1114. Type traits underspecified

Section: 23.15 [meta] Status: C++11 Submitter: Daniel Krügler Opened: 2009-05-12 Last modified: 2016-02-10

Priority: Not Prioritized

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Discussion:

Related to 975 and 1023.

The current wording in 23.15.1 [meta.rqmts] is still unclear concerning it's requirements on the type traits classes regarding ambiguities. Specifically it's unclear

[ Batavia (2009-05): ]

Alisdair would prefer to factor some of the repeated text, but modulo a corner case or two, he believes the proposed wording is otherwise substantially correct.

Move to Open.

[ 2009-10 post-Santa Cruz: ]

Move to Tentatively Ready.

Proposed resolution:

[ The usage of the notion of a BaseCharacteristic below might be useful in other places - e.g. to define the base class relation in 23.14.5 [refwrap], 23.14.12 [func.memfn], or 23.14.13.2 [func.wrap.func]. In this case it's definition should probably be moved to Clause 17 ]

  1. Change 23.15.1 [meta.rqmts] p.1 as indicated:

    [..] It shall be DefaultConstructible, CopyConstructible, and publicly and unambiguously derived, directly or indirectly, from its BaseCharacteristic, which is a specialization of the template integral_constant (20.6.3), with the arguments to the template integral_constant determined by the requirements for the particular property being described. The member names of the BaseCharacteristic shall be unhidden and unambiguously available in the UnaryTypeTrait.

  2. Change 23.15.1 [meta.rqmts] p.2 as indicated:

    [..] It shall be DefaultConstructible, CopyConstructible, and publicly and unambiguously derived, directly or indirectly, from an instance its BaseCharacteristic, which is a specialization of the template integral_constant (20.6.3), with the arguments to the template integral_constant determined by the requirements for the particular relationship being described. The member names of the BaseCharacteristic shall be unhidden and unambiguously available in the BinaryTypeTrait.

  3. Change 23.15.4 [meta.unary] p.2 as indicated:

    Each of these templates shall be a UnaryTypeTrait (20.6.1), publicly derived directly or indirectly from true_type if the corresponding condition is true, otherwise from false_type where its BaseCharacteristic shall be true_type if the corresponding condition is true, otherwise false_type.

  4. Change 23.15.6 [meta.rel] p.2 as indicated:

    Each of these templates shall be a BinaryTypeTrait (20.6.1), publicly derived directly or indirectly from true_type if the corresponding condition is true, otherwise from false_type where its BaseCharacteristic shall be true_type if the corresponding condition is true, otherwise false_type.