==================
Registration order
==================

.. contents:: Table of contents

------------
Introduction
------------

"... I would very much like to pass booleans from Python to C++ and have them
accepted as boils. However I cannot seem to do this. ..."

"... My class has 2 "append" functions. The first one, has single argument with
type "const char*", the second one also has single argument with type "char".
It seems, that I am not able to call the first function. ..."

If you have problem similar to described ones, than I am almost sure you have
registration order problem.

-------
Example
-------

.. code-block:: C++

  struct tester_t{
      tester_t() {}

      const char* append(const char*)
      { return "append(const char *)"; }

      const char* append(const char)
      { return "append(const char)"; }

      const char* do_smth( bool )
      { return "do_smth( bool )"; }

      const char* do_smth( int )
      { return "do_smth( int )"; }
  };

`Py++`_ generates code, that register functions in the order they appear in the
source code:

.. code-block:: C++

  namespace bp = boost::python;

  BOOST_PYTHON_MODULE(my_module){
    bp::class_< tester_t >( "tester_t" )
        .def( bp::init< >() )
        .def( "append"
            , (char const * ( ::tester_t::* )( char const * ) )( &::tester_t::append ) )
        .def( "append"
            , (char const * ( ::tester_t::* )( char const ) )( &::tester_t::append ) )
        .def( "do_smth"
            , (char const * ( ::tester_t::* )( bool ) )( &::tester_t::do_smth ) )
        .def( "do_smth"
            , (char const * ( ::tester_t::* )( int ) )( &::tester_t::do_smth ) );
  }

---------------------------
Registration order pitfalls
---------------------------

Do you want to guess what is the output of the following program:

.. code-block:: Python

  import my_module
  tester = my_module.tester_t()
  print tester.do_smth( True )
  print tester.do_smth( 10 )
  print tester.append( "Hello world!" )

?

The output is:

  do_smth( int )

  do_smth( int )

  append(const char)

Unexpected, right? The registration order of exposed overloaded functions is
important. `Boost.Python`_ tries overloads in reverse order of definition.

If I understand right, `Boost.Python`_ tries to match in reverse order the
overloaded functions, if it can convert `Python`_ arguments to C++ ones, it does
this and calls the function.

Now, when you understand the behavior, it should be pretty simple to provide
a correct functionality:

1. You can change alias of the function, by mangling the type of the argument
   into it:

  .. code-block:: Python

    mb = module_builder_t( ... )
    for f in mb.class_( 'tester_t' ).member_functions():
        f.alias = f.alias + f.arguments[0].type.decl_string

2. You can reorder the declarations within the source file.

3. You can ask `Py++`_ to generate code, which takes into account the order of
   declarations:

   .. code-block:: Python

     from pyplusplus.module_creator import sort_algorithms

     sort_algorithms.USE_CALLDEF_ORGANIZER = True
     # The functionality is available from version 0.8.3

4. The last and the perfect solution. `Py++`_ will let you know, when your code
   has such problem. The functionality is available from version 0.8.3. After
   this you can change the aliases of the functions. The third step is to create
   small "dispatch" function in Python:

   .. code-block:: Python

     import my_module

     def tester_t_do_smth( self, value ):
         if isinstance( value, bool ):
             self.do_smth_bool( value ):
         else:
             self.do_smth_int( value )

     tester_t.do_smth = tester_t_do_smth

   The technique shown here described pretty good in `Boost.Python`_
   `Extending Wrapped Objects in Python tutorials`_ .

   May be in future, `Py++`_ will generate this code for you. Anyway, if you have
   a lot of use cases like this consider to generate `Python`_ code and not to
   write it manually.

.. _`Extending Wrapped Objects in Python tutorials` : http://boost.org/libs/python/doc/tutorial/doc/html/python/techniques.html#python.extending_wrapped_objects_in_python




.. _`Py++` : ./../../pyplusplus.html
.. _`Boost.Python`: http://www.boost.org/libs/python/doc/index.html
.. _`Python`: http://www.python.org
.. _`GCC-XML`: http://www.gccxml.org
