Question

当我想在Python中使用C ++多态时，我对pybind11有一个奇怪的行为。这是我的问题的一个简单示例：

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

但预期的输出是

[MyBase，MyDerived]

MyBase

MyDerived

因为mylist返回一个std :: vector，它包含一个派生类（MyDerived）的实例作为第二个成员。奇怪的是，当我打印整个列表时，MyDerived被识别出来。

这是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

这是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＆＃34; print＆＃34;对于MyDerived，我收到以下错误消息

[MyBase，MyDerived]

MyBase

追踪（最近一次呼叫最后一次）：

文件＆＃34; test.py＆＃34;，第8行，

一个[1] .PRINT（）

AttributeError：＆＃39; polymorphism.MyDerived＆＃39;对象没有属性＆＃39; print＆＃39;

这条消息似乎意味着MyDerived在被调用错误版本的打印时得到了很好的认可（如果我理解的话）。

编辑2：这是一个使用trampoline类的版本。但是，此版本导致相同的错误输出。

/* 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

这是相应的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pybind11似乎在mylist()中的原始指针有问题。如果将返回类型更改为vector<unique_ptr<MyBase>>，则示例可正常工作。以下示例编译为python模块example并生成预期输出。

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 shell：

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

多态性和pybind11

1 个答案: