This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of C++14 status.

2317. The type property queries should be UnaryTypeTraits returning size_t

Section: 19.15.5 [meta.unary.prop.query] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-16

Priority: 0

View all issues with C++14 status.


The sibling sections 19.15.4 [meta.unary], 19.15.6 [meta.rel], and 19.15.7 [meta.trans] respectively specify UnaryTypeTraits, BinaryTypeTraits, and TransformationTraits, as stated by each /2 paragraph. However, 19.15.5 [meta.unary.prop.query] is underspecified. alignment_of, rank, and extent are said to produce "Values", but the type of that Value is not specified, and the struct templates are not required to derive from integral_constant. Such derivation is more than stylistic — it allows the structs to be passed to functions taking integral_constant.

alignment_of returns alignof(T) which is size_t ( [expr.alignof]/2). extent returns an array bound, which is clearly size_t. rank returns "the number of dimensions" of an array, so any type could be chosen, with size_t being a reasonable choice. (Another choice would be unsigned int, to match extent's template parameter I.)

[Issaquah 2014-02-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

  1. Following 19.15.5 [meta.unary.prop.query]/1 add a new paragraph as indicated:

    Each of these templates shall be a UnaryTypeTrait (19.15.1 [meta.rqmts]) with a BaseCharacteristic of integral_constant<size_t, Value>.