boost:python passing an pointer to pointer as parameter - c++

Question

First, i'm not a python programmer, so excuse my silly mistakes.
In C++ I have this public method from MyClass that creates a image dynamically and returns its size.
int MyClass::getImg(uchar *uimg[])
{
int size = variable_size;
*_uimg = new uchar[size];
memcpy(*_uimg, imageOrigin->data(), size);
uimg = _uimg;
return size;
}
And the boost:python:
BOOST_PYTHON_MODULE(mymodule)
{
class_<MyClass>("MyClass")
.def("getImg", &MyClass::getImg)
;
}
and when i try to use it in python:
def getImg():
img = c_char_p()
size = mymodule.MyClass.getImg(byref(img))
I'm getting this error:
Boost.Python.ArgumentError: Python argument types in
MyClass.getImg(MyClass, CArgObject, CArgObject)
did not match C++ signature:
getImg(class MyClass {lvalue}, unsigned char * *)
In python I also tried declaring
img = POINTER(c_ubyte)()
but that did'nt help either.
I googled it around for hours and i didn't came up with any good solution.
I just need access to that image in python, how can i get this done?

Answer 1

Revised and working version, exposing the data as a python list.
I am not sure how to implement this, but there may be a better way of implementing it, if you can choose your python interface freely. I think creating a list may be a better way to expose it. What do you expect from the return value? Maybe you can also wrap it a s a numpy array for better performance (there are also some questions on stackoverflow if you search for boost python and numphy)
class TestPtr
{
public:
int getImg(uchar **uimg)
{
int size = 9045;
uchar *_uimg = new uchar[size];
for( int i = 0; i < size; ++i )
_uimg[i] = i;
*uimg = _uimg;
return size;
}
};
boost::python::list getImgList( TestPtr &p )
{
uchar *uimg;
int rv = p.getImg(&uimg);
boost::python::list l;
for ( int i = 0; i < rv; ++i)
l.append( uimg[i] );
return l;
}
BOOST_PYTHON_MODULE(mymodule)
{
class_<TestPtr>("TestPtr")
.def("getImg",&getImgList)
;
}
Another hint, in your question, you have used the following line to call the method:
mymodule.MyClass.getImg()
This will make a class call (more or less equivalent to a static call) to your object. This is a mor ecorrect way to do it:
img = mymodule.MyClass()
data = img.getImg()