doc/namespace.texi

@node A C++ namespace for gnulib
@section A C++ namespace for gnulib

The function definitions provided by Gnulib (@code{.c} code) are meant
to be compiled by a C compiler.  The header files (@code{.h} files),
on the other hand, can be used in either C or C++.

By default, when used in a C++ compilation unit, the @code{.h} files
declare the same symbols and overrides as in C mode, except that functions
defined by Gnulib or by the system are declared as @samp{extern "C"}.

It is also possible to indicate to Gnulib to provide many of its symbols
in a dedicated C++ namespace.  If you define the macro
@code{GNULIB_NAMESPACE} to an identifier, many functions will be defined
in the namespace specified by the identifier instead of the global
namespace.  For example, after you have defined
@smallexample
#define GNULIB_NAMESPACE gnulib
@end smallexample
@noindent
at the beginning of a compilation unit, Gnulib's @code{<fcntl.h>} header
file will make available the @code{open} function as @code{gnulib::open}.
The symbol @code{open} will still refer to the system's @code{open} function,
with its platform specific bugs and limitations.

The symbols provided in the Gnulib namespace are those for which the
corresponding header file contains a @code{_GL_CXXALIAS_RPL} or
@code{_GL_CXXALIAS_SYS} macro invocation.

The benefits of this namespace mode are:
@itemize
@item
Gnulib defines fewer symbols as preprocessor macros.  For example, on a
platform where @code{open} has to be overridden, Gnulib normally does
@code{#define open rpl_open}.  If your package has a class with a member
@code{open}, for example a class @code{foo} with a method @code{foo::open},
then if you define this member in a compilation unit that includes
@code{<fcntl.h>} and use it in a compilation unit that does not include
@code{<fcntl.h>}, or vice versa, you will get a link error.  Worse: You
will not notice this problem on the platform where the system's @code{open}
function works fine.  This problem goes away in namespace mode.

@item
It provides a safety check whether the set of modules your package requests
from Gnulib is sufficient.  For example, if you use the function
@code{gnulib::open} in your code, and you forgot to request the module
@samp{open} from Gnulib, you will get a compilation error (regardless of
the platform).
@end itemize

The drawback of this namespace mode is that the system provided symbols in
the global namespace are still present, even when they contain bugs that
Gnulib fixes.  For example, if you call @code{open (...)} in your code,
it will invoke the possibly buggy system function, even if you have
requested the module @samp{open} from gnulib-tool.

You can turn on the namespace mode in some compilation units and keep it
turned off in others.  This can be useful if your package consists of
an application layer that does not need to invoke POSIX functions and
an operating system interface layer that contains all the OS function
calls.  In such a situation, you will want to turn on the namespace mode
for the application layer---to avoid many preprocessor macro
definitions---and turn it off for the OS interface layer---to avoid
the drawback of the namespace mode, mentioned above.