[C++-sig] why does the "shared_ptr<X> const&" silently become 0xCCCCCCCC

[Roman Yakovenko]

2009/3/11 ZaeX <zaexage at gmail.com>:
> Hi, All:
>
> I used a "shared_ptr<A> const&" to store a ref of A in B;
> Well, the constructor of B seems ok, the share_ptr<A> seems to be well
> initialized;
> but after the constructor, I found the the pointer silently become
> 0xCCCCCCCC
>
> ///////////////////////////////////// below are the simplified class
> definitions
> class A{};
>
> class B
> {
> private:
>    shared_ptr<A> const& m_ptr;
> public:
>    B(shared_ptr<A> const& ptr):m_ptr(ptr) {cout<<m_ptr.get()<<endl;}
>    void ShowPtr(cout<<m_ptr.get()<<endl;)
> }
> /////////////////////////////////// below are my exporting code
>
>     class_<B, auto_ptr<B> >("B",  init<shared_ptr<A> const&>())
>        .def("ShowPtr", &B::ShowPtr)
>     ;
>     class_<A, shared_ptr<A>, noncopyable >("A")
>     ;
> /////////////////////////////////// below are python code
>  a = A()
>  b = B(a)  #cout output the ptr of a
>  b.ShowPtr()  #cout output 0xCCCCCCCC
>
> ============================================
> I'm quite confused by the behaviors here. Could you tell me what goes wrong?

You have to bind lifetime argument to the lifetime of the created
object: http://www.boost.org/doc/libs/1_38_0/libs/python/doc/tutorial/doc/html/python/functions.html#python.call_policies

Constructors also have call policies and you can use them.

Anyway, you will save to yourself a lot of time, if you will use
shared_ptr by value. I suggest you to change your code.

-- 
Roman Yakovenko
C++ Python language binding
http://www.language-binding.net/