[C++-sig] TypeError: No to_python (by-value) converter found for C++ type: class std::basic_ostream<char, struct std::char_traits<char> >

[Simon Warg]

And I forgot to tell you, the python class in an extension of a c++  
class. Hence I have the serialise and deserialise function defined in  
python.

// Simon

On 29 sep 2010, at 16.02, Jakub Zytka <kubol at kormoran.net> wrote:

> On 09/29/10 14:11, Simon W wrote:
>> Thank you for the answer. Is there any way I can prevent it from  
>> copying the
>> stream object even though std::ostream::write() returns a  
>> std::ostream& ?
> You can use other return policy, or define one on your own.
>
> http://www.boost.org/doc/libs/1_44_0/libs/python/doc/tutorial/doc/html/python/functions.html#python.call_policies
>
> But honestly, I do not think it is the correct way to go. Read below.
>
> > I'm trying to serialise some data. I have a c++ class:
> >          class ostream
> >          {
> > ...
> > could to the c++ way and add a integer before the byte stream that  
> reveal how
> > big it is. But How can i serialise an int from python that takes  
> up constant
> > "byte-size" for an integer between lets say 0-10000!?
> Everything can be done, but I have a different question:
> I haven't followed all your previous emails, but do you *really*  
> need to implement serialization code in python?
> Somehow I feel it is very weird that on one hand you want to have  
> serialization code in python, and yet use ostream.
>
> What would be wrong with coding serialization purely in C++, and  
> then providing appropriate pickle suite to python:
> struct MyPickleSuite
> {
> // ...
> boost::python::tuple MyPickleSuite::getstate(MyClass & object)
> {
> std::ostringstream oStream(ios_base::binary);
> serialize(oStream, object);
> return boost::python::make_tuple(oStream.str());
> }
>
> void MyPickleSuite::setstate(MyClass & object, boost::python::tuple  
> state)
> {
> // verify tuple correctness etc.
> std::string serializedData = extract<std::string>(state[0]);
> std::istringstream iStream(serializedData, ios_base::binary);
> serialize(iStream, object); // NOTE: you can use the same code for  
> serialization and deserialization. stream type should determine  
> actual behavior
> }
> }
>
> That way you do not have to expose ostream to python at all. In fact  
> if you wanted to switch to another type of stream (imagine that you  
> have a custom stream implementation, which provides eg. type  
> checking) it is cheap. You only change a few functions in your  
> pickle suite, nothing more.
>
> Even if you do really need to define serialization in python I see  
> no reason to play with streams. I'd rather had a wrapper which hides  
> underlying implementation (eg. std::streams) and operates on strings.
>
> Please reconsider your design.
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