Improved Proposed Wording for LWG 2114
(contextually convertible to bool)

• Introduction
• Revision History
• Discussion
• Rationale
• Resolved Issues
• Proposed Resolution
• Acknowledgements
• Bibliography

Introduction

This proposal is intended to provide wording to resolve the existing library issue LWG 2114.

Revision History

Changes since P2167R0:

• Remove changes suggested for 26.7.3.2 [valarray.comparison]. For the same reason as e.g. for 20.6.6 [optional.relops] and 23.5.1.8 [reverse.iter.cmp] the result is just converted to bool and nothing else is done with it.

• Rebase wording to N4892.

• Improve document title.

Discussion

Issue LWG 2114 has already a long history and a number of wording revisions that went backward and forward. But with the introduction of the boolean-testable exposition-only concept by P1964R2 adopted during the Prague 2020 meeting we have now an officially accepted useful tool that can be used to fix this issue.

Since the edits of the working draft are not very small, this paper has been written.

Rationale

The boolean-testable has all the properties that LWG 2114 tried to specify without having language-concepts available. Our existing terminology of "modeling" a concept should allow us to use this wording even when we specify something within existing named requirements sets. So the approach of this paper is just to impose the boolean-testable concept requirements to those places that the issue identifies, now updated to the most recent working draft.

The following guidelines have been chosen to decide for applying the boolean-testable concept requirements below:

• Type families that are potentially influenced by user-defined types, such as types meeting the Cpp17NullablePointer requirements, are usually good candidates, unless the bool-like expressions are purposely intended to reflect the exact behaviour as presented (The design of the comparison functions of optional belong into this category)

• Type families or specifications that solely exist to shorten the specification of library-provided types, should usually not be good candidates, unless the intention exists here to provide Library implementors more freedom. An example for this case are the Container requirements which are actually invented to shorten the specification of the existing concrete container templates, where all of them use a concrete type bool for operator==/!= and for the empty function

  It should be pointed out that in the past we had considered to take advantage of "convertible to bool" requirements for allocator equality by allowing to implement them as constexpr with std::true/false_type as return types, but with the introduction of the is_always_equal query type of an allocator we had withdrawn this idea, so at the moment we have no need for such a customization point. If we really want to return to a similar point again, we can easily decide to change the return type e.g. for container equality again to model boolean-testable, but now specifically to realize a concrete purpose.

Resolved Issues

If the proposed resolution will be accepted, the following library issues will be resolved:

Proposed resolution

The proposed wording changes refer to N4892.

1. Change Table [tab:cpp17.equalitycomparable] as indicated:

   Table 25: Cpp17EqualityComparable requirements [tab:cpp17.equalitycomparable]

   Expression	Return type	Requirement
   a == b	convertible to bool decltype(a == b) models boolean-testable	== is an equivalence relation, that is, it has the following properties: […]

2. Change Table [tab:cpp17.lessthancomparable] as indicated:

   Table 26: Cpp17LessThanComparable requirements [tab:cpp17.lessthancomparable]

   Expression	Return type	Requirement
   a < b	convertible to bool decltype(a < b) models boolean-testable	< is a strict weak ordering relation (25.8 [alg.sorting])

3. Change in [nullablepointer.requirements] Table [tab:cpp17.nullablepointer] as indicated:

   Table 33: Cpp17NullablePointer requirements [tab:cpp17.nullablepointer]

   Expression	Return type	Operational semantics
   …
   a != b	contextually convertible to bool decltype(a != b) models boolean-testable	!(a == b)
   a == np np == a	contextually convertible to bool decltype(a == np) and decltype(np == a) both model boolean-testable	a == P()
   a != np np != a	contextually convertible to bool decltype(a != np) and decltype(np != a) both model boolean-testable	!(a == np)

4. Change 17.11.6 [cmp.alg] as indicated:

   -4- The name compare_strong_order_fallback denotes a customization point object (16.3.3.3.6 [customization.point.object]). Given subexpressions E and F, the expression compare_strong_order_fallback(E, F) is expression-equivalent (3.21 [defns.expression-equivalent]) to:

   1. […]

   2. (4.3) — Otherwise, if the expressions E == F and E < F are both well-formed and convertible to boolthe types decltype(E == F) and decltype(E < F) both model boolean-testable,

      E == F ? strong_ordering::equal :
      E < F  ? strong_ordering::less :
               strong_ordering::greater
      

      except that E and F are evaluated only once.

   3. […]

   […]

   -5- The name compare_weak_order_fallback denotes a customization point object (16.3.3.3.6 [customization.point.object]). Given subexpressions E and F, the expression compare_weak_order_fallback(E, F) is expression-equivalent (3.21 [defns.expression-equivalent]) to:

   1. […]

   2. (5.3) — Otherwise, if the expressions E == F and E < F are both well-formed and convertible to boolthe types decltype(E == F) and decltype(E < F) both model boolean-testable,

      E == F ? weak_ordering::equivalent :
      E < F  ? weak_ordering::less :
               weak_ordering::greater
      

      except that E and F are evaluated only once.

   3. […]

   […]

   -6- The name compare_partial_order_fallback denotes a customization point object (16.3.3.3.6 [customization.point.object]). Given subexpressions E and F, the expression compare_partial_order_fallback(E, F) is expression-equivalent (3.21 [defns.expression-equivalent]) to:

   1. […]

   2. (6.3) — Otherwise, if the expressions E == F and E < F are both well-formed and convertible to boolthe types decltype(E == F) and decltype(E < F) both model boolean-testable,

      E == F ? partial_ordering::equivalent :
      E < F  ? partial_ordering::less :
      E > F  ? partial_ordering::greater :
               partial_ordering::unordered
      

      except that E and F are evaluated only once.

   3. […]

5. Change 20.5.8 [tuple.rel] as indicated:

   template<class... TTypes, class... UTypes>
     constexpr bool operator==(const tuple<TTypes...>& t, const tuple<UTypes...>& u);
   

   -1- Mandates: For all i, where 0 ≤ i < sizeof...(TTypes), get<i>(t) == get<i>(u) is a valid expression returning a type that is convertible to booland the type decltype(get<i>(t) == get<i>(u)) models boolean-testable. sizeof...(TTypes) equals sizeof...(UTypes).

6. Keep 20.6.6 [optional.relops] unchanged: These operations just evaluate what they get and say, and no further requirements are imposed.

7. Keep 20.6.8 [optional.comp.with.t] unchanged: These operations just evaluate what they get and say, and no further requirements are imposed.

8. Keep 20.7.6 [variant.relops] unchanged: These operations just evaluate what they get and say, and no further requirements are imposed.

9. Keep 20.15.8 [meta.logical] unchanged: These logical type traits do already the boolean logic for you, and no further requirements are imposed.

10. Change Table [tab:container.req] as indicated:

    [Drafting note: Given that the containers are no user-provided types it seems like an unnecessary generic allowance to support types that are convertible to bool below. In particular, because the container requirements tables would then be in (weak) conflict with the header and class synopses of the corresponding containers, which all specify concrete bool for these operations.

    For this specific wording change here, the author has a mild preference to get rid of the "convertibility" freedom. The alternative would be declare these types as "modeling boolean-testable".]

    Table 73: Container requirements [tab:container.req]

    Expression	Return type	Operational semantics	Assertion/note pre-/post-condition	Complexity
    …
    a == b	convertible to bool	== is an equivalence relation. equal(a.begin(), a.end(), b.begin(), b.end())	[…]	[…]
    a != b	convertible to bool	Equivalent to !(a == b)	[…]	[…]
    …
    a.empty()	convertible to bool	a.begin() == a.end()	[…]	[…]

11. Keep 23.3.4.4 [iterator.concept.winc] unchanged: It seems to me that sufficient wording exists to exclude funny integer-class types, and they are all under control by the implementation.

12. Change in [iterator.cpp17] Table [tab:inputiterator] and Table [tab:randomaccessiterator] as indicated:

    Table 85: Cpp17InputIterator requirements (in addition to Cpp17Iterator) [tab:inputiterator]

    Expression	Return type	Operational semantics	Assertion/note pre-/post-condition
    a != b	contextually convertible to bool decltype(a != b) models boolean-testable	!(a == b)	[…]
    …

    […]

    Table 89: Cpp17RandomAccessIterator requirements (in addition to Cpp17BidirectionalIterator) [tab:randomaccessiterator]

    Expression	Return type	Operational semantics	Assertion/note pre-/post-condition
    …
    a < b	contextually convertible to bool decltype(a < b) models boolean-testable	Effects: Equivalent to: return b - a > 0;	[…]
    a > b	contextually convertible to bool decltype(a > b) models boolean-testable	b < a	[…]
    a >= b	contextually convertible to bool decltype(a >= b) models boolean-testable	!(a < b)	[…]
    a <= b	contextually convertible to bool decltype(a <= b) models boolean-testable	!(a > b)	[…]

13. Keep 23.5.1.8 [reverse.iter.cmp] unchanged: These operations just evaluate what they get and say, and no further requirements are imposed.

14. Keep 23.5.3.8 [move.iter.op.comp] unchanged: These operations just evaluate what they get and say, and no further requirements are imposed.

15. Change 25.2 [algorithms.requirements] as indicated:

    [Drafting note: The wording changes below also fix (a) unusual wording forms used ("should work") which are unclear in which sense they are imposing normative requirements and (b) the problem, that the current wording seems to allow that the predicate may mutate a call argument, if that is not a dereferenced iterator. Upon applying the new wording it became obvious that the both the previous and the new wording has the effect that currently algorithms such as adjacent_find, search_n, unique, and unique_copy are not correctly described (because they have no iterator argument named first1), which could give raise to a new library issue. ]

    -7- When not otherwise constrained, the Predicate parameter is used whenever an algorithm expects a function object (20.14 [function.objects]) that, when applied to the result of dereferencing the corresponding iterator, returns a value testable as true. In other words, if an algorithm takes Predicate pred as its argument and first as its iterator argument with value type T, it should work correctly in the construct pred(*first) contextually converted to bool (7.3 [conv])the expression pred(*first) shall be well-formed and the type decltype(pred(*first)) shall model boolean-testable (18.5.2 [concept.booleantestable]). The function object pred shall not apply any non-constant function through the dereferenced iteratorits argument. Given a glvalue u of type (possibly const) T that designates the same object as *first, pred(u) shall be a valid expression that is equal to pred(*first).

    -8- When not otherwise constrained, the BinaryPredicate parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing two corresponding iterators or to dereferencing an iterator and type T when T is part of the signature returns a value testable as true. In other words, if an algorithm takes BinaryPredicate binary_pred as its argument and first1 and first2 as its iterator arguments with respective value types T1 and T2, it should work correctly in the construct binary_pred(*first1, *first2) contextually converted to bool (7.3 [conv])the expression binary_pred(*first1, *first2) shall be well-formed and the type decltype(binary_pred(*first1, *first2)) shall model boolean-testable. Unless otherwise specified, BinaryPredicate always takes the first iterator's value_type as its first argument, that is, in those cases when T value is part of the signature, it should work correctly in the construct binary_pred(*first1, value) contextually converted to bool (7.3 [conv])the expression binary_pred(*first1, value) shall be well-formed and the type decltype(binary_pred(*first1, value)) shall model boolean-testable. binary_pred shall not apply any non-constant function through the dereferenced iteratorsany of its arguments. Given a glvalue u of type (possibly const) T1 that designates the same object as *first1, and a glvalue v of type (possibly const) T2 that designates the same object as *first2, binary_pred(u, *first2), binary_pred(*first1, v), and binary_pred(u, v) shall each be a valid expression that is equal to binary_pred(*first1, *first2), and binary_pred(u, value) shall be a valid expression that is equal to binary_pred(*first1, value).

16. Change 25.8.1 [alg.sorting.general] as indicated:

    [Drafting note: The existing wording inherits all the good wording from BinaryPredicate, that we fixed above, so there is only little to do but specifying the conversion to bool less strict, since we already know that it is a type that models boolean-testable]

    -2- Compare is a function object type (20.14 [function.objects]) that meets the requirements for a template parameter named BinaryPredicate (25.2 [algorithms.requirements]). The return value of the function call operation applied to an object of type Compare, when contextually converted to bool (7.3 [conv]), yields true if the first argument of the call is less than the second, and false otherwise. Compare comp is used throughout for algorithms assuming an ordering relation.

17. Change in 29.5.4.2 [fpos.operations] Table [tab:fpos.operations] as indicated:

    Table 121: Position type requirements [tab:fpos.operations]

    Expression	Return type	Operational semantics	Assertion/note pre-/post-condition
    …
    p != q	convertible to bool decltype(p != q) models boolean-testable	!(p == q)
    …

18. Change 32.2.1 [thread.req.paramname] as indicated:

    [Drafting note: The following performs some minor drive-by fixes to fix minor wording issues that would e.g. exclude normal function pointers to be used as predicate. Note that we intentionally do not describe a const lvalue pred, since there is nothing in the specification that would imply or require that.]

    -1- Throughout this Clause, the names of template parameters are used to express type requirements. If a template parameter is named Predicate, operator() applied to the template argument shall return a value that is convertible to boolPredicate is a function object type (20.14 [function.objects]). Let pred denote an lvalue of type Predicate. Then the expression pred() shall be well-formed and the type decltype(pred()) shall model boolean-testable (18.5.2 [concept.booleantestable]). The return value of pred(), converted to bool, yields true if the corresponding test condition is satisfied, and false otherwise. […]

Acknowledgements

Thanks to Barry Revzin, Tim Song, and Tomasz Kamiński for reviewing this proposal and providing helpful improvement suggestions.

Bibliography