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LoneDwarf

Locked container

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Does anyone have a nice solution to detecting when an STL container is being modified while it's being iterated over? In the following code, I would like to assert if listpObjects gets modified as a side effect of pObject->update(). I don't want to put an if inside the loop either.
typedef std::list< Object* >	ObjectPtrList;
typedef ObjectPtrList::iterator	ObjectPtrListIter;
//==================================================================================================
//==================================================================================================
void UpdateAll( ObjectPtrList& listpObjects )
{
	for( ObjectPtrListIter iter = listpObjects.end(); iter != listpObjects.end(); ++iter )
	{
		Object*	pObject = *iter;
		pObject->update();
	}
}

I wrote a very simple wrapper for std::list that acquires the list but it's not all that hot, plus I will have to write it for other containers.
//==================================================================================================
//==================================================================================================
template < typename TYPE >
class STL_LockList
{
private:
	//----------------------------------------------------------------------------------------------
	typedef	std::list< TYPE >	ListType;

	//----------------------------------------------------------------------------------------------
	bool			m_bLocked;
	ListType		m_list;

public:
	//----------------------------------------------------------------------------------------------
	STL_LockList() : m_bLocked( false ) 
	{

	}
	~STL_LockList()
	{
		T_Assert( m_bLocked == false );
	}

	//----------------------------------------------------------------------------------------------
	ListType&		lock()
	{
		T_Assert( m_bLocked == false );
		m_bLocked = true;
		return m_list;
	}
	//----------------------------------------------------------------------------------------------
	void			unlock()
	{
		T_Assert( m_bLocked == true );
		m_bLocked = false;
	}

	//----------------------------------------------------------------------------------------------
	void			push_front( const TYPE& item )
	{
		T_Assert( m_bLocked == false );
		m_list.push_front( item );
	}
	//----------------------------------------------------------------------------------------------
	void			push_back( const TYPE& item )
	{
		T_Assert( m_bLocked == false );
		m_list.push_back( item );
	}
	//----------------------------------------------------------------------------------------------
	void			remove( const TYPE& item )
	{
		T_Assert( m_bLocked == false );
		m_list.remove( item );
	}
};

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I'm assuming this is for concurrency work.

the STL is not threadsafe. you should look into using a thread-safe library for those containers that need to support concurrent access; and use the STL for those that you know will only be accessed in a single thread.

intel released a multi-thread safe library with some container primitives that are simple to use:
http://www.intel.com/cd/software/products/asmo-na/eng/294797.htm

-me

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Quote:
Original post by Palidine
I'm assuming this is for concurrency work.


No it's not. It's just to protect against bugs, hence the assert. These lists are for stuff like Physics::UpdateAll(), AI::UpdateAll() etc. It can be a hard bug to trace down if somehow one of these lists is modified, say an object being destroyed, while I am iterating over it.

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Quote:
Original post by Tesshu
Quote:
Original post by Palidine
I'm assuming this is for concurrency work.


No it's not. It's just to protect against bugs, hence the assert. These lists are for stuff like Physics::UpdateAll(), AI::UpdateAll() etc. It can be a hard bug to trace down if somehow one of these lists is modified, say an object being destroyed, while I am iterating over it.


No, there is no reliable way.

Use containers where iterators aren't invalidated, or structure your code in such a way that it cannot happen. std::list is one way, although performance penalty is likely unacceptable.

This is a considerable problem with events and callbacks, where callback may modify subscriptions while the dispatcher is iterating through the same collection.

There's are solutions, but neither is the silver bullet.

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Quote:
Original post by Antheus


There's are solutions, but neither is the silver bullet.


Thank you for the link, it's a very good read. I actually had the method of using a vector and instead of removing the element, I assigned NULL to it. Thing is that you have to put an if NULL in the loop, which makes me feel dirty. I decided to make it policy not not allow updating the list at all while iterating over it.

I was just hoping that there would be version of STL that had this kind of support :(

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