Table 102 in [associative.reqmts]/8 states on expression a.key_comp() that it "returns the comparison object out of which a was constructed". At the same time, [container.requirements.general]/8 states (starting in the third line) that "...Any Compare, Pred, or Hash objects belonging to a and b shall be swappable and shall be exchanged by unqualified calls to non-member swap...". This is problematic for any compliant implementation, since once swapped the container cannot return the comparison object out of which it was constructed unless incurring in storing an otherwise needless object.

The simple solution is to correct that statement in Table 102, but I believe this is part of a larger problem of underspecified behavior: The new standard has made an effort in regards to allocators and now fully specifies what happens to stateful allocator objects. It has even specified what happens to stateful hasher and key_equal members of unordered containers (they propagate), but it says nothing about stateful comparison objects of (ordered) associative containers, except for the statement in [container.requirements.general]/8 referred above and only related to swap.

For example, it is unclear to me what is specified to happen on an assignment: should the comparison object be copied/moved along with the elements, or should the left-hand side object keep its own? Maybe this has been intentionally left unspecified with the purpose of compatibility with C++98, which I understand it specified that comparison objects were kept for the entire life of the container (like allocators) — an unfortunate choice. But anyway, the segment of [container.requirements.general] quoted above seems to break any possible backwards compatibility with C++98 in this regard.

Therefore, taking into consideration consistency with how this is dealed with for unordered associative containers, I propose that Table 102 is modified as follows:

• The row for expression a.key_comp() is changed so that its "assertion/note pre-/post-condition" reads "Returns a's comparison object."

• A new row is added at the appropriate location (which I believe would be after "X(il)" row), with:

  Table 102 — Associative container requirements (in addition to container)

  Expression	Return type	Assertion/note pre-/post-condition	Complexity
  X(b) X a(b)	X	Copy constructor. In addition to the requirements of Table 96, copies the comparison object.	Linear in b.size()
  a = b	X&	Copy assignment operator. In addition to the requirements of Table 96, copies the comparison object.	Linear in a.size() and b.size()