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<H1><A NAME="SECTION02330000000000000000"></A><A NAME="1514"></A>
<BR>
Writing the Implementation
</H1>
<P>
We will write the implementation in the lib/ subdirectory of hello/.
The first step is to run the Babel shell script to generate the library
implementation code for the SIDL file.<A NAME="1515"></A>
We will implement the library in C++.
The simplified command to generate the Babel library code (assuming Babel is in
your PATH) is <A NAME="tex2html23"
HREF="footnode.html#foot1580"><SUP><SPAN CLASS="arabic">5</SPAN>.<SPAN CLASS="arabic">1</SPAN></SUP></A>:
<P>
<BLOCKQUOTE>
<TT> <TT>%</TT> <TT><B>babel -sC++ -olib ../hello.sidl</B></TT>
<BR>
</TT></BLOCKQUOTE>
<P>
In this Babel command, the ``<TT>-sC++</TT>'' flag, or its long form
``<TT>-server=C++</TT>'', indicates that we wish to generate C++ bindings
for an implementation<A NAME="tex2html24"
HREF="footnode.html#foot1581"><SUP><SPAN CLASS="arabic">5</SPAN>.<SPAN CLASS="arabic">2</SPAN></SUP></A>. The ``<TT>-olib</TT>''
flag, or its long form ``<TT>-output-dir=lib</TT>'', defines the root
directory of where the generated code should be placed.
<P>
This command will generate a large number of C and C++ header
and source files. It is often surprising to newcomers just how
much code is generated by Babel. Rest assured, each file
has a purpose and there is a lot of important things being
done as efficiently as possible under the hood.
<P>
Files are named after the fully-qualified
class-name. For instance, a package <TT><I CLASS="slanted">Hello</I></TT> and class
<TT><I CLASS="slanted">World</I></TT> would have a fully qualified name (in SIDL) as
<TT><I CLASS="slanted">Hello.World</I></TT>. This corresponds to file names
beginning with <TT>Hello_World</TT><A NAME="tex2html25"
HREF="footnode.html#foot1529"><SUP><SPAN CLASS="arabic">5</SPAN>.<SPAN CLASS="arabic">3</SPAN></SUP></A>. For each class,
there will be files with <TT>_IOR</TT>,
<TT>_skel</TT>, <TT>_stub</TT>, or <TT>_impl</TT> appended after the
fully qualified name.
<SPAN CLASS="textit">IOR files</SPAN><A NAME="1535"></A> are always in ANSI C (source and headers),
containing Babel's Intermediate Object Representation.
<SPAN CLASS="textit">Impl files</SPAN><A NAME="1537"></A> contain the actual implementation, and
can be in any language that Babel supports, in this
case, they're C++ files. Impl files are the only files
that a developer need look at or touch after generating
code from the SIDL source. <SPAN CLASS="textit">Skel files</SPAN><A NAME="1539"></A> perform translations
between the IORs and the Impls. In some cases (like Fortran)
the Skels are split into a few files: some in C, some in
the Impl language. In the case of C++, the Skels are pure
C++ code wrapped in <TT>extern "C" {}</TT> declarations.
If the file is neither an IOR, Skel, nor Impl, then it
is likely a <SPAN CLASS="textit">Stub</SPAN><A NAME="1542"></A>. Stubs are the proxy classes
of Babel, performing translations
between the caller language and the IOR.
Finally, the file babel.make is a Makefile fragment that will
simplify writing the Makefile necessary to compile the library.
You may ignore the babel.make file if you wish.
<P>
The only files that should be modified by the developer
(that's you since you're implementing Hello World)
are the ``Impls'', which are in this case
files ending with <TT>_Impl.hh</TT> or <TT>_Impl.cc</TT>
Babel generates these implementation files as a starting
point for developers.
These files will contain the implementation of the Hello library.
Every implementation file contains many pairs of comment ``splicer''
lines<A NAME="1545"></A>
such as the following:
<P>
<BR>
<PRE CLASS="verbatim">std::string
Hello::World_impl::getMsg()
throw ()
{
// DO-NOT-DELETE splicer.begin(Hello.World.getMsg)
// Insert code here...
// DO-NOT-DELETE splicer.end(Hello.World.getMsg)
}
</PRE></td></tr></table></blockquote>
<P>
Any modifications between these splicer lines will be
saved after subsequent invocations of the Babel tool.
Any changes outside the splicer lines will be lost.
This splicer feature was developed to make it
easy to do incremental development using Babel.
By keeping your edits within the splicer blocks,
you can add new methods to the hello.sidl file
and rerun Babel without the loss of your previous
method implementations. You shouldn't ever need to edit the file
outside the splicer blocks.
<P>
For our hello application, the implementation is trivial.
Add the following return statement between the splicer lines
in the <TT>lib/Hello_World_Impl.cc</TT> file:
<P>
<BR>
<PRE CLASS="verbatim">std::string
Hello::World_impl::getMsg()
throw ()
{
// DO-NOT-DELETE splicer.begin(Hello.World.getMsg)
return std::string("Hello World!");
// DO-NOT-DELETE splicer.end(Hello.World.getMsg)
}
</PRE></td></tr></table></blockquote>
<P>
To keep the Makefile<A NAME="1549"></A> simple, we will use some GNU Make features. This
Makefile may not work with other make implementations. The GNU gcc and
g++ compilers are used in this example. The following Makefile in the
lib/ subdirectory will compile the library files and create a shared
library named <TT>libhello.so</TT>:
<A NAME="1551"></A>
<P>
<BR>
<PRE CLASS="verbatim">.cc.o:
g++ -fPIC -I$(HOME)/babel/include -c $<
.c.o:
gcc -fPIC -I$(HOME)/babel/include -c $<
include babel.make
OBJS = ${IMPLSRCS:.cc=.o} ${IORSRCS:.c=.o} \
${SKELSRCS:.cc=.o} ${STUBSRCS:.cc=.o}
libhello.so: ${OBJS}
g++ -shared -o $@ ${OBJS}
clean:
${RM} *.o libhello.so
</PRE></td></tr></table></blockquote>
<P>
You do not necessarily need to create a shared library for this example;
you may generate a standard static library (e.g., libhello.a).
However, in general, you must generate a shared library if you will be
calling your library from Python or Java.
To create the shared library archive libhello.so, simply execute make as
follows:
<P>
<BLOCKQUOTE>
<TT> <TT>%</TT> <TT><B>cd lib/</B></TT>
<BR><TT>%</TT> <TT><B>make libhello.so</B></TT>
<BR>
</TT></BLOCKQUOTE>
<P>
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<br><br>babel-0.10.2<br>users_guide Last Modified 2005-03-23<br><br><a href="http://www.llnl.gov/CASC/components">http://www.llnl.gov/CASC/components</a><br><a href="mailto:components@llnl.gov">components@llnl.gov</a>
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