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STL: Copying vectors with different allocators

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Started by Luke1410 February 22, 2009 10:50 PM

15 comments, last by magic_man 15 years, 1 month ago

Luke1410

101

Author

March 19, 2009 04:33 PM

Quote:Original post by stonemetal
have you tried boxing the int?

somthing like

struct PoolID{
PoolID(int a): Pool(a){}
int Pool;
};

vector<int, myalloc<int, PoolID(5)> > foo;
That way you have a unified type for your template.

I'm a bit lost with this suggestion. How should the myalloc template class look like? I mean if I declare it as:
template<typename T, typename T2> class myalloc {} it won't compile, because it would expect a typename not an instance of an actual class for T2.

If I tried template<typename T, PoolID T2> class myalloc {} that won't work either, because PoolID is not a constant type and therefore can't be resolved as a template parameter, or am I missing the point here?

Luke1410

101

Author

March 19, 2009 04:36 PM

Quote:Original post by magic_man
Instead of using the assignment operator you may want to define your own function maybe something like the following.

*** Source Snippet Removed ***

That looks merely like a variation of the suggestions already given above by chairthrower and SiCrane, or did I miss something?
Thanks for the reply, anyway.

magic_man

March 19, 2009 04:51 PM

It is yes but the important part is the lhs.clear() otherwise it would give different behaviour from the standard. This could be used in place of a user called assignment and could be generalised again to work with any container. As far as I can recall there is no work around to using the assignment operator which works correctly.

"You insulted me!" I did not say that in the private message Tom Sloper!

visitor

643

March 19, 2009 05:23 PM

But then wouldn't vector::assign do the exact same thing (clear what is already there?).

Vector should have all kinds of constructors that take a const reference to the allocator, so if you removed the second template parameter from the allocator, this would work:

int main(){	std::vector<int, TrackedAllocator<int> > vec1((TrackedAllocator<int>(Pool1)));	std::vector<int, TrackedAllocator<int> > vec2((TrackedAllocator<int>(Pool2)));	vec1 = vec2;	return 0;}

But in any case, assign seems a wise thing to do for assigning vectors (I don't think I've ever assigned one container to another anyway, so I think it shouldn't require that much extra typing).

stonemetal

288

March 19, 2009 11:09 PM

Quote:Original post by Luke1410

I'm a bit lost with this suggestion. How should the myalloc template class look like? I mean if I declare it as:
template<typename T, typename T2> class myalloc {} it won't compile, because it would expect a typename not an instance of an actual class for T2.

If I tried template<typename T, PoolID T2> class myalloc {} that won't work either, because PoolID is not a constant type and therefore can't be resolved as a template parameter, or am I missing the point here?

Sorry, I was in the middle of something else at the time I wrote that. you would obviously have to pass the poolID into the constructor of myalloc not as a template parameter.

Luke1410

101

Author

March 20, 2009 04:46 AM

Thanks for all the answers... I poked a bit deeper into the STL and it is as we suspected initially: Since the standard does not directly require that containers using different allocators are interchangeable, it's not supported by the implementation. There would be one obvious possibility I can think of to add that support:
Add a cpy ctor to any container class, which accepts any kind of allocator and compare the two allocators using the ==-operator to see if the allocators are interchangable.

However, this would have to be done by Dinkumware to be consistent, so we'll stick with the assign-cpy-suggestion provided here.

magic_man

March 20, 2009 04:59 AM

Quote:Original post by visitor
But then wouldn't vector::assign do the exact same thing (clear what is already there?).

Of course it would, I do not know what I was thinking. Ignore me.

"You insulted me!" I did not say that in the private message Tom Sloper!

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