Aleksey Gurtovoy 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'? What is it good for?
Personally, I don't think that providing it is any more weird than providing 'begin'.
'begin/end' are needed in order to iterate the sequence, and therefore required for for any sequence. 'front', as I see it (which I hope is the right way to see it) is only relevant for sequences where the order of the elements is user-defined, i.e. vector and list. Where the order is not guaranteed (set/map), there is no meaning to 'front'. Of course, everything I say is analogous to std containers, and should sound familiar.
'front' is a shortcut for 'deref< begin<s>::type >'
The fact that 'front' is just a shortcut for 'deref< begin<s>::type >' is another reason why it's not really needed for Forward Sequence. Providing 'front' without 'back' sounds stridently asymmetric to me.
3. The docs say that the size of joint_view is the arithmetic sum of the sizes of the sequences of which it's composed.
Yep.
Is that true also in the case of set/map?
Yes.
For an mpl newbie it sounds reasonable that it will removed duplicates and might end up with a size that is smaller than the sum of its components. If it's not, then it sounds quite useless to use it for set/map.
Informally, 'joint_view' provides us a way to iterate through the elements of two sequences without paying the upfront cost of their concatenation. While technically it could be specialized for associative sequences to eliminate duplicates, it would (a) break genericity of the adaptor and (b) deprive us of the basic, unspecialized functionality. If I was to implement the suggested behavior, I'd go for a separate 'unique_view' component which would have none of the downsides and an advantage of being applicable to non-associative sequences as well.
Then I think a word of warning in the docs about using it with set/map is a good idea. People like me could get confused...
HTH
Yes it does! Thanks.