Re: throw exception in c++ and return it to python

> I'm trying to build a C++ library doing the complex mathematics with a
> Python interpreter on top of it.
>
> In the application it occasionally occurs that exceptions are thrown.
> I'd like to catch these exceptions in Python, but a really would like to
> catch not the standard exceptions, but my own exception written in c++
> and - via swig - wrapped to a Python exception.
>
> This is illustrated for a small example.
>
> I've got a class Matrix with a setValue method. If the position is out
> of the range of the allocated memory of the matrix, an exception of the
> type MyException is thrown.
>
> //File Matrix.h
> #ifndef MATRIX_H
> #define MATRIX_H
>
> #include <vector>
> #include "MyException.h"
> class Matrix
> {
>
> public:
>     Matrix(int numRows_, int numColumns_, std::vector<double> entries_)
> {
>     int numEntries = numRows_ * numColumns_;
>     if (numEntries!=entries_.size())
>         throw MyException(std::string("Size of vector does not
> correspond to number of rows/colums\n"));
> entries = entries_;
> numRows = numRows_;
> numColumns = numColumns_;
> }
>
>     void setValue(int row_, int col_, double value)
> {
> if (row_<0 || col_<0 || row_>=numRows || col_>=numColumns)
> {
> throw MyException(std::string("Position out of matrix dimension."));
> }
> else
>    entries[col_*numRows+row_] = value;
> }
>
> protected:
> int numRows, numColumns;
> std::vector<double> entries;
> };
>
>
> #endif // MATRIX_H
>
>
>
>
> //File MyException.h
> #ifndef MYEXCEPTION_H
> #define MYEXCEPTION_H
>
> #include <string>
> #include <exception>
>
>
> class MyException : public std::exception
> {
>   protected:
> std::string message;
>
>   public:
> explicit MyException(const std::string&  m) : message(m) {}
> virtual ~MyException() throw() {}
> virtual const char* what() const throw()
> {
> return message.c_str();
> }
> };
>
> #endif  // MYEXCEPTION_H
>
>
> Using swig, I've created the wrapper files using the MyProject.i to
> create the module MyProject
>
> //file MyProject.i
> %module MyProject
> %{
> /* Put headers and other declarations here */
> #include "Matrix.h"
> #include "MyException.h"
> %}
>
> // user std::vectors on python level
> %include "std_vector.i"
> %include "std_string.i"
>
> // Instantiate templates used by example
> namespace std
> {
>    %template(DoubleVector) vector<double>;
> }
>
> %include "MyException.h"
> %include "exception.i"
>
> %exception
> {
>   try
>   {
>     $action
>   }
>   catch (MyException)
>   {
>     SWIG_exception_fail(SWIG_ValueError, "MyException");
>   }
>
>   SWIG_CATCH_STDEXCEPT // catch std::exception
>   catch (...)
>   {
>     SWIG_exception_fail(SWIG_UnknownError, "Unknown exception");
>   }
> }
>
> // provide the public interface
> %include "Matrix.h"
>
>
> Finally, I've applied this in a Python routine test.py
>
> //file test.py
> import MyProject
>
> doubleentries = MyProject.DoubleVector(0)
> doubleentries.resize(4)
> doubleentries[0] = 1
> doubleentries[1] = 2
> doubleentries[2] = 3
> doubleentries[3] = 4.5
>
> a=MyProject.Matrix(2,2,doubleentries)
> try:
>     a.setValue(-1,-1,-1e12)
>
> except MyProject.MyException as e:
>     print e
>     print "That was MyException"
> except ValueError as e:
>     print e
>     print "That was a Value Error"
> except :
>     print "Ohh, some undefined Error occured"
>
>
> What actually happens in the MyProject.i is, that the MyException is
> catched from the c++ file and swig creates a ValueError-exception, but
> instead of passing a ValueError-exception (or any other standard
> exception) to Python, I'd like to pass directly Myexception to Python.
> Myexception is via SWIG already a data type known in Python, but how can
> a actually modify the line
>   catch (MyException)
>   {
>     SWIG_exception_fail(SWIG_ValueError, "MyException");
>   }
>
> to something like
>   catch (MyException)
>   {
>     SWIG_exception_fail(SWIG_MyException "MyException");
>   }
>