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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.