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2467. is_always_equal has slightly inconsistent default

Section: 16.4.4.6 [allocator.requirements], 20.2.9.2 [allocator.traits.types] Status: C++17 Submitter: Howard Hinnant Opened: 2015-01-18 Last modified: 2017-07-30

Priority: 0

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Discussion:

Table 28 — "Allocator requirements" says that X::is_always_equal has a default value of is_empty<X>, and this is consistent with the return type description:

Identical to or derived from true_type or false_type

is_empty<X> is guaranteed to be derived from either true_type or false_type. So far so good.

20.2.9.2 [allocator.traits.types]/p10 says: 

typedef see below is_always_equal;

Type: Alloc::is_always_equal if the qualified-id Alloc::is_always_equal is valid and denotes a type (14.8.2); otherwise is_empty<Alloc>::type.

This is subtly different than what Table 28 says is the default: is_empty<Alloc>::type is not is_empty<Alloc>, but is rather one of true_type or false_type.

There are two ways to fix this:

  1. Change Table 28 to say: is_empty<X>::type.

    or

  2. Change 20.2.9.2 [allocator.traits.types]/p10:

    Type: Alloc::is_always_equal if the qualified-id Alloc::is_always_equal is valid and denotes a type (14.8.2); otherwise is_empty<Alloc>::type.

Both options are correct, and I see no reason to prefer one fix over the other. But Table 28 and 20.2.9.2 [allocator.traits.types]/p10 should be consistent with one another.

[2015-02 Cologne]

DK: We should accept the first bullet. GR: Why does is_empty even have a type member? AM: All type traits have a type member. I agree with DK's preference for the first type.

Proposed resolution:

This wording is relative to N4296.

  1. Change 16.4.4.6 [allocator.requirements], Table 28 — "Allocator requirements" as presented:

    Table 28 — Allocator requirements
    Expression Return type Assertion/note
    pre-/post-condition    		 Default
    …
    X::is_always_equal Identical to or derived
    from true_type or
    false_type     		[…] is_empty<X>::type
    …