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This is a discussion on Linked list within the C++ Programming forums, part of the General Programming Boards category; This is driving me nuts, so I came here. Code: class CTestTask : public ITask { public: bool Start(); void ...

  1. #1
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    Linked list

    This is driving me nuts, so I came here.

    Code:
    class CTestTask : public ITask
    {
    public:
    
    	bool Start();
    	void Update();
    	void Stop();
    
    private:
    
    	std::list < particle * > particles;
    
    };
    This is called every frame.

    Code:
    void CTestTask::Update()
    {
    
    for( int i = 0; i < (PARTICLES_ADDED_PER_SECOND+1; i++ )
            particles.push_back( new SnowParticle );
    
    for (std::list<particle*>::iterator part=particles.begin(); part != particles.end(); part++)
        {   
        // We first test the particle to see if it should remain alive 
    		//if (*part)
    		{
    			//(*part) = new SnowParticle;
    			if( (*part)->TestParticle() )
    			{
    				// If it should, we update and draw the particle 
    				(*part)->UpdateParticle();
    				(*part)->DrawParticle();
    				// And then move onto the next particle 
    			}
    			else
    			{
    				// If we end it here, then it continues until the point that it would have crashed, and stops making particles???
    
    				// Otherwise we cut the particle out of the list, and trash it. 
    				delete (*part);
    				//particle * oldparticle = (*part);
    				particles.erase( part, part );
    				//delete oldparticle;
    			}
    		}
        }
    if (*part)->TestParticle() returns false, it should delete the memory for the particle and then delete the pointer from the linked list.

    This causes a crash . If I don't free the memory from the new, then it works, but there's a memory leak...

  2. #2
    Registered User jlou's Avatar
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    If the crash is at the line where you delete the memory, then the problem might be in the destructor (I assume that particle base class has a virtual destructor). However, it looks like the erase call is what could be causing you problems. You are erasing an element in a list, then using the iterator pointing at the erased element in your next iteration. I don't think that will work. You should save a copy of the position to be erased, then increment part, then erase the saved iterator.

  3. #3
    Registered User jlou's Avatar
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    Actually, a better solution might be:

    part = particles.erase(part);
    part--;


    Since erase returns an iterator to the next element, but you have a for loop that is incrementing on its own.

  4. #4
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    class Particle does NOT have a destructor other than the default. So why is deleting the memory causing a crash?

    i.e.

    Code:
    part = particles.erase(part);
    part--;
    works, but

    Code:
    delete (*part);
    part = particles.erase( part );
    part--;
    and

    Code:
    Particle * deadparticle = (*part);
    part = ParticleList.erase(part);
    part--;
    delete (deadparticle);
    do not.
    Last edited by Trauts; 03-12-2004 at 07:04 PM.

  5. #5
    Registered User jlou's Avatar
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    Since particle is a base class, you must give it a virtual destructor. Deleting an object of a derived type from a pointer to the base class is undefined (meaning it could cause a crash) if the base class does not have a virtual destructor. Try adding the following to the particle class and see if it helps.
    Code:
    virtual ~particle() { }
    EDIT - Also, if that doesn't solve it, the problem could be in any of the destructors up the inheritance chain all the way to SnowParticle.
    Last edited by jlou; 03-12-2004 at 07:53 PM.

  6. #6
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    thanks.

    Thats gotta be it. I didn't realize thats why everybody uses virtual constructors in their classes

    Works like a charm.

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