Re: [boost] Proposal: Monotonic Containers

Subject: Re: [boost] Proposal: Monotonic Containers
From: Christopher Jefferson (chris_at_[hidden])
Date: 2009-06-09 10:18:18

On 9 Jun 2009, at 13:34, Christian Schladetsch wrote:

>> The following code assertss for me, g++ 4 on Mac OS X. Notice how I  
>> only
>> need to do 33 iterations.
>
>
> I will get back to you. No, I havent done exhaustive cross-platform  
> testing
> but this is suprising and indicates a basic fault. I use Ubuntu by  
> default.

I find the code breaks identically on Ubuntu and Windows.

Furthermore, I'm afraid to say I'm sure this is unfixable. vector  
requests a new block of memory, copies data across, and then frees the  
old block. You can't make it do what you want to.

You could, either in your wrapper do a .reserve() for the appropriate  
amount of memory, but that's certainly not guaranteed by the standard  
to work in all cases -- I don't know if it would work in practice  
across multiple compilers. This would only work for vector, not the  
other containers, which would need their own fixes (for example most  
std::deques allocate in blocks of their own, large, predefined size,  
and also need a separate allocation for a "master list" of pointers).  
I imagine with std::set you might be better off, but then again that  
always does a fixed size allocation, so arguably there you would be  
better with an optimised fixed-size allocator, which can efficiently  
handle frees anyway.

Chris
>
>
>>
>> Ddo you have even a basic testing suite for this code? I couldn't  
>> find one,
>> and this seems like a fairly fundamental bug. Perhaps it doesn't  
>> occur on
>> windows?
>>
>> #include "boost/monotonic/vector.h"
>>
>> int main()
>> {
>>   boost::monotonic::inline_storage<100*sizeof(int)> storage;   //  
>> create
>> local storage on the stack
>>   boost::monotonic::vector<int> deathrow(storage);   // create a
>> std::vector that uses this storage
>>
>>   for(int i = 0; i < 33; ++i)
>>     deathrow.push_back(i);
>>
>>
>> }
>>
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>
> Christian.
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