I have a C++ class with a member function that takes an unsigned char* buffer and an unsigned int length as arguments and operates on them. I've wrapped this class with Boost::Python and would like to pass a pre-populated buffer to the class from a Python script. The Python-side buffer is created with struct.pack. I can't figure out how to make the argument type match and keep getting Boost.Python.ArgumentError.
#ifndef EXAMPLECLASS_H_
#define EXAMPLECLASS_H_
#include <cstdio>
class ExampleClass
{
public:
ExampleClass() {}
virtual ~ExampleClass() {}
void printBuffer(unsigned char* buffer, unsigned int length)
{
for (unsigned int i = 0; i < length; ++i)
{
printf("%c", buffer[i]);
}
printf("\n");
}
};
#endif
#include "Example.h"
int main(int argc, char** argv)
{
unsigned char buf[4];
buf[0] = 0x41;
buf[1] = 0x42;
buf[2] = 0x43;
buf[3] = 0x44;
ExampleClass e;
e.printBuffer(buf, 4);
return 0;
}
#include <boost/python.hpp>
#include "Example.h"
using namespace boost::python;
BOOST_PYTHON_MODULE(example_py)
{
class_<ExampleClass>("ExampleClass")
.def("printBuffer", &ExampleClass::printBuffer)
;
}
#!/usr/bin/env python
import example_py
import struct
import ctypes
buf = struct.pack('BBBB', 0x41, 0x42, 0x43, 0x44)
print 'python:'
print buf
e = example_py.ExampleClass()
print 'c++:'
print e.printBuffer(ctypes.cast(ctypes.c_char_p(buf), ctypes.POINTER(ctypes.c_ubyte)), len(buf))
include_directories(
include
${Boost_INCLUDE_DIRS}
${PYTHON_INCLUDE_DIRS}
)
add_library(example_py
src/Example_py.cpp
)
target_link_libraries(example_py ${Boost_LIBRARIES} ${PYTHON_LIBRARIES})
set_target_properties(example_py PROPERTIES PREFIX "")
add_executable(example src/example.cpp)
target_link_libraries(example example_py)
$ ./example
ABCD
$ ./scripts/example.py
python: ABCD
c++:
Traceback (most recent call last):
File "/home/dustingooding/example/scripts/example.py", line 13, in <module>
print 'c++:', e.printBuffer(ctypes.cast(ctypes.c_char_p(buf), ctypes.POINTER(ctypes.c_ubyte)), len(buf))
Boost.Python.ArgumentError: Python argument types in
ExampleClass.printBuffer(ExampleClass, LP_c_ubyte, int)
did not match C++ signature:
printBuffer(ExampleClass {lvalue}, unsigned char*, unsigned int)
I've tried a number of different approaches (passing 'buf' directly, passing 'buf' as a ctypes.c_char_p, creating a ctypes.ubyte array and populating it with the contents of 'buf' and passing it), but none seem to work.
I don't understand why 'LP_c_ubyte' and 'unsigned char*' don't match.
Here's a Github project with a ready-to-go codebase. Feel free to use this. I've added @Tanner's fix. https://github.com/dustingooding/boost_python_ucharp_example
It may be worth considering exposing a Pythonic auxiliary function as the ExampleClass.printBuffer
method to Python, that delegates to the c-ish ExampleClass::printBuffer
member function. For instance, this would allow the Python users to invoke:
import example
import struct
buf = struct.pack('BBBB', 0x41, 0x42, 0x43, 0x44)
e.printBuffer(buf)
Rather than requiring the user to perform the correct ctypes
cast and sizing.
The struct.pack()
method returns a str
object in Python2 and a bytes
object in Python3, so the auxiliary C++ function would need to populate a continuous block of memory with the elements of from either str
or bytes
. The boost::python::stl_input_iterator
can provide a convenient way to construct C++ containers, such as std::vector<char>
, from a Python object, such as str
or bytes
. The only oddity is that stl_input_iterator
expects the Python type to support the iterable protocol, which str
does not do. However, the builtin iter()
Python method can be used to create an iterable object.
/// @brief Auxiliary function used to allow a Python iterable object with char
/// elements to be passed to ExampleClass.printBuffer().
void example_class_print_buffer_wrap(
ExampleClass& self,
boost::python::object py_buffer)
{
namespace python = boost::python;
// `str` objects do not implement the iterator protcol (__iter__),
// but do implement the sequence protocol (__getitem__). Use the
// `iter()` builtin to create an iterator for the buffer.
// >>> __builtins__.iter(py_buffer)
python::object locals(python::borrowed(PyEval_GetLocals()));
python::object py_iter = locals["__builtins__"].attr("iter");
python::stl_input_iterator<char> begin(
py_iter(py_buffer)), end;
// Copy the py_buffer into a local buffer with known continguous memory.
std::vector<char> buffer(begin, end);
// Cast and delegate to the printBuffer member function.
self.printBuffer(
reinterpret_cast<unsigned char*>(&buffer[0]),
buffer.size());
}
With the auxiliary function created, one just needs to expose it as the ExampleClass.printBuffer
method:
BOOST_PYTHON_MODULE(example)
{
namespace python = boost::python;
python::class_<ExampleClass>("ExampleClass")
.def("printBuffer", &example_class_print_buffer_wrap)
;
}
Here is a complete example demonstrating this approach:
#include <cstdio>
#include <vector>
#include <boost/python.hpp>
#include <boost/python/stl_iterator.hpp>
// Mocks...
/// @brief Legacy class that cannot be changed.
class ExampleClass
{
public:
void printBuffer(unsigned char* buffer, unsigned int length)
{
for (unsigned int i = 0; i < length; ++i)
{
printf("%c", buffer[i]);
}
printf("\n");
}
};
/// @brief Auxiliary function used to allow a Python iterable object with char
/// elements to be passed to ExampleClass.printBuffer().
void example_class_print_buffer_wrap(
ExampleClass& self,
boost::python::object py_buffer)
{
namespace python = boost::python;
// `str` objects do not implement the iterator protcol (__iter__),
// but do implement the sequence protocol (__getitem__). Use the
// `iter()` builtin to create an iterator for the buffer.
// >>> __builtins__.iter(py_buffer)
python::object locals(python::borrowed(PyEval_GetLocals()));
python::object py_iter = locals["__builtins__"].attr("iter");
python::stl_input_iterator<char> begin(
py_iter(py_buffer)), end;
// Copy the py_buffer into a local buffer with known continguous memory.
std::vector<char> buffer(begin, end);
// Cast and delegate to the printBuffer member function.
self.printBuffer(
reinterpret_cast<unsigned char*>(&buffer[0]),
buffer.size());
}
BOOST_PYTHON_MODULE(example)
{
namespace python = boost::python;
python::class_<ExampleClass>("ExampleClass")
.def("printBuffer", &example_class_print_buffer_wrap)
;
}
Interactive usage:
>>> import example
>>> import struct
>>> buf = struct.pack('BBBB', 0x41, 0x42, 0x43, 0x44)
>>> print 'python:', buf
python: ABCD
>>> e = example.ExampleClass()
>>> e.printBuffer(buf)
ABCD
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