the _NATIVE_WCHAR_T_DEFINED macro or perhaps I just missed it. But this is
good for Boost. Now there is a way to determine whether which of the wchar_t
types in VC++ are being used at compile time.

My suggestion for Boost is that it use library implementors use this in
order to alert the end-user at compile time that a particular usage of
wchar_t is not supported, or use it to correctly link in the correct version
of the library when both are supported. For VC++, the #pragma
comment(lib,"SomeLib") specifies the name of the library to link, either
import or static. I know John Maddock does this in Regex++ but the general
usage for our wchar_t, reiterated here, would be:

#if defined(_NATIVE_WCHAR_T_DEFINED)
#pragma comment(lib,"NativeWchartVersion.lib"
#elif defined(_WCHAR_T_DEFINED)
#pragma comment(lib,"VCWchartVersion.lib")
#endif

Of course I expect Boost may want to create BOOST_ macros for these VC++
specific macros in the correct configuration file and use them instead, but
one gets the general idea from this.