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https://github.com/marekjm/gengen

GNU Gengen - A parameterized-text-generator generator based on a template
https://github.com/marekjm/gengen

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GNU Gengen - A parameterized-text-generator generator based on a template

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README 

        
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=), 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 

#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