Yuval Ronen <ronen_yuval@yahoo.com> writes:
David Abrahams wrote:
1. The docs assert there is a 'front' algorithm for set and map. This sounds weird to me, as the order of the types is them unspecified.
The 'front's availability is dictated by the Forward Sequence concept requirements (http://www.boost.org/libs/mpl/doc/refmanual/forward-sequence.html).
Then perhaps the Forward Sequence concept should not require 'front'?
Well, there's a default implementation that works for anything providing begin, so I suppose it doesn't need to be part of the concept from that point of view. However, some sequences might provide a more efficient one, and generic code that wants to use front should be able to count on it.
More efficient than constant time? If 'begin' is required to have constant time, and 'deref' is constant time (deref is not documented to be constant time, but I assume it is - is this assumtion wrong?), and front can be implemented using begin and deref, then front is also constant time. Can any sequence beat it?
Sure, with a smaller constant. one instantiation instead of two or more, for example.
What is it good for?
Isn't it obvious? Getting the first element of the sequence.
But there is no 'first element' in Associative Sequences. From the Associative Sequence doc:
"Unlike associative containers in the C++ Standard Library, MPL associative sequences have no associated ordering relation"
Come on, man, use common sense. That doesn't mean there's no first element. That just means there's no guarantee which of the elements it will turn out to be. If it's a set, for example, you can look at front, then iterate over [next<begin<S>::type>::type, end<S>::type) and never encounter that value again.
There are 'begin' and 'end' for iteration, but there's no guaranteed order, as far as I understand, so there's no meaning to 'front'.
Then your understanding is flawed.
Providing 'front' without 'back' sounds stridently asymmetric to me.
See any singly-linked list implementation.
Singly-linked containers were excluded from the standard library, and probably for a good reason,
Yeah, the committee only had so much time to process what Alex gave them.
so I don't see why they should be re-introduced in MPL.
Um, it's a little late for that. Type lists (mpl::list) are one of the most basic kinds of type sequences. -- Dave Abrahams Boost Consulting www.boost-consulting.com