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shared_ptr usage

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Started by bkt July 18, 2005 05:00 PM

10 comments, last by bkt 18 years, 9 months ago

bkt

178

Author

July 18, 2005 05:00 PM

I've never used boost::shared_ptr before, but I think that I'm getting problems at compile time with the boost::shared_ptr library. Here's the errors I get, and the lines that cooresponds with it.


c:\Documents and Settings\Johnny\Desktop\engine\core\engine.cpp(31): error C2440: 'delete' : cannot convert from 'boost::shared_ptr<T>' to 'void *'
        with
        [
            T=logFile
        ]
c:\Documents and Settings\Johnny\Desktop\engine\core\engine.cpp(27): error C2440: 'delete' : cannot convert from 'boost::shared_ptr<T>' to 'void *'
        with
        [
            T=CKernel
        ]
c:\Documents and Settings\Johnny\Desktop\engine\core\engine.cpp(15): error C2679: binary '=' : no operator found which takes a right-hand operand of type 'logFile *' (or there is no acceptable conversion)
c:\Documents and Settings\Johnny\Desktop\engine\core\engine.cpp(14): error C2679: binary '=' : no operator found which takes a right-hand operand of type 'CKernel *' (or there is no acceptable conversion)

Here are some of the lines:


	m_pKernel = new CKernel;
	log		  = new logFile("log.txt");
---
	delete m_pKernel;
//	CEngineVar::release();

	log->close();
	delete log;

-John "bKT" Bellone [homepage] [[email=j.bellone@flipsidesoftware.com]email[/email]]

chollida1

532

July 18, 2005 05:14 PM

Well if m_pKernel is a shared_ptr you wouldn't delete it, you let it go out of scope to delete it.you can also call m_pKernel.Release() on it.

Cheers
Chris

CheersChris

nilkn

960

July 18, 2005 05:22 PM

boost::shared_ptr has weird symantics. You need to use the constructor to initialize it:

// assuming m_pKernel and log are boost::shared_ptrsm_pKernel = boost::shared_ptr<CKernel>(new CKernel);log = boost::shared_ptr<logFile>(new logFile("log.txt"));

And like chollida1 said, you shouldn't call delete on smart pointers. shared_ptr will destroy itself automatically when it's ref count reaches 0.

gumpy

795

July 18, 2005 05:25 PM

boost::shared_ptr<logFile> log( new logFile("log.txt") );

when the shared_ptr (log) goes out of scope, the logFile instance is automatically deleted for you. keep in mind that shared_ptr is also reference counted and can be stored in stl containers(unlike auto_ptr).

This space for rent.

bkt

178

Author

July 18, 2005 05:45 PM

I have a linked list of shared_ptr<ITask> and I'm looking to typecast down from a shared_ptr<VideoTask>, shared_ptr<TimerTask>, shared_ptr<LogicTask> into the list (they all devrive from the ITask object). Here's the code I've been trying to get working:

	taskList.push_back( shared_ptr<ITask> (reinterpret_cast<ITask> (&m_pTimer)) );	taskList.push_back( shared_ptr<ITask> (reinterpret_cast<ITask> (&m_pVideo)) );	taskList.push_back( shared_ptr<ITask> (reinterpret_cast<ITask> (&m_pLogic)) );

Here's the linked list object:

class CTaskList : public list< shared_ptr<ITask> >{};

But that doesn't seem to be liking me too much. I'll fiddle around with a little bit more, but from what I'm gathering, the ampersand operator doesn't return a pointer to the object inside the shared_ptr?

-John "bKT" Bellone [homepage] [[email=j.bellone@flipsidesoftware.com]email[/email]]

MaulingMonkey

1,729

July 18, 2005 06:21 PM

Quote:Original post by bkt
But that doesn't seem to be liking me too much. I'll fiddle around with a little bit more, but from what I'm gathering, the ampersand operator doesn't return a pointer to the object inside the shared_ptr?

Nope, it's getting the addrses of the shared_ptr structure itself (similarly, using the ampersand operator on a pointer would get a pointer to a pointer, not return the original pointer). Use the .get() member to get a raw pointer. Note: Constructing a second shared_ptr from that raw pointer will in fact cause the item to be deleted twice!!! So don't do this:

boost::shared_ptr< foo > foo1( new foo );boost::shared_ptr< foo > foo2( foo1.get() ); //ERROR: We'll delete foo twice!!!

Or do so indirectly using casts.
I'm curious as to the types of m_pTimer, m_pVideo, and m_pLogic are. If they are of types that derive from ITask, then there's no need to do any casting jumbo:

struct ITask {};struct TimerTask : public ITask {};boost::shared_ptr< TimerTask > m_pTimer( new TimerTask );boost::shared_ptr< ITask > foo( m_pTimer );

For 3 of the 4 _cast<> operators, there is a boost equivilant. Let's assume we want to use dynamic_cast to ensure a pointer to an ITask is in fact a TimerTask. We could write:

boost::shared_ptr< ITask > foo( ... );boost::shared_ptr< TimerTask > Timer( dynamic_cast< TimerTask * >( foo.get() ) );

This would be bad, as we would have constructed Timer from a raw pointer (causing the double delete problem originally outlined). The alternative:

boost::shared_ptr< ITask > foo( ... );boost::shared_ptr< TimerTask > Timer( boost::dynamic_pointer_cast< TimerTask >( foo ) );

This code will not delete our task twice, and will in fact function as expected.

The full list of casts available are:

static_cast< T * >  -> boost::static_pointer_cast< T >const_cast< T * >   -> boost::const_pointer_cast< T >dynamic_cast< T * > -> boost::dynamic_pointer_cast< T >

More Documentation

bkt

178

Author

July 18, 2005 06:36 PM

No, actually, m_pVideo, m_pLogic, and m_pTimer are setup just as you've shown here:

struct ITask {};struct TimerTask : public ITask {};boost::shared_ptr< TimerTask > m_pTimer( new TimerTask );boost::shared_ptr< ITask > foo( m_pTimer );

But the problem is that when I do that (because it does compile fine, that was the first thing that I tried) the code crashed and gave me a NULL pointer assertion when I attempted to use the shared_ptr from the list< shared_ptr<ITask> >.

-John "bKT" Bellone [homepage] [[email=j.bellone@flipsidesoftware.com]email[/email]]

Andrew Russell

1,394

July 18, 2005 08:52 PM

Quote:Original post by bkt
But the problem is that when I do that (because it does compile fine, that was the first thing that I tried) the code crashed and gave me a NULL pointer assertion when I attempted to use the shared_ptr from the list< shared_ptr<ITask> >.

This is odd, because this code is perfect. Your problem will be elsewhere then.

struct ITask {};struct TimerTask : public ITask {};boost::shared_ptr<TimerTask> m_pTimer(new TimerTask);boost::shared_ptr<ITask> foo(m_pTimer);

Your problem probably has something to do with:

taskList.push_back( shared_ptr<ITask> (reinterpret_cast<ITask> (&m_pTimer)) );taskList.push_back( shared_ptr<ITask> (reinterpret_cast<ITask> (&m_pVideo)) );taskList.push_back( shared_ptr<ITask> (reinterpret_cast<ITask> (&m_pLogic)) );

Which it utterly terrible. Try this code:

// definitionsstd::list<shared_ptr<ITask> > tasklist;shared_ptr<TimerTask> m_pTimer(new TimerTask);// etc...// the pushingtaskList.push_back(shared_ptr<ITask>(m_pTimer));taskList.push_back(shared_ptr<ITask>(m_pVideo));taskList.push_back(shared_ptr<ITask>(m_pLogic));

You probably want to read the manual on reinterpret_cast. It has absolutly no business being used in that code.

AndrewRussell.net

bkt

178

Author

July 18, 2005 09:54 PM

That's what I changed it to afterwords when it didn't work, before hand though, this is what it looked like:

taskList.push_back( m_pTimer );taskList.push_back( m_pVideo );taskList.push_back( m_pLogic );

I only started tampering with it when that gave me the assert fail.

-John "bKT" Bellone [homepage] [[email=j.bellone@flipsidesoftware.com]email[/email]]

bkt

178

Author

July 19, 2005 08:20 AM

I've had a chance to run everything through the debugger, and for some reason the pointers inside the list seem to be NULL after I pass them on to the boot function of my kernel object. Here's the code:

void CKernel::boot(CTaskList taskList){  m_TaskList = taskList;  for(CTaskList::iterator it=m_TaskList.begin();	it != m_TaskList.end(); it++)		LINK_TASK_TO_KERNEL( (*it) );    run();}CEngine::CEngine(void){	m_pKernel = shared_ptr<CKernel> (new CKernel);	log		  = shared_ptr<logFile> (new logFile("log.txt"));	CTaskList taskList; // used to pass into the kernel booting process	// we need to add the tasks to the task list in order for the kernel to run	// them. this makes everything as abstract as possible so we can cut and paste	taskList.push_back( m_pTimer );	taskList.push_back( m_pVideo );	taskList.push_back( m_pLogic );	m_pKernel->boot( taskList );	// *** we're running at this point, and should only exit afrer the loop dies ***}

I'm sorry about all these questions, I've just never used these shared pointers before and they're all a little it confusing. I'm not sure if I can use them with the STL, if I can't (although I believe someone said its safe to use these, and not auto_ptr?). Thanks for everyone's help.

-John "bKT" Bellone [homepage] [[email=j.bellone@flipsidesoftware.com]email[/email]]

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