多态性与pybind11

Question

当我想在Python中使用C++多态性时，我对C++有一种奇怪的行为。下面是我的问题的一个简单例子：

import polymorphism as plm

a = plm.mylist()

print(a)

a[0].print()
a[1].print()

此脚本的输出是

MyBase，MyDerived MyBase MyBase

但预期的产出是

MyBase，MyDerived MyBase MyDerived

因为mylist返回一个std::vector，它包含派生类(MyDerived)的一个实例作为第二个成员。奇怪的是，当我作为一个整体打印列表时，MyDerived就被识别了。

下面是C++代码的头文件：

/* polymorphism.hpp */

#ifndef POLYMORPHISM_HPP
#define POLYMORPHISM_HPP

#include <vector>

class MyBase
{
  public:
    virtual void print() const;
};


class MyDerived : public MyBase
{
  public:
   virtual void print() const;
};


std::vector<MyBase*> mylist();

#endif

以下是cpp文件：

#include "polymorphism.hpp"

#include <iostream>
#include <pybind11/stl.h>
#include <pybind11/pybind11.h>

void MyBase::print() const
{ std::cout << "MyBase" << std::endl; }

void MyDerived::print() const
{ std::cout << "MyDerived" << std::endl; }

std::vector<MyBase*> mylist()
{
  std::vector<MyBase*> list(2);
  list[0] = new MyBase();
  list[1] = new MyDerived();
  return list;
}

PYBIND11_MODULE(polymorphism, m) 
{
  pybind11::class_<MyBase>(m, "MyBase")
    .def(pybind11::init<>())
    .def("print", &MyBase::print)
    .def("__repr__", [](const MyBase &a) { return "MyBase"; });

  pybind11::class_<MyDerived, MyBase>(m, "MyDerived")
    .def(pybind11::init<>())
    .def("print", &MyDerived::print)
    .def("__repr__", [](const MyDerived &a) { return "MyDerived"; });

  m.def("mylist", &mylist, "return a list");
}

编辑:更令人惊讶的是，当我删除MyDerived的"print“绑定时，我会得到以下错误消息

MyBase，MyDerived MyBase 回溯(最近一次调用)： 文件"test.py"，第8行 A1.() AttributeError：'polymorphism.MyDerived‘对象没有属性'print’

这个消息似乎意味着MyDerived被很好地识别，而错误的打印版本被调用(如果我理解的话)。

编辑2:这是一个使用蹦床类的版本。但是，这个版本会导致同样的错误输出。

/* polymorphism.hpp */
#ifndef POLYMORPHISM_HPP
#define POLYMORPHISM_HPP

#include <vector>
#include <pybind11/stl.h>
#include <pybind11/pybind11.h>

class MyBase
{
  public:
    virtual void print() const;
};


class MyDerived : public MyBase
{
  public:
    virtual void print() const;
};


std::vector<MyBase*> mylist();

class PyMyBase : public MyBase 
{
  public:
    using MyBase::MyBase; // Inherit constructors
    void print() const override { PYBIND11_OVERLOAD(void, MyBase, print ); }
};

class PyMyDerived : public MyDerived 
{
  public:
    using MyDerived::MyDerived; // Inherit constructors
    void print() const override { PYBIND11_OVERLOAD(void, MyDerived, print);}
};

#endif

下面是对应的cpp文件：

/* polymorphism.cpp */
#include "polymorphism.hpp"

#include <iostream>

void MyBase::print() const
{ std::cout << "MyBase" << std::endl; }


void MyDerived::print() const
{ std::cout << "MyDerived" << std::endl; }


std::vector<MyBase*> mylist()
{
  std::vector<MyBase*> list(2);
  list[0] = new MyBase();
  list[1] = new MyDerived();
  return list;
}


PYBIND11_MODULE(polymorphism, m) 
{
   pybind11::class_<MyBase, PyMyBase>(m, "MyBase")
     .def(pybind11::init<>())
     .def("print", &MyBase::print)
     .def("__repr__", [](const MyBase &a) { return "MyBase"; });

   pybind11::class_<MyDerived, PyMyDerived>(m, "MyDerived")
     .def(pybind11::init<>())
     .def("print", &MyDerived::print)
     .def("__repr__", [](const MyDerived &a) { return "MyDerived"; });

   m.def("mylist", &mylist, "return a list");
}

Answer 1

我不知道为什么，但是pybind11似乎对mylist()中的原始指针有问题。如果将返回类型更改为vector<unique_ptr<MyBase>>，则示例将正确工作。下面的示例编译为python模块example并生成预期的输出。

example.cpp：

#include <pybind11/stl.h>
#include <pybind11/pybind11.h>
#include <iostream>
#include <memory>
#include <vector>

class MyBase {
public:
    virtual void print() const {
        std::cout << "MyBase::print()" << std::endl;
    }
};

class MyDerived : public MyBase {
public:
    virtual void print() const override {
        std::cout << "MyDerived::print()" << std::endl;
    }
};

std::vector<std::unique_ptr<MyBase>> mylist() {
    std::vector<std::unique_ptr<MyBase>> v;
    v.push_back(std::make_unique<MyBase>());
    v.push_back(std::make_unique<MyDerived>());
    return v;
}

PYBIND11_MODULE(example, m) {
    pybind11::class_<MyBase>(m, "MyBase")
        .def(pybind11::init<>())
        .def("print", &MyBase::print)
        .def("__repr__", [](MyBase const&) { return "MyBase"; });

    pybind11::class_<MyDerived>(m, "MyDerived")
        .def(pybind11::init<>())
        .def("print", &MyDerived::print)
        .def("__repr__", [](MyDerived const&) { return "MyDerived"; });

    m.def("mylist", &mylist, "returns a list");
}

python外壳：

>>> import example
>>> l = example.mylist()
>>> l[0].print()
MyBase::print()
>>> l[1].print()
MyDerived::print()