Re: [boost] Re: Boost::Python iterators are off by 1?

From: Dan Eloff (dan.eloff_at_[hidden])
Date: 2004-09-02 19:36:41

>Just use boost::iterator_facade to implement your iterator and myriad
>issues will disappear.

Almost, it cut the code down to about 1/3 of the size and  1/6 the
complexity, allowing me to much more easily see where the problems
were and how to fix them.

However, one problem did not go away. Boost::Python does the C++ equivelent of:

for( iter it = ctn.begin(); !(it == ctn.end()); )
     *it++;

The choice of post-increment was evil in my scenario. I had a C
third-part function returning a pointer to a heap allocated structure
or NULL if there is no more data. Calling the function a second time
frees the previous results memory.

No matter how I looked at the problem, I kept ending up with 2 of
these structures at any given point in time, one being held by the
temporary iterator being returned from the post increment, and one by
the modified original iterator.

One was of course invalid. I could have copied it into a new memory
block before obtaining the second, but doing that would double the
copying and memory usage of an already memory intensive operation.

I did eventually get it working, but through a blatant hack only.
Knowing the order the operations, I realised that if I put the code
from the increment() function into equal() function before the
comparison is made then everything will work out fine.

I was unable to think of a better solution, perhaps you can think of one?

May I ask why Boost::Python uses post-increment and then derefencing
instead of pre-increment after derefencing. I don't understand why it
was done this way, but I'm sure there was a good reason.

Cheers,
-Dan