On Wed, Jan 7, 2015 at 5:16 PM, Keean Schupke <[email protected]> wrote:
> I'm probably being slow, but I don't see that. With sortBy you pass the
> comparison function explicitly. With a 'view' the comparison function is
> passed implicitly (by they type). That is the only difference. A view is
> simply type that overrides the comparator.
>
> Where is the extra memory etc coming from?
For concreteness, here's what it looks like in C++:
template <class RandomAccessIterator>
void sort (RandomAccessIterator first, RandomAccessIterator last);
template <class RandomAccessIterator, class Compare>
void sort (RandomAccessIterator first, RandomAccessIterator last,
Compare comp);
The first is sort, the second is sortBy. Say the comparison function looks like
class SaltedCompare {
HugeClass salt;
bool operator()(int x, int y) const { return hash(salt,x) < hash(salt,y); }
}
where the salt is (for some reason) huge. In C++, we pass a single
instance of SaltedCompare to the second version of sort, and the first
two arguments can point directly into a flat array of ints.
Instead, you propose making a SaltedInt type. At best, this would
look something like
class SaltedInt {
HugeClass* salt;
int x;
bool operator<(...) { ... }
}
We either add another memory indirection or store a copy of HugeClass
in every entry. In either case, we need to copy the array.
Apologies if what I'm describing is different from the views you had in mind.
Geoffrey
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