Re: equivalent of realloc in C++
Martin T. wrote:
Bo Persson wrote:
Martin T. wrote:
.....
I do not claim to understand how modern operating system
distribute memory(-blocks) to requests from a process, but the
example was explicitly using a rather small number (few thousands
of elements) and, say, given a vector of 1KB (or 10KB, or 50KB)
in size, why should it be so unlikely that there's another n KB
mem available after that vector? The savings will add up for many
moderately sized vector objects, where we can possibly expect an
expand operation to work because of vector size vs. memory
granularity.
The potential savings in not copying 1kB must be terribly small.
Hrmpf. But *if* (note the conditional) providing a mechanism for the
reallocation to happen if it is possible without measurable
additional cost, then why shouldn't it be done? Plus - we are not
only talking of copying 1KB but also calling a copy-ctor (or a
move-ctor) n times each time we reallocate.
*If* it is free, or *always* a win, and does something that is hard or
impossible with the existing tools - then yes. If it is of
questionable value, or just adds a new way of doing something - then
no.
Note that move constructors, a general purpose feature, already
improves the vector<string> case. That's a good addition to the
language.
Some of us also argue that you can already avoid this by adding a
vec.reserve(expected_size) before using the vector. Or have the
implementation reserve a huge chunk at the first push_back. This
can be done without changing the existing interfaces.
Obviously. The expand scheme would be for the cases where you don't
know how big your container should be beforehand.
But if you have *any* idea of what the size could be - a 1000, a
million, 10 million, 100 million - then
vec.reserve(general_idea_of_the_size), will improve the performance -
even if the estimate is off by an order of magnitude. The number of
copies are reduced if you give the vector just a reasonable first
allocation.
Bo Persson
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