Re: vector<> can probably never grow to it's maximum size!

[Paolo Carlini <pcarlini at suse dot de>]

Dhruv Matani wrote:

> Hello,
> 	I would like to discuss an issue which deals with vector's memory
> management. Consider a 16 machine.
>
> Here, max_size() would typically be say: ((2^16)-1)/sizeof(_Tp). Now,
> consider that the current size is max_size()/2. If I want to add say 3
> more elements, the vector to get space for totally max_size() elements,
> and will fail because operator new will probably fail because of such a
> large request. Can't we have the vector settle for a slightly smaller
> return from the allocator at the expense of not sticking to the standard
> strictly about the exponential increase policy?
>  
If I understand correctly your question, you are right and indeed we are 
already
implementing this logic for std::string: see _M_mutate in 
basic_string.tcc: we
always check not to exceed max_size and in that case we simply stay with it.

BUT BUT BUT

Be warned that in the std::vector case things are slightly more tricky than
computing min(max_size, new_size), since max_size == size_type(-1) /
sizeof(value_type) and when sizeof(value_type) == 1, max_size is already
at maximum rapresentable unsigned value, therefore, you cannot really
exceed it with new_size...

Also, I want you to audit *all* the varios places in std::vector where 
we grow,
since someone pointed out time ago that there are inconsistencies. Also, 
I want
to see nice testcases, with custom allocators.

Paolo.