[C++-sig] Re: Howto wrap operator[]? (boost::python)

[Ralf W. Grosse-Kunstleve]

--- Mike Rovner <mike at bindkey.com> wrote:
> 
> "Nicodemus" <nicodemus at globalite.com.br> wrote in message
> news:3EEA3FE1.3080601 at globalite.com.br...
> 
> > template <class T>
> > void vector_setitem(std::vector<T>& v, int index, T value)
> > {
> >     if (index >= 0 && index < v.size()) {
> >         v[index] = value;
> >     }
> >     else {
> >         PyErr_SetString(PyExc_IndexError, "index out of range");
> >         throw_error_already_set();
> >     }
> > }
> 
> That will forbid very useful Python feature - negative index :(
> So better include
> 
>      if( index < 0 ) index+=v.size();
> 
> before your if.

One more nit: replace int by long to support very large arrays.
FWIW: here is my little helper function, carefully tailored to
mirror indexing of builtin lists:

  inline
  std::size_t
  positive_getitem_index(long i, std::size_t size)
  {
    if (i >= 0) {
      if (i >= size) raise_index_error();
      return i;
    }
    if (-i > size) raise_index_error();
    return size + i;
  }

The size parameter is the size of the indexed object.
raise_index_error() is another tiny helper equivalent to the
two lines for rasing the exception in the quoted section above.

Ralf


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