Document C++ namespace mode.

diff --git a/ChangeLog b/ChangeLog
index 28eab583e250e7bd19dfcbacd1a52a773446ffeb..bde1e828d38cd6d0ec0cafb5241b86efa98475e3 100644 (file)
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,8 @@
 2010-03-07  Bruno Haible  <bruno@clisp.org>
 
+       Document C++ namespace mode.
+       * doc/gnulib.texi (A C++ namespace for gnulib): New section.
+
        wctype: Avoid #define replacements in C++ mode.
        * lib/wctype.in.h: Include c++defs.h, warn-on-use.h.
        (iswalnum, iswalpha, iswblank, iswcntrl, iswdigit, iswgraph, iswlower,

diff --git a/doc/gnulib.texi b/doc/gnulib.texi
index 0a8469194340d8be5086d1d26db489d99f2494a8..d3c811bad7e134e22738d4d5f62e0a5366d8e09b 100644 (file)
--- a/doc/gnulib.texi
+++ b/doc/gnulib.texi
@@ -614,6 +614,7 @@ before every release.
 @menu
 * Out of memory handling::
 * Obsolete modules::
+* A C++ namespace for gnulib::      A different way of using Gnulib in C++
 * Library version handling::
 * Windows sockets::
 * Libtool and Windows::
@@ -688,6 +689,72 @@ This module is obsolete.
 @end example
 
 
+@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.
+
+
 @node Library version handling
 @section Library version handling