Boost Python: Error when passing variable by reference in a function

问题

I would like to understand why the following function does not work in python:

#include<boost/python.hpp>
#include<iostream>
#include<string>

void hello(std::string& s) {

   std::cout << s << std::endl;
}

BOOST_PYTHON_MODULE(test)
{
   boost::python::def("hello", hello);
}

When I import library into python

import test
test.hello('John')

I get an error:

test.hello(str)
did not match C++ signature:
   hello(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > {lvalue})

Everything works with just 'std::string s', but I would like to the object by reference without copying it. I noticed that the error pops up for any other function with references like e.g. int&.

回答1:

As kindall mentioned, python strings are immutable. one solution can be a stl::string wrapper. your python side will be affected, but it's a sipmle solution and a small price to pay for that interface

class_<std::string>("StlString")
        .def(init<std::string>())
        .def_readonly("data", &std::string::data);

note that the assign operator, python to c++ stl will be given by boost, but you'll have to expose the data member in order to access the stored data. Iv'e added another costructor for the look and feel of c++ object.

>>> import test
>>> stlstr = test.StlString()
>>> stlstr
<test.StlString object at 0x7f1f0cda74c8>
>>> stlstr.data
''
>>> stlstr = 'John'
>>> stlstr.data
'John'
>>> initstr = test.StlString('John')
>>> initstr.data
'John'

来源：https://stackoverflow.com/questions/47296287/boost-python-error-when-passing-variable-by-reference-in-a-function