如何将返回C ++自定义类的函数与Cython接口？

Question

I have this C++ function prototype:

 Bar & getBar()

where Bar is a user-defined C++ class. Now I would like to interface this function with Cython, in the simpliest way possible.

My .pyx file looks like this:

cdef class PyBar:
   cdef Bar* thisptr 
   (...)

def pygetBar():
   cdef Bar bar = getBar()  
   cdef PyBar pybar = PyBar()

   (*pybar.thisptr) =  bar  # <-- too bad, do not work

Whatever I try I cannot copy a C++ Bar instance in a 'thisptr'. For the line above, I get this Cython error message:

 Cannot assign type 'Bar' to 'Bar *'

The only solution I found is to define a C++ helper function that just does the copy:

void helper_copy_bar(Bar* source, Bar* destination)
{ *destination = *source }

While this works, it's pretty annoying to create a new C++ function each time I just want to interface such a simple function. There must be something I'm missing from the docs.

Edit :

I found a workaround which has other disadvantages:

 def pygetBar():
    cdef Bar * bar = new Bar(getBar())  
    cdef PyBar pybar = PyBar()
    del pybar.thisptr
    pybar.thisptr = bar

    return pybar

It still involves a useless new/delete of a c++ object but at least there is no need for an external function (good for the lazy). I'm still wondering which solution (an external function or the second one) is better, and if there is a way to use the first solution without an external function.

Answer 1

Bar& is reference to Bar, it is not a pointer to Bar, although internally, compilers may implement it as a pointer. Easiest way to simplify this would be to write a helper C function, I'm not aware that Cython has explicit support for C++? Your "workaround" looks suitable.