.. _exceptions_guide:

Exceptions in PCL
-----------------
There have been a multitude of discussions in the past regarding exceptions in
PCL (see http://www.pcl-developers.org/to-throw-or-not-to-throw-td4828759.html
for an example). Herein, we discuss the major points with respect to writing
and using exceptions.

Adding a new Exception 
======================

Any new exception should inherit from the :pcl:`PCLException <pcl::PCLException>` class in
``pcl/exceptions.h``

.. code-block:: cpp

  /** \class MyException
    * \brief An exception that is thrown when I want it.
    */
		
  class PCL_EXPORTS MyException : public PCLException
  {
    public:
      MyException (const std::string& error_description,
                   const char* file_name = NULL,
                   const char* function_name = NULL,
                   unsigned line_number = 0)
        : pcl::PCLException (error_description, file_name, function_name, line_number) { }  
  };

Using Exceptions
================

For ease of use we provide this macro

.. code-block:: cpp

  #define PCL_THROW_EXCEPTION (ExceptionName, message)
  {
    std::ostringstream s;
    s << message;
    throw ExceptionName (s.str (), __FILE__, BOOST_CURRENT_FUNCTION, __LINE__);
  }

Then in your code, add:

.. code-block:: cpp

  if (my_requirements != the_parameters_used_)
    PCL_THROW_EXCEPTION (MyException, "my requirements are not met " << the_parameters_used);

This will set the file name and the line number thanks to the macro definition.
Take care of the message: it is the most important part of the exception. You
can profit of the std::ostringstream used in the macro, so you can append
variable names to variable values and so on to make it really explicit.  Also
something really important is when the method you are writing ``can`` throw an
exception, please add this to the the function documentation:

.. code-block:: cpp

  /** Function that does cool stuff
    * \param nb number of points
    * \throws MyException
    */
  void 
  myFunction (int nb);

This will be parsed by Doxygen and made available in the generated API
documentation so the person that would use your function knows that they have
to deal with an exception called ``MyException``.

Exceptions handling
===================

To properly handle exceptions you need to use the ``try``... ``catch`` block.

.. code-block:: cpp

  // Here we call myFunction which can throw MyException
  try
  {
    myObject.myFunction (some_number);
    // You can put more exceptions throwing instruction within same try block
  }
  // We catch only MyException to be very specific
  catch (pcl::MyException& e)
  {
    // Code to deal with the exception maybe changing myObject.the_parameters_used_
  }

  // Here we catch any exception
  #if 0
  catch (exception& e)
  {
    // Code to deal with the exception maybe changing myObject.the_parameters_used_
  }
  #endif

Exceptions handling is really context dependent so there is no general
rule that can be applied but here are some of the most used guidelines:

  * exit with some error if the exception is critical
  * modify the parameters for the function that threw the exception and recall it again
  * throw an exception with a meaningful message saying that you encountered an exception
  * continue (really bad)

