GNU Gengen

A parameterized-text-generator generator based on a template

by Lorenzo Bettini, http://www.lorenzobettini.it

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Table of Contents

   * Download
   * Simple example
   * Gengen manual

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GNU Gengen (GENerator GENerator) is a tool that, starting from a parameterized
text, called template, generates a text generator that can substitute
parameters with values.

At the moment Gengen generates C++ or C code; 
however other target languages are
under development.

Gengen is free software; you are free to use, share and modify it under the
terms of the GNU General Public License (see COPYING).

The home page of Gengen is
http://www.gnu.org/software/gengen/index.html

See NEWS file for a summary of new features in this release and ChangeLog
for the complete list of changes sources

Download

Gengen is delivered with sources that have to be compiled. It can be
downloaded from the above site.

The steps to perform for the installation are standard for packages in
source form: once you have unpacked the sources in a directory, `cd' to the
directory containing the package's source code and execute the following
steps:

./configure
make
make install

Note: unless you specify a different install directory by --prefix option of
configure (e.g. ./configure --prefix=<your home>), you must be root to 'make
install'.

I do not distribute Windows binaries anymore; since, they can be
easily built by using Cygnus C/C++ compiler, available at
http://www.cygwin.com/. However, if you don't feel like downloading
such compiler, you can request such binaries directly to me, by e-mail
and I'll send them to you.

Simple example

Say you are writing a C++ program and at some point your program has to
generate the following code:

if (i < 10)
  printf("the value of i is %d", i);

Suppose now that the previous piece of code has to be generated many times
by your program, and every time instead of i another symbol has to be
generated (decided at run time). In this case, supposing that this value is
contained in a variable symb, the code for generating this code would be a
little bit complex:

cout << "if (" << symb << "< 10)" << endl;
cout << "  printf(\"the value of " << symb << " is %d\", "
     << symb << ");" << endl;

Probably you didn't even realize that you forgot to leave a space before the
< 10; basically this is due to the fact that this piece of code mixes the
code that has to be generated with the code that generates it, and this
tends to make this part of program less easy to maintain. Especially if some
day you have to change the code that has to be generated, you'll have to act
on this part of the program, and probably you'll have to execute some tests
in order to be sure that you did it right.

If the code that you have to generate is a slightly more complex, the task
may easily become a pain in the neck!

Wouldn't it be nice if you could write the code to be generated in a
separate file, let's call it template, say `test1.cc_skel' this way

if (@i@ < 10)
  printf("the value of @i@ is %d", @i@);

and have a tool that generates a generator, that you can instantiate at
run-time with the value that has to be substituted to the parameteri?

Well, Gengen does right this! I simply had to run the following command:

gengen -i test1.cc_skel --file-name test1.h --gen-name test1

and it generates a file `test1.h' with a class test1_gen_class, then you
could write simply this code, in another file, say `test1_gen.cc':

#include <iostream.h>
#include "test1.h"

int
main()
{
  test1_gen_class gen_class;
  gen_class.set_i("foo");
  gen_class.generate_test1(cout);
  cout << endl;
  gen_class.set_i("bar");
  gen_class.generate_test1(cout);
  cout << endl;

  return 0;
}

and when you run it you would obtain the expected output:

if (foo < 10)
  printf("the value of foo is %d", foo);
if (bar < 10)
  printf("the value of bar is %d", bar);

Notice that the method generate_test1 accepts an output stream (indeed in
this example the standard output stream cout is used), thus the stream
abstraction facilities can be exploited.

Now the code that has to be generated and the code that generates it are
separated and they can be maintained more easily: if you want to change the
code that has to be generated you act on the file `test1.cc_skel';
alternatively, say you need to change the value that will be substituted for
i, you just change the file `test1_gen.cc'.

Gengen manual

For installation and all the advanced features of Gengen and more involved
examples, please read the manual that comes both in info version and html
version.

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Please send me any suggestion, http://www.lorenzobettini.it
You may also send them to Raman Gopalan <ramangopalan@gmail.com>
