Pyth.Python：参数类型与C++的签名不匹配

Question

当Python调用C++函数时，我遇到了一个奇怪的问题。

我公开了一个要从中调用函数的类：

class_<MyClass, std::shared_ptr<MyClass>>("MyClass", init<>())
    // ...
    .def("someFunc", &MyClass::someFunc)
;

我从另一个通过.def_readonly(...)公开的类的成员变量中获取std::shared_ptr<MyClass>

当我试图调用该函数时，出现以下错误：

File "pytest.py", line 27, in test_func
    cu.someFunc("string")
Boost.Python.ArgumentError: Python argument types in
    MyClass.someFunc(MyClass, str)
did not match C++ signature:
    result(MyClass{lvalue}, std::string)

据我所见，签名是匹配的。 有人看到问题了吗？

Answer 1

如本文ticket所述，Boost.Python不完全支持std::shared_ptr。

简而言之，两个简单的解决方案是：

• 使用boost::shared_ptr而不是std::shared_ptr。
• 使用via add_property()公开std::shared_ptr成员变量，提供具有^{}类型的^{}。

---

虽然异常中的签名看起来相同，但微妙的细节是PythonMyClass对象嵌入了一个std::shared_ptr<MyClass>。因此，Boost.Python必须执行从std::shared_ptr<MyClass>到左值MyClass的转换。但是，Boost.Python目前不支持custom lvalue conversions。因此，抛出ArgumentError异常。

使用def_readonly("spam", &Factory::spam)公开成员变量时，相当于通过以下方式公开：

add_property("spam", make_getter(&Factory::spam, return_internal_reference()))

当以这种方式公开的类型是boost::shared_ptr时，Python有特殊的代码。由于它是只读属性并且std::shared_ptr是要复制的，因此可以安全地公开具有返回值策略且类型为return_by_value的std::shared_ptr的副本。

这里有一个完整的例子，其中Factory公开了std::shared_ptr持有的Spam对象和boost::shared_ptr持有的Egg对象：

#include <iostream>
#include <memory> // std::shared_ptr, std::make_shared
#include <string>
#include <boost/make_shared.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/python.hpp>

/// @brief Mockup Spam type.
struct Spam
{
  ~Spam() { std::cout << "~Spam()" << std::endl; }

  void someFunc(std::string str)
  {
    std::cout << "Spam::someFunc() " << this << " : " << str << std::endl;
  }
};

/// @brief Mockup Egg type.
struct Egg
{
  ~Egg() { std::cout << "~Egg()" << std::endl; }

  void someFunc(std::string str)
  {
    std::cout << "Egg::someFunc() " << this << " : " << str << std::endl;
  }
};

/// @brief Mockup Factory type.
struct Factory
{
  Factory()
    : spam(std::make_shared<Spam>()),
      egg(boost::make_shared<Egg>())
  {
    spam->someFunc("factory");
    egg->someFunc("factory");
  }

  std::shared_ptr<Spam>   spam;
  boost::shared_ptr<Egg>  egg;
};

BOOST_PYTHON_MODULE(example)
{
  namespace python = boost::python;

  // Expose Factory class and its member variables.
  python::class_<Factory>("Factory")
    // std::shared_ptr<Spam>
    .add_property("spam", python::make_getter(&Factory::spam,
        python::return_value_policy<python::return_by_value>()))
    // boost::shared_ptr<Egg>
    .def_readonly("egg", &Factory::egg)
    ;

  // Expose Spam as being held by std::shared_ptr.
  python::class_<Spam, std::shared_ptr<Spam>>("Spam")
    .def("someFunc", &Spam::someFunc)
    ;

  // Expose Egg as being held by boost::shared_ptr.
  python::class_<Egg, boost::shared_ptr<Egg>>("Egg")
    .def("someFunc", &Egg::someFunc)
    ;
}

交互式Python演示用法和对象生存期：

>>> import example
>>> factory = example.Factory()
Spam::someFunc() 0x8d73250 : factory
Egg::someFunc() 0x8d5dbc9 : factory
>>> factory.spam.someFunc("python")
Spam::someFunc() 0x8d73250 : python
>>> factory.egg.someFunc("python")
Egg::someFunc() 0x8d5dbc9 : python
>>> factory = None
~Egg()
~Spam()
>>> factory = example.Factory()
Spam::someFunc() 0x8d73250 : factory
Egg::someFunc() 0x8d06569 : factory
>>> spam = factory.spam
>>> factory = None
~Egg()
>>> spam.someFunc("python")
Spam::someFunc() 0x8d73250 : python
>>> spam = None
~Spam()
>>> factory = example.Factory()
Spam::someFunc() 0x8d73250 : factory
Egg::someFunc() 0x8ce10f9 : factory
>>> egg = factory.egg
>>> factory = None
~Spam()
>>> egg.someFunc("python")
Egg::someFunc() 0x8ce10f9 : python
>>> egg = None
~Egg()