[C++-sig] problems with typedef void *

[Stefan Seefeld]

On 12/12/2010 04:02 PM, Jacob Davis wrote:
> Howdy!
>
> I'm wrapping a C libary with Boost Python and having a really 
> difficult time with void * typedefs. For example, here's a typedef and 
> the function header that uses it:
>
> typedef void * EmoEngineEventHandle;
> EmoEngineEventHandle EE_EmoEngineEventCreate();
>
> And my undoubtedly overly simplistic wrapper for the function:
>
> def("EE_EmoEngineEventCreate", EE_EmoEngineEventCreate);
>
> The compiler complains with lots of similar messages:
>
> 1>C:\boost_1_45_0\boost/python/detail/caller.hpp(223) : error C2027: 
> use of undefined type 
> 'boost::python::detail::specify_a_return_value_policy_to_wrap_functions_returning<T>'

There are a couple of issues with your approach. Note that the line 
above is a trick to pass a message through a (very noisy) compiler 
error. The "specify_a_return_value_policy..." piece above is telling you 
that you can't wrap a function returning a void pointer without 
explicitly telling boost.python what to do with the pointer. Normally it 
would attempt to pass by-value, but that obviously doesn't work (you 
can't dereference a void pointer !).
But, if you want to pass by-reference, how should the pointer be treated 
? Who owns the data ?

More importantly, you don't use a strong type system in your API, since 
"types" are aliasing void pointers. Thus, the compiler can't help you 
distinguish e.g. function overloads, and there is no way to 
auto-generate a type registry.

You may wrap your API with a C++ wrapper, though I honestly doubt this 
is practical. In fact, I'm in doubt that boost.python is the right 
choice for your Python wrapping effort. Have you considered alternatives ?

     Stefan

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