vector lightweight debug mode

[François Dumont]

Thanks for the explanation, it is much clearer.

So here is a the more limited patch.

Tested under Linux x86_64.

Ok to commit ?

François


On 18/11/2015 13:27, Jonathan Wakely wrote:
> On 17/11/15 20:49 +0100, François Dumont wrote:
>> On 16/11/2015 11:29, Jonathan Wakely wrote:
>>> Not doing the checks is also an option. That minimizes the cost :-)
>>
>> This is controlled by a macro, users already have this option.
>
> True, but we're talking about maybe enabling these checks by default
> when building linux distributions.
>
>>>
>>> For the full debug mode we want to check everything we can, and accept
>>> that has a cost.
>>>
>>> For the lightweight one we need to evaluate the relative benefits. Is
>>> it worth adding checks for errors that only happen rarely? Does the
>>> benefit outweigh the cost?
>>>
>>> I'm still not convinced that's the case for the "valid range" checks.
>>> I'm willing to be convinced, but am not convinced yet.
>>
>> Ok so I will remove this check. And what about insert position check ? I
>> guess this one too so I will remove it too. Note that will only remain
>> checks on the most basic operations that is to say those on which the
>> check will have the biggest impact proportionally.
>
> Yes, that's a good point.
>
> But my unproven assumption is that it's more common to use operator[]
> incorrectly, rather than pass invalid iterators to range insert, which
> is a relatively "advanced" operation.
>
>
>> I would like we push the simplest version so that people can start
>> experimenting.
>>
>> I would also prefer concentrate on _GLIBCXX_DEBUG mode :-)
>>
>>>
>>>>    It would be great to have it for gcc 6.0. I am working on the same
>>>> for other containers.
>>>
>>> Please don't do the valid range checks for std::deque, the checks are
>>> undefined for iterators into different containers and will not give a
>>> reliable answer.
>>
>> But debug mode is full of those checks, no ?
>
> They're supposed to be guarded by checks for _M_can_compare, if they
> aren't that's a regression. For debug mode we can tell whether two
> iterators are comparable, because they store a pointer back to their
> parent container. We can't check that in normal mode.
>
>

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