Boost shared_ptr question

Hello all,

I have a class called Wheel, and I created a boost shared_ptr to this class in the following way:

Code:

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typedef boost::shared_ptr<Wheel> WheelPtr;

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I also have a vector of these shared_ptr's to Wheels as so:

Code:

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vector<WheelPtr> wheels;

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In the wheel class I have a variable called "flat" which is initialized to true.

I have one thread that keeps iterating through the list looking for flat wheels like so:

Code:

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vector<WheelPtr>::iterator it;
for(it=wheels.begin(); it!=wheels.end();)
{
    if(!(*it)->Flat())
    {
      (*it) = WheelPtr(); // set to NULL here
        it=wheels.erase(it);
    }
    else
          it++;
}

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In another thread I am constantly doing something else to the wheels. What I have noticed is that when I execute the line:

Code:

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      (*it) = WheelPtr(); // set to NULL here

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in the debugger the handle is 0x00000000. But in my other thread the handle is something else like 0x00104f08.

Why is the shared_ptr not NULL in the other thread?

Regards,
Ellay K.

Quote:

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Originally Posted by ekhule

I also have a vector of these shared_ptr's to Wheels as so:

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Where do you do any synchronization? Writing a threaded program doesn't just mean you call the function to launch a thread.

A vector operations are not thread safe. If you're altering the vector at the same time you're reading from the vector, what do you think may/will happen?

http://stackoverflow.com/questions/3...race-condition

Regards,

Paul McKenzie

Also, this accomplishes ridding yourself of flat tires:

Code:

---

#include <algorithm>
//...
bool IsTireFlat(const WheelPtr& theWheel)
{  return theWheel->Flat(); }
//...
wheels.erase(std::remove_if(wheels.begin(), wheels.end(), IsTireFlat), wheels.end());

---

No loops needed.

But it still is the case that while you're altering the vector in one thread, no other threads should be attempting to read (or write) to the vector.

Regards,

Paul McKenzie