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2010. is_* traits for binding operations can't be meaningfully specialized

Section: 22.10.15.2 [func.bind.isbind] Status: C++14 Submitter: Sean Hunt Opened: 2010-07-19 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

22.10.15.2 [func.bind.isbind] says for is_bind_expression:

Users may specialize this template to indicate that a type should be treated as a subexpression in a bind call.

But it also says:

If T is a type returned from bind, is_bind_expression<T> shall be publicly derived from integral_constant<bool, true>, otherwise from integral_constant<bool, false>.

This means that while the user is free to specialize, any specialization would have to be false to avoid violating the second requirement. A similar problem exists for is_placeholder.

[ 2010 Batavia (post meeting session) ]

Alisdair recognises this is clearly a bug introduced by some wording he wrote, the sole purpose of this metafunction is as a customization point for users to write their own bind-expression types that participate in the standard library bind protocol. The consensus was that this should be fixed in Madrid, moved to Open.

[2011-05-13 Jonathan Wakely comments and provides proposed wording]

The requirements are that is_bind_expression<T>::value is true when T is a type returned from bind, false for any other type, except when there's a specialization involving a user-defined type (N.B. 16.4.5.2.1 [namespace.std] means we don't need to say e.g. is_bind_expression<string> is false.)

The obvious way to meet the requirements is for the primary template to derive from integral_constant<bool, false> and for implementations to provide specializations for the unspecified types returned from bind. User-defined specializations can do whatever they like, as long as is_bind_expression::value is sane. There's no reason to forbid users from defining is_bind_expression<user_defined_type>::value=false if that's what they want to do.

Similar reasoning applies to is_placeholder, but a further issue is that 22.10.15.2 [func.bind.isbind] contains wording for is_placeholder but contains no definition of it and the sub-clause name only refers to is_bind_expression. The wording below proposes splitting paragraphs 3 and 4 of 22.10.15.2 [func.bind.isbind] into a new sub-clause covering is_placeholder.

If the template specializations added by the proposed wording are too vague then they could be preceded by "for exposition only" comments

[2011-05-18 Daniel comments and provides some refinements to the P/R]

Both bind-related type traits should take advantage of the UnaryTypeTrait requirements. Additionally, the updated wording does not imply that the implementation provides several specializations. Wording was used similar to the specification of the uses_allocator type trait (which unfortunately is not expressed in terms of BinaryTypeTrait requirements).

[Bloomington, 2011]

Move to Ready

Proposed resolution:

This wording is relative to the FDIS.

  1. Change 22.10.15.2 [func.bind.isbind] to:

    namespace std {
  template<class T> struct is_bind_expression; // see below
    : integral_constant<bool, see below> { };
}

    -1- is_bind_expression can be used to detect function objects generated by bind. bind uses is_bind_expression to detect subexpressions. Users may specialize this template to indicate that a type should be treated as a subexpression in a bind call.

    -2- If T is a type returned from bind, is_bind_expression<T> shall be publicly derived from integral_constant<bool, true>, otherwise from integral_constant<bool, false>Instantiations of the is_bind_expression template shall meet the UnaryTypeTrait requirements ([meta.rqmts]). The implementation shall provide a definition that has a BaseCharacteristic of true_type if T is a type returned from bind, otherwise it shall have a BaseCharacteristic of false_type. A program may specialize this template for a user-defined type T to have a BaseCharacteristic of true_type to indicate that T should be treated as a subexpression in a bind call..

    -3- is_placeholder can be used to detect the standard placeholders _1, _2, and so on. bind uses is_placeholder to detect placeholders. Users may specialize this template to indicate a placeholder type.

    -4- If T is the type of std::placeholders::_J, is_placeholder<T> shall be publicly derived from integral_constant<int, J>, otherwise from integral_constant<int, 0>.

  2. Insert a new sub-clause immediately following sub-clause 22.10.15.2 [func.bind.isbind], the suggested sub-clause tag is [func.bind.isplace]:

    20.8.9.1.? Class template is_placeholder [func.bind.isplace]

    namespace std {
  template<class T> struct is_placeholder; // see below
}

    -?- is_placeholder can be used to detect the standard placeholders _1, _2, and so on. bind uses is_placeholder to detect placeholders.

    -?- Instantiations of the is_placeholder template shall meet the UnaryTypeTrait requirements ([meta.rqmts]). The implementation shall provide a definition that has a BaseCharacteristic of integral_constant<int, J> if T is the type of std::placeholders::_J, otherwise it shall have a BaseCharacteristic of integral_constant<int, 0>. A program may specialize this template for a user-defined type T to have a BaseCharacteristic of integral_constant<int, N> with N > 0 to indicate that T should be treated as a placeholder type.