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3268. memory_order::memory_order_foo broken in C++20

Section: 32.5.4 [atomics.order] Status: NAD Submitter: Eric Fiselier Opened: 2019-08-31 Last modified: 2026-06-15

Priority: 4

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Discussion:

P0439R0 renamed the std::memory_order enumerators when making it a scoped enumeration. The paper makes the old unscoped names available in the global namespace, but not within the scope of the enumeration.

For example:

std::memory_order::memory_order_consume is no longer well-formed but std::memory_order::consume and std::memory_order_consume are.

In order to prevent unnecessary breakage, we should re-add the memory_order_foo names to the enumeration.

[2019-10 Priority set to 4 after reflector discussion]

[Brno 2026-06-12; Status changed: New → NAD.]

LWG noted that two major implementations have removed these enumerators. One of them reported that they have not received any reports relating to this removal after 8 years. LWG does not believe that it is necessary to re-add these names to the standard.

Proposed resolution:

This wording is relative to N4830.

1. Add a new sub-clause at the end of Clause D [depr] as indicated:

   D.?? Deprecated memory_order enumerators

   -?- The following enumerators are declared in addition to those specified in 32.5.4 [atomics.order]:

   namespace std {
     enum class memory_order : unspecified {
       memory_order_relaxed = relaxed, memory_order_consume = consume, memory_order_acquire = acquire, 
       memory_order_release = release, memory_order_acq_rel = acq_rel, memory_order_seq_cst = seq_cst
     };
   }