P1960R0
NB Comment Changes Reviewed by SG1

1. Abstract

All of the changes that resulted from NB comments reviewed by SG1 in Belfast that are not already covered by existing papers.

2. US355

2.1. Poll

Recommend to make notify_one, notify_all, and operator=(float) of atomic_ref const, in response to US 358 and US 355: Unanimous consent

2.2. Wording

Change all the declarations of member functions notify_one and notify_all in class atomic_ref to be const.

In 31.7 "Class template atomic_ref" [atomics.ref.generic] near the end of the class synopsis:

void notify_one() const noexcept;
void notify_all() const noexcept;

In 31.7.1 "Operations" [atomics.ref.ops] near the end of the list of functions:

void notify_one() const noexcept;

[ ... skip ... ]

void notify_one() const noexcept;

In 31.7.2 "Specializations for integral types" [atomics.ref.int] near the end of the class synopsis:

void notify_one() const noexcept;
void notify_all() const noexcept;

In 31.7.3 "Specializations for floating-point types" [atomics.ref.float] near the end of the class synopsis:

void notify_one() const noexcept;
void notify_all() const noexcept;

In 31.7.4 "Partial specializations for pointers" [atomics.ref.pointer] near the end of the class synopsis:

void notify_one() const noexcept;
void notify_all() const noexcept;

3. US358

3.1. Poll

Recommend to make notify_one, notify_all, and operator=(float) of atomic_ref const, in response to US 358 and US 355: Unanimous consent

3.2. Wording

In 31.7.3 "Specializations for floating-point types" [atomics.ref.float] near the middle of the class synopsis:

floating-point operator=(floating-point) const noexcept;

4. US359

4.1. Poll

Recommend adopting the comment’s proposed wording (i.e. ignoring the alternative), in response to US 359:
SF F N A SA
5 12 1 0 0
Unanimous consent

LWG recommended using value_type as the return type, and making the same change to the identical wording in atomic.

4.2. Wording

Change 31.7.5 "Member operators common to integers and pointers to objects" [atomics.ref.memop] as follows:

T* value_type operator++(int) const noexcept;

Effects: Equivalent to return fetch_add(1);

T* value_type operator--(int) const noexcept;

Effects: Equivalent to return fetch_sub(1);

T* value_type operator++() const noexcept;

Effects: Equivalent to return fetch_add(1) + 1;

T* value_type operator--(int) const noexcept;

Effects: Equivalent to return fetch_sub(1) - 1;

Change 31.8.5 "Member operators common to integers and pointers to objects" [atomics.types.memop] as follows:

T value_type operator++(int) volatile noexcept;
T value_type operator++(int) noexcept;

Effects: Equivalent to return fetch_add(1);

T value_type operator--(int) volatile noexcept;
T value_type operator--(int) noexcept;

Effects: Equivalent to return fetch_sub(1);

T value_type operator++() volatile noexcept;
T value_type operator++() noexcept;

Effects: Equivalent to return fetch_add(1) + 1;

T value_type operator--() volatile noexcept;
T value_type operator--() noexcept;

Effects: Equivalent to return fetch_sub(1) - 1;

5. US356

5.1. Polls

Recommend removing the instance-dependence of atomic_ref ::is_lock_free(), in response to US 356:
SF F N A SA
3 3 5 1 0
Consensus for change.

Recommend adding an instance-dependent atomic_ref ::is_instance_lock_free(), in response to US 356:
SF F N A SA
2 5 3 1 2
Not consensus for change in an NB ballot process

Recommend making atomic_ref ::is_lock_free() static, in response to US 356:
SF F N A SA
3 5 3 1 2
Not consensus for change in an NB ballot process

5.2. Wording

Change the specification of atomic_ref<T>::is_lock_free in 31.7.1 "Operations" [atomics.ref.ops] p4:

bool is_lock_free() const noexcept;

Returns: true if the object’s operations on all objects of the type atomic_ref<T> are lock-free, false otherwise.

6. US364

6.1. Poll

No object to unanimous consent to adopting the wording in this paper as the resolution to US364.

6.2. Wording

Change the specification of counting_semaphore<N>::try_acquire in 32.7.2 "Class template counting_semaphore" [thread.sema.cnt] as follows:

bool try_acquire() noexcept;

Effects:

• With low probability, returns immediately. An implementation should ensure that try_acquire does not consistently return false in the absence of contending acquisitions.

• Otherwise, atomically check whether counter is greater than zero and, if so, decrement counter by one.

Effects: Attempts to atomically decrement counter if it is positive, without blocking. If counter is not decremented, there is no effect and try_acquire immediately returns. An implementation may fail to decrement counter even if it is positive. [ Note: This spurious failure is normally uncommon, but allows interesting implementations based on a simple compare and exchange ([atomic]). -- end note] An implementation should ensure that try_acquire does not consistently return false in the absence of contending semaphore operations.

Returns: true if counter was decremented, otherwise false.