Re: Using templates to wrap OS APIs

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <>
Thu, 15 Oct 2009 07:12:31 CST
On 15 Okt., 10:12, Scott Meyers <> wrote:

Somewhere, I figure this has got to be a FAQ. But I don't know where.

I want to wrap OS-dependent code in an OS-independent API to shield


from having to know that, e.g., what's called Sleep under Windows is


usleep under Linux. No problem, I can use ifdefs:

   #ifdef WINDOWS
     inline void sleepMilliseconds( unsigned int uMilliseconds )
     { Sleep( uMilliseconds ); }
     inline void sleepMilliseconds( unsigned int uMilliseconds )
     { usleep( 1000 * uMilliseconds ); }

Except the preprocessor is gross. I'd rather use templates:

   struct Windows;
   struct Linux;
   template<typename OS> struct OSWrapper;

   struct OSWrapper<Windows>
     inline static void sleepMilliseconds( unsigned int uMilliseconds )
     { Sleep( uMilliseconds ); }

   struct OSWrapper<Linux>
     inline static void sleepMilliseconds( unsigned int uMilliseconds )
     { Sleep( uMilliseconds ); }

Unfortunately, this won't compile. Even with the appropriate
platform-specific #include directives, there is no Sleep on Linux and no
usleep on Windows, so one of the template specializations won't compile,
regardless of the platform on which I am compiling.

The names Sleep and usleep are non-dependent names in the template, hence
looked up prior to instantiation. If I could somehow make them dependent
names, they'd be looked up only after instantiation, which would be fine.
However, I can't make them dependent by prefixing the calls with "this->",
because the functions I'm in are static. I can't find a way to move them
into another class (thus allowing me to make the names dependent by
prefixing them with "OS::" in the general template) without facing the


name-lookup problem in whatever class I introduce.

Wrapping platform-dependent APIs has got to be a common C++ activity, and
it seems like templates should be well-suited to the job. Yet I keep
coming back to ifdefs. Can somebody please point out the error of my


I would try to get rid of the multi-configuration-implementation in a
header. In fact it is far much more easier to configure the build tool
to select
the proper directory for configuration-specific code. This way there
is no
need for any template code, because you client-header would just

#include <os-config.h>

// implicitly include
// inline void sleepMilliseconds( unsigned int uMilliseconds )
// with the correct implementation

where depending on the build-tool configuration <os-config.h>
would now point either to




This way you have a much better encapsulation of system-specific
code. Each such header is independent from the sum of all
configurations you need to support.

HTH & Greetings from Bremen,

Daniel Kr?gler

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