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2345. integer_sequence should have a self-typedef ::type

Section: 21.2.2 [intseq.intseq] Status: NAD Submitter: Stephan T. Lavavej Opened: 2013-11-01 Last modified: 2016-01-28

Priority: 2

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Discussion:

21.3.4 [meta.help] says that integral_constant<T, v> provides ::value_type (for T) and ::type (for itself).

21.2.2 [intseq.intseq] says that integer_sequence<T, I...> provides ::value_type (for T), but nothing for itself.

Self-typedefs can be useful when users create chains of derived classes, then want to get the Standard base type. This is especially relevant to integer_sequence, as variadic templates encourage recursive inheritance.

[2014-02-13 Issaquah: Close as NAD]

AJM: My own implementation used a different alias for types representing parameter packs, and specifically did not define type. I tried it both ways, and found bugs more quickly when type was not defined.

Proposed resolution:

This wording is relative to N3797.

1. Edit 21.2.2 [intseq.intseq] as indicated:

   namespace std {
     template<class T, T... I>
     struct integer_sequence {
       typedef T value_type;
       typedef integer_sequence<T, I...> type;
       static constexpr size_t size() noexcept { return sizeof...(I); }
     };
   }