麻烦析构函数，用boost.python中的指针调用python的对象

Question

when I call python's function with pointer as an argument in boost.python, there are some troubles in destructor. The following is a sample code

c++

#include <boost/python.hpp>
#include <boost/python/module.hpp>
#include <boost/python/def.hpp>
#include <string>
#include <iostream>
using namespace boost::python;

class A {
public:
    A() {
        std::cout<< "A start"<<std::endl;
    }
    ~A() {std::cout<< "A end" <<std::endl;}
}
class B {
public:
    B() { aa=new A; }
    ~B() { delete aa; }
    void run(object ct) {
        _obj=ct();              //creat a python object
        _obj.attr("fun1")(aa);  //call a function named "fun1" with a pointer arg
        _obj.attr("fun2")(aa);  //call a function named "fun2" with a pointer arg 
    }
    A *aa;
    object _obj;
}

BOOST_PYTHON_MODULE(ctopy)
{
    class_<A> ("A",init<>())
    ;
    class_<b> ("B",init<>())
    .def("run",&B::run)
    ;
}

python:

import ctopy
class tc:
    def fun1(self,tt):
        print "fun1"
    def fun2(self,tt):
        print "fun2"
bb=ctopy.D()
bb.run(tc)

this result:

A start
fun1
A end
fun2
A end
A end

note:

The "A end" has been printed three.I try it in "valgrind"，there are some errors.I just want to run the destructor once.How to do?

Answer 1

To understand what's really happening, you're missing a copy-constructor:

B()
  A()
B::run()
  A(a)     <- copy construct A to pass by value
    fun1() <- arg passed by value
  ~A()     <- delete copy
  A(a)     <- copy construct A to pass by value
    fun2() <- arg passed by value
  ~A()     <- delete copy
~B()
  ~A()

In your example, aa is passed by value, that's where the copies come from. It does not matter if aa is a pointer or not, boost will convert it to an object that is passed to your python methods.

If you want to avoid additional copies of A you can pass aa by reference rather than by value :

_obj.attr("fun1")(boost::ref(aa));
_obj.attr("fun2")(boost::ref(aa));

Which will result in:

B()
  A()    <- constructed once
B::run()
  fun1() <- arg passed by reference
  fun2() <- arg passed by reference
~B()
  ~A()   <- destroyed once

See Calling Python Functions and Methods for more info.