I was wondering:
how come the stl map class does not have a reserve function like the
useful vector::reserve function?
I was thinking about implementing my own reserve class, by providing
my own allocator, inherting a new class from map, and add it a reserve
function that will work on my allocator and ask it to reserve memory
for future use.
however, I don't know how to call the map's Allocator instance in
order to reserve the memory. can anyone please help?
thanks,
liran 3 17071
"Liran Shaul" <li*****@gmail. com> wrote in message
news:4c******** *************** ***@posting.goo gle.com... I was wondering: how come the stl map class does not have a reserve function like the useful vector::reserve function?
When a std::vector reallocates its storage, it can be a very costly
operation. A simple call to push_back() may cause every element in the
std::vector to be copied to a newly allocated block of memory. A call to
reserve() can avoid these unnecessary allocations and copy operations.
std::map, on the other hand, never needs to copy all of the
existing/remaining elements simply because a new element was inserted or
removed.
I was thinking about implementing my own reserve class, by providing my own allocator, inherting a new class from map, and add it a reserve function that will work on my allocator and ask it to reserve memory for future use. however, I don't know how to call the map's Allocator instance in order to reserve the memory. can anyone please help?
Every container has a "get_alloca tor" member function that returns a copy of
its allocator. You might be able to come up with something that retrieves
the allocator and gives it a "hint" that the map will grow in size (like
"myMap.get_allo cator().reserve (...)"), but the effort may not pay off
performance-wise. If std::map's allocations are a performance bottleneck,
then I could see how that might be a reasonable approach to make your
application faster.
--
David Hilsee
> Every container has a "get_alloca tor" member function that returns a copy of its allocator. You might be able to come up with something that retrieves the allocator and gives it a "hint" that the map will grow in size (like "myMap.get_allo cator().reserve (...)"), but the effort may not pay off performance-wise. If std::map's allocations are a performance bottleneck, then I could see how that might be a reasonable approach to make your application faster.
Sounds like a pool allocator might be a better solution to the OP's problem. http://www.boost.org/libs/pool/doc/index.html
john
"John Harrison" <jo************ *@hotmail.com> wrote in message
news:2t******** *****@uni-berlin.de... Every container has a "get_alloca tor" member function that returns a
copy of its allocator. You might be able to come up with something that
retrieves the allocator and gives it a "hint" that the map will grow in size (like "myMap.get_allo cator().reserve (...)"), but the effort may not pay off performance-wise. If std::map's allocations are a performance
bottleneck, then I could see how that might be a reasonable approach to make your application faster.
Sounds like a pool allocator might be a better solution to the OP's
problem. http://www.boost.org/libs/pool/doc/index.html
I agree. I only mentioned a reserve() member function on an allocator to
indulge the OP's idea.
--
David Hilsee This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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