Functional Code Breaks When Used Twice.

Question

I'm still working on my Field class, and tried to improve my piss-poor insertion/erase performance.

However, the new function works once, then breaks catastrophically when I use it a second time.

This is the code:

template <class T>
T *Field<T>::insert(const T *pPos, const T& data)
{
    // Special case:  field is empty. insert should still succeed.
    // Special case: Pointing to one past the end. insert should still succeed
    if( empty() || pPos == last() )
    {
        this->push_back(data);
        return (this->last() - 1);
    }

    /* Explanation:  Find cell before which to insert new value. Push_back new
      new value, then keep swapping cells until reaching *pPos and swapping it
      with data. The while fails, we exit, insert successful. */
    T *p = ( std::find( this->first(), this->last(), *pPos ));
    if( p != last() )
    {
        this->push_back(data);

        T *right = (this->last() - 1);
        T *left  = (this->last() - 2);
        while( *pPos != data )
            std::iter_swap( left--, right-- ); 

    // pPos *has* to be the destination of new value, so we simply return param.
        return const_cast<T*>(pPos);
    }
    else
        throw std::range_error("Not found");
}

Calling code from main

// Field already has push_back()ed values 10, 20, 30.
field->insert( &(*field)[2], 25 ); // field is a ptr (no reason, just bad choice)

Produces this output when printed on the console.

Field: 10 20 30    // Original Field
25                 // Function return value
Field: 10 20 25 30 // Correct insertion.

New calling code from main

// Field already has push_back()ed values 10, 20, 30
field->insert( &(*field)[2], 25 );
field->insert( &(*field)[3], 35 );

Produces this output when printed on the console.

Field: 10 20 30
25
35
-4.2201...e+37, 10, 15, 20, 30

Windows has triggered a breakpoint in Pg_1.exe. 
This may be due to a corruption in the heap (oh shit).

No symbols are loaded for any call stack frame. 
The source code cannot be displayed.

The console then proceeds to never shutdown again until I close VSC++08 itself.

What? Why? How? What is my code doing!?

Additional Info

The Field has a size of three before the push, and a capacity of four. After two insertions, the Field is correctly increased to have a capacity of 8 (doubled), and stores five elements.

It doesn't matter where I insert my second element with insert(), it will fail the exact same way. Same output, even same number (I think) at the first cell.

Additional Code

Push_Back()

Note: This code was not changed during my refactoring. This function has always worked, so I highly doubt that this will be the problem-cause.

/* FieldImpl takes care of memory management. it stores the values v_, vused_,
  and vsize_. Cells in the Field are only constructed when needed through a 
  placement new that is done through a helper function. */
template <class T>
void Field<T>::push_back(const T& data)
{
    if( impl_.vsize_ == impl_.vused_ )
    {
        Field temp( (impl_.vsize_ == 0) ? 1 
                                        : (impl_.vsize_ * 2) );

        while( temp.impl_.vused_ != this->impl_.vused_ )
            temp.push_back( this->impl_.v_[temp.size()] );

        temp.push_back(data);
        impl_.Swap(temp.impl_);
    }
    else
    {
// T *last()  const { return &impl_.v_[impl_.vused_]; }
// Returns pointer to one past the last constructed block.
// variant:     T *last()  const { return impl_.v_; }
        Helpers::construct( last(), data );
        ++impl_.vused_;
    }
}

Answer 1

// ...
if( p != last() )
{
    this->push_back(data);

After this line pPos may not be a valid pointer anymore.

The console then proceeds to never shutdown again until I close VSC++08 itself.

Tried clicking the Stop button in the debugger?

Answer 2

From the Debugger, and from ybungalobill, it is possible to see that pPos is invalidated after a special case in the

if( p != last()
{
    this->push_back(data);

part of the code. If the array is resized, the pointer is invalidated. To bridge this, I simply stored const T pos = *pPos before the push and therefore removed the use of the *pPos pointer after a push.

Updated code:

const T pos = *pPos;
T *p = ( std::find( this->first(), this->last(), pos ) );
if( p != last() )
{
    this->push_back(data);
    p = ( std::find( this->first(), this->last(), pos ) );

    T *right = (this->last() - 1);
    T *left  = (this->last() - 2);
    while( *p != data )
        std::iter_swap( left--, right-- ); 

    return const_cast<T*>(p);
}