Pybind11：将字符串*参数传递给构造函数 [英] Pybind11: passing a string* argument to a constructor

问题描述

在不允许更改的C ++库中，我有一个类似如下的构造函数：

In a C++ library that I'm not allowed to change I have a constructor that looks like this:

Dfa(const int n_state, const int dim_alf, const string *alf);

如果我只是绑定

.def(py::init<const int, const int, const std::string*>())

它编译成功。
问题是我无法通过python传递字符串*，因为例如，如果我尝试在python上执行

it compiles succesfully. The problem is that I can't pass a string* by python, because for example if I try to execute on python

alph=['x','y']
z=Dfa(3,2,alph)

它返回以下错误：

TypeError: __init__(): incompatible constructor arguments. The
following argument types are supported:
gi_gipy.Dfa(arg0: int, arg1: int, arg2: unicode)

R zu用户建议我写一个包装器，但我不知道该怎么做。鉴于python中的内容类似于： ['x'，'y'] ，因此c ++中的
被接受为 std :: list< std :: string>
，我尝试编写以下代码：

User "R zu" kindly suggested me to write a wrapper, but I can't figure out how. Given that what in python is something like: ['x','y'] , in c++ is accepted as std::list<std::string> , I tried writing the following code:

.def(py::init([](int n_state,int dim_alf, std::list<std::string> alph){
         std::string* alfabeto=new std::string[dim_alf];
         std::list<string>::iterator it=alph.begin();
         for(int i=0;it!=alph.end();++i,++it)  alfabeto[i]=*it;
         Dfa::Dfa(n_state,dim_alf,alfabeto);
}))

但它会向我返回2个错误：

but it returns to me 2 errors:

cannot pass expression of type 'void' to variadic function
construct<Class>(v_h, func(std::forward<Args>(args)...)

和

static_assert failed "pybind11::init(): init function must return a compatible pointer,
  holder, or value"
static_assert(!std::is_same<Class, Class>::value /* always false */

很明显，我对如何克服感到困惑我认为这个问题与使用字符串指针作为构造函数的参数有关。重复一遍，我无法更改库，只能创建适当的绑定。谢谢您的关注

It is clear that I'm a bit confused on how to overcome this problem, that I think is connected to the use of a pointer to string as a parameter to a constructor. I repeat that I can't change the library, I can only create the appropriate binding. Thank you for your attention

推荐答案

main.cpp：

#include <iostream>
#include <list>
#include "pybind11/pybind11.h"
#include <pybind11/stl.h>
namespace py = pybind11;

class Dfa{
public:
    Dfa(const int n_state, const std::size_t size, const std::string* alpha)
            : alpha_(*alpha) {
        std::cout << "n_state: " << n_state << "\n";
        std::cout << "size: " << size << "\n";
        std::cout << "*alpha: " << *alpha << "\n";
    }
    // copy the std::string, not the reference or pointer.
    std::string alpha_; 
};

Dfa make_dfa(int n_state, std::string alpha) {
    // Copies the python unicode str to a local std::string
    // Modifying the local copy won't change the python
    // str.
    return Dfa(n_state, alpha.size(), &alpha);
    // Problem: Once the program leaves this function,
    // This local std::string is destroyed.
    // If the Dfa class just copies the pointer to this
    // std::string instead of the std::string, the Dfa
    // class will use a destroyed string.
    // Unless the Dfa object copies the string, this will
    // cause big trouble.
}

void print_char_list(std::list<char> alpha) {
    for (auto c: alpha) std::cout << c << ", ";
    std::cout << "\n";
    std::cout << "length of list is: " << alpha.size() << "\n";
}

PYBIND11_MODULE(_cpp, m) {
    py::class_<Dfa>(m, "Dfa")
            .def_readwrite("alpha", &Dfa::alpha_);;
    m.def("make_dfa", &make_dfa, "Create a Dfa object");
    m.def("print_char_list", &print_char_list, "Print a list of chars");
}

CMakeLists.txt：

CMakeLists.txt:

cmake_minimum_required(VERSION 3.9)
project(test_pybind11)

set(CMAKE_CXX_STANDARD 11)

# Find packages.
set(PYTHON_VERSION 3)
find_package( PythonInterp ${PYTHON_VERSION} REQUIRED )
find_package( PythonLibs ${PYTHON_VERSION} REQUIRED )

# Download pybind11
set(pybind11_url https://github.com/pybind/pybind11/archive/stable.zip)

set(downloaded_file ${CMAKE_BINARY_DIR}/pybind11-stable.zip)
file(DOWNLOAD ${pybind11_url} ${downloaded_file})
execute_process(COMMAND ${CMAKE_COMMAND} -E tar xzf ${downloaded_file}
        SHOW_PROGRESS)
file(REMOVE ${downloaded_file})

set(pybind11_dir ${CMAKE_BINARY_DIR}/pybind11-stable)
add_subdirectory(${pybind11_dir})
include_directories(${pybind11_dir}/include)

# Make python module
pybind11_add_module(_cpp main.cpp)

Python 3测试：

Python 3 test:

>>> import _cpp
>>> s = "xyz"
>>> d = _cpp.make_dfa(1, s)
n_state: 1
size: 3
*alpha: xyz
>>> print(d.alpha)
xyz
>>> d.alpha = "abc"
>>> d.alpha
'abc'
>>> _cpp.print_char_list(['x', 'y', 'z'])
x, y, z, 
length of list is: 3

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