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2731. Existence of lock_guard<MutexTypes...>::mutex_type typedef unclear

Section: 30.4.4.1 [thread.lock.guard] Status: Open Submitter: Eric Fiselier Opened: 2016-06-13 Last modified: 2018-03-18

Priority: 3

View all other issues in [thread.lock.guard].

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Discussion:

In the synopsis of 30.4.4.2 [thread.lock.scoped] the mutex_type typedef is specified as follows:

template <class... MutexTypes>
class scoped_lock {
public:
  typedef Mutex mutex_type; // If MutexTypes... consists of the single type Mutex
  […]
};

The comment seems ambiguous as it could mean either:

  1. sizeof...(MutexTypes) == 1.
  2. sizeof...(MutexTypes) >= 1 and every type in MutexTypes... is the same type.

I originally took the language to mean (2), but upon further review it seems that (1) is the intended interpretation, as suggested in the LEWG discussion in Lenexa.

I think the language should be clarified to prevent implementation divergence.

[2016-07, Toronto Saturday afternoon issues processing]

General feeling that sizeof(MutexTypes...) == 1 is a better way to state the requirement.

Reworked the text to refer to scoped_lock instead of lock_guard

Marshall and Eric to reword and discuss on reflector. Status to Open

[2018-3-14 Wednesday evening issues processing; general agreement to adopt once the wording is updated.]

2018-03-18 Marshall provides updated wording.

Previous resolution: [SUPERSEDED]

This wording is relative to N4594.

  1. Edit 30.4.4.1 [thread.lock.guard]/1, class template lock_guard synopsis, as indicated:

    template <class... MutexTypes>
    class lock_guard {
    public:
      typedef Mutex mutex_type; // Only iIf MutexTypes... consists of theexpands to a single type Mutex
      […]
    };
    

Proposed resolution:

This wording is relative to N4727.

  1. Edit 30.4.4.1 [thread.lock.guard]/1, class template lock_guard synopsis, as indicated:

    template <class... MutexTypes>
    class scoped_lock {
    public:
      using mutex_type = Mutex; // Only iIf sizeof(MutexTypes...) == 1 MutexTypes... consists of the single type Mutex
      […]
    };