Re: std::vector: Memory allocation in copy constructor for empty vector

[Paolo Carlini <pcarlini at suse dot de>]

Hi,

interesting issue...

> in the default constructor of the std::vector a zero sized vector is
> created. If this vector is copied
> with the copy constructor a memory block of size zero is allocated,
> resulting in a real memory allocation.
>
> This behavior has changed as the std::allocator changed from a separate
> implementation to the
> __glibcxx_base_allocator somewhere around 2004 (we recently changed from
> gcc 3.3.2 to 4.1.0).
>
> Because we use something like:
> vector <vector <int> > v;
> v.resize (1000000);
>
> That gave use a huge bunch of memory allocations :-(
>  
Yes, previously, __simple_alloc::allocate avoided calling 
_Alloc::allocate when __n == 0. Therefore this intermediate layer took 
care of avoiding pointless allocations.

> So here are my questions:
> 1) Is there an easy standard way to change the default allocator? Not per
> instance - this would be
>   way to much to change :-(
> 2) Is there a chance that the copy constructor of the vector (or the
> allocator) will be changed to
>   the former behavior? (I guess the standard don't say anything about
> which of both is correct)
>  
I think we should do 2), seems doable. I'm looking into it.

You could certainly do 1) and choose one among the various available 
allocators at configure time 
(http://gcc.gnu.org/onlinedocs/libstdc++/configopts.html), but even in 
the best cases the library would still waste the time of the 
Alloc::allocate call, even when the latter returns immediately 
(__pool_alloc for instance).

Paolo.