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

14 May 2005


      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 
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