Re: [Boost-users] shared_ptr, intrusive_ptr ...

From: Stefan Strasser (sstrasser_at_[hidden])
Date: 2005-05-14 15:05:26

Jean-François Brouillet schrieb:
> I'm confused again :(
> 
> I've read the note about shared_ptr & this in the documentation,
> but I'm still a bit lost as to how I'm supposed to overcome the
> problem.
> 
> The code below would work fine if I were to remove all this smart
> pointer business and handle new & delete by hand. But because
> this kind of code is just so typical of the huge Java library
> I'm porting, there's not much choice but to get an as "surprise
> free" object model as possible.
> 

in case you've not left out anything important:
I think overriding a virtual function for callback is more appropriate 
here than storing a "cross-casted" version of the this-pointer.

however, if the solution may be intrusive you can derive your classes 
from boost::shared_from_this, and use "shared_from_this()", which 
returns a shared_ptr, instead of "this".

> 
> #include <iostream>
> 
> #include <boost/shared_ptr.hpp>
> 
> struct TaskDoneStruct {
>     virtual void    success() = 0 ;
>     virtual void    failure() = 0 ;
> } ;
> 
> typedef boost::shared_ptr<TaskDoneStruct> TaskDone ;
> 
> struct LongTaskStruct {
> 
>     TaskDone    callback ;
> 
>     virtual void    setCallback(const TaskDone& td) {
>         this->callback = td ;
>     }
> 
>     virtual void run() {
>         start() ;
>         complete() ;
>     }
> 
>     virtual void start() = 0 ;
>     virtual void complete() {
>         if (callback) {
>             callback->success() ;
>         }
>     }
> 
>     virtual     ~LongTaskStruct() {}
> } ;
> 
> struct MyTaskStruct : public LongTaskStruct, TaskDoneStruct {
> 
>     MyTaskStruct() {
>         setCallback(TaskDone(this)) ;  // this is the root of the
>                                        // double free problem
>     }
> 
>     virtual void start() {
>         std::cout << "Starting lengthy process ..." << std::endl ;
>     }
> 
>     virtual void success() {
>         std::cout << "Done lengthy process ..." << std::endl ;
>     }
> 
>     virtual void failure() {
>         std::cout << "Aborted lengthy process ..." << std::endl ;
>     }
> } ;
> 
> typedef boost::shared_ptr<MyTaskStruct> MyTask ;
> 
> void
> tastTest() {
>     MyTask  myTask(new MyTaskStruct) ;
> 
>     myTask->run() ;
> }
> 
> int
> main(
>     int     argc
> ,   char *  argv) {
> 
>     std::cout << "Tests starting ..." << std::endl ;
> 
>     tastTest() ;
> 
>     std::cout << "Tests done ..." << std::endl ;
> 
>     return 0 ;
> }
> 
> /*
> 
> BoostTests(1270) malloc: ***  Deallocation of a pointer not
> malloced: 0x300180; This could be a double free(), or free()
> called with the middle of an allocated block;
> 
> */
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> 
> 

-- 
Stefan Strasser