What wrong for I failing call Visual c++ class's method from python

Question

My environment and Version:

    Windows 10,
    Microsoft Visual Studio Professional 2017:
            Visual Project: 'hello2' / Debug / x86,
    Python 3.9.6 (tags/v3.9.6:db3ff76, Jun 28 2021, 15:04:37) [MSC v.1929 32 bit (Intel)],
    Boost 1.77.0:
            Boost Builds command:
            > b2 --build-type=complete address-model=32 --toolset=msvc-14.1  stage >build.log 2>&1
                    Remarks:
                    I also download and try Boost prebuild (binary) from:
                    https://sourceforge.net/projects/boost/files/boost-binaries/1.77.0/
                    but having same problem (as describe below) while I includes and link with this prebuild Boost.

The project's code copied from Boost Example/document: https://www.boost.org/doc/libs/1_77_0/libs/python/doc/html/tutorial/tutorial/exposing.html

Steps summary:

1. Build Visual DLL project hello2 DLL includes: C++ class, Boost.Python wrapper

2. Copied 'hello2.dll' to hello2.pyd

3. python interpreter: import hello2 (actual hello2.pyd) as a python module

   (try to access the class method - but faile - see below) … ….

Detailing:

Visual Project: 'hello2', File: hello2.cpp:

#include "pch.h"
    
#include <boost/python.hpp>
using namespace boost::python;

struct World
{
    void set(std::string msg) { this->msg = msg; }
    std::string greet() { return msg; }

    std::string msg;
};
    
BOOST_PYTHON_MODULE(hello2)
{
    class_<World>("World")
    .def("set", &World::set)
    .def("greet", &World::greet)
    ;
}

On Windows Commands line:

C:\Users\haimh\AppData\Local\Programs\Python\OnGointHH\SlnForTestProjects\Debug> copy hello2.dll hello2.pyd 1 file(s) copied.

Try-1 (On Windows Commands line):

C:\Users\haimh\AppData\Local\Programs\Python\OnGointHH\SlnForTestProjects\Debug> py

Python 3.9.6 (tags/v3.9.6:db3ff76, Jun 28 2021, 15:04:37) [MSC v.1929 32 bit (Intel)] on win32 Type "help", "copyright", "credits" or "license" for more information.

>>>
>>> import hello2
>>> planet = hello2.World()

[Error message thrown in popup window: Mrcrosoft Visual C++ Runtime Library Assertion failed] [1]: https://i.stack.imgur.com/Fc1wo.jpg

Try-2 (On Windows Commands line):

C:\Users\haimh\AppData\Local\Programs\Python\OnGointHH\SlnForTestProjects\Debug> py

Python 3.9.6 (tags/v3.9.6:db3ff76, Jun 28 2021, 15:04:37) [MSC v.1929 32 bit (Intel)] on win32 Type "help", "copyright", "credits" or "license" for more information.

>>>
>>> import hello2
>>> planet = hello2.World
>>> planet.set('howdy')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
Boost.Python.ArgumentError: Python argument types in
    World.set(str)
did not match C++ signature:
    set(struct World {lvalue}, class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >)
>>>

Answer 1

I'm not sure why your first try ( pl.net = hello2.World() ) didn't work. My best (not-so-good) guess is a version mismatch between the python headers and the python interpreter you're using. You can check this using the python version macros .

As for the second version - this doesn't work because set is not a static method. Only static methods can be called directly on the class World . Regular methods need to be called on instances of the class. Trying to do that in pure Python will get you a similar error:

>>> class Klass:
...     def f(self):
...         pass
...
>>> Klass.f()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: f() missing 1 required positional argument: 'self'

In the C++ case, you're missing the argument struct World {lvalue} which is the equivalent of self.

Answer 2

I found - at least how to make settings create a working code/project...

The answer is: Change Visual C++ settings from 'Debug' to 'Release'... Then the program run fine without any problem.

Thanks to everyone who tried to help.