Re: [Boost-users] Boost shared pointers, PThreads and void* casts

From: Andrew Holden (aholden_at_[hidden])
Date: 2007-12-19 12:33:50

Andrew Holden [aholden_at_[hidden]] wrote:
> STenyaK (Bruno Gonzalez) [stenyak_at_[hidden]] wrote:
>> Ok. This is how my code structure looks:
>> ----------------------------
>> typedef shared_ptr<MyClass> pMyClass;
>> void* MyClass::actionThread(void *objectPointer) {
>>     pMyClass * _obj = static_cast<pMyClass*>(objectPointer);    
>>     pMyClass obj = *_obj; obj->action(false);
>>     delete _obj;
>> }
>> void MyClass::action(bool threaded)
>> {
>>     if (threaded)
>>     {
>>         pthread_t tid;
>>         pthread_create(&tid, 0, actionThread, new pMyClass(this))
>>     } else {
>            //TODO: actually perform the action
>>     }
>> }
>> ----------------------------
>
> ----------------------------
> typedef shared_ptr<MyClass> pMyClass;
> void* MyClass::actionThread(void *objectPointer) {
>     MyClass * _obj = static_cast<MyClass*>(objectPointer);
>     pMyClass obj = _obj->shared_from_this();
>     obj->action(false);
> }
> 
> void MyClass::action(bool threaded)
> {
>     if (threaded)
>     {
>         pthread_t tid;
>         pthread_create(&tid, 0, actionThread, this)     } else {
>         //TODO: actually perform the action
>     }
> }
> ----------------------------

I just realized you should ignore this implementation because it
contains a race condition (main thread may delete the object before
actionThread can call shared_from_this.  Here is a tweak for your
version that uses shared_from_this:

----------------------------
typedef shared_ptr<MyClass> pMyClass;
void* MyClass::actionThread(void *objectPointer) {
    pMyClass * _obj = static_cast<pMyClass*>(objectPointer);
    pMyClass obj = *_obj;
    delete _obj;
    obj->action(false);
}
void MyClass::action(bool threaded)
{
    if (threaded)
    {
        pthread_t tid;

//Change next line:
        pthread_create(&tid, 0, actionThread, new
pMyClass(shared_from_this()))
    } else {
        //TODO: actually perform the action
    }
}
----------------------------