N4498
Mike Spertus, Symantec
mike_spertus@symantec.com
revision of N4470
2015-05-04

Variadic lock_guard (Rev. 2)

The basic idea of this proposal is that std::lock_guard would benefit from being variadic to support multiple locks analogously to how std::lock does.

lock_guard is a very useful and widely used way to manage the lifetimes of lock ownership. std::mutex mtx; void f(){ std::lock_guard<mutex> lck(mtx); // Mutex will be unlocked however scope is exited   /* ... */ }

Unfortunately, if more than one lock needs to be required, life gets unnecessarily complicated

This is a lot more advanced and error-prone than the single lock case. For example, you often see the deadlock-invitingvoid swap(MyType const &l, MyType const &r) { std::lock_guard<std::mutex> llck(l.mtx); std::lock_guard<std::mutex> rlck(r.mtx); /* ... */ } or the exception-unsafe void swap(MyType const &l, MyType const &r) { std::lock(l.mtx, r.mtx); /* ... */ l.mtx.unlock(); r.mtx.unlock(); }

These are exactly the kinds of complexities that are easily avoided by std::lock_guard in the single-lock case. Wouldn't it be great if std::lock_guard was variadic so it also worked with multiple locks?void swap(MyType const &l, MyType const &r) { std::lock_guard<std::mutex, std::mutex> lck(l.mtx, r.mtx); /* ... */ } And that is exactly what we are proposing!

This can work in the presence of shared locking as well (just like std::lock does) as shown in the following example shared by Howard Hinnant#include <mutex> #include <shared_mutex> class X { using Mutex = std::shared_timed_mutex; using ReadLock = std::shared_lock<Mutex>; mutable Mutex mut_; // more data public: // ... X& operator=(const X& x) { if (this !=&x) { ReadLock rl(x.mut_, std::defer_lock); std::lock_guard<Mutex, ReadLock> lck(mut_, rl); // assign data ... } return *this; } // ... }; which improves clarity while eliminating three lines of code

We do not propose creating a “make function” because std::lock_guard is not movable (which we suspect is the reason std::lock_guard didn't have a make function in the first place), because we believe the two-lock case is most important in practice, because we want to keep this proposal simple, and because something along the lines of N3602/N4471 may offer a more general solution that obviates the entire issue. For now, we think that the already useful std::lock_guard class is strictly better when variadic.

Wording

namespace std {
  template <class Mutex>
  template <typename ...MutexTypes>
  class lock_guard {
  public:
    typedef Mutex mutex_type;  // If MutexTypes... consists of the single type Mutex
    
    explicit lock_guard(mutex_type& m);
    lock_guard(mutex_type& ...m, adopt_lock_t);
    explicit lock_guard(MutexTypes&... m);
    lock_guard(MutexTypes&... m, adopt_lock_t);
    ~lock_guard();
    
    lock_guard(lock_guard const&) = delete;
    lock_guard& operator=(lock_guard const&) = delete;
  private:
    mutex_type&tuple<MutexTypes*...> pm; // exposition only
  };
}

An object of type lock_guard controls the ownership of a lockable objects within a scope. A lock_guard object maintains ownership of a lockable objects throughout the lock_guard object's lifetime (3.8). The behavior of a program is undefined if the lockable objects reference by pm does not exist for the entire lifetime of the lock_guard object. When sizeof...(MutexTypes) is 1, the supplied Mutex type shall meet the BasicLockable requirements. Otherwise, each of the mutex types shall meet the Lockable requirements. (30.2.5.2).

explicit lock_guard(mutex_type&MutexTypes&... m);

Requires: If mutex_typea MutexTypes type is not a recursive mutex, the calling thread does not own the corresponding mutex element of m.
Effects:If sizeof...(MutexTypes) is 1, then m.lock(). Otherwise if sizeof...(MutexTypes) is greater than 1, then lock(m...). Otherwise, no effects.
Postcondition: &pm == &mmake_tuple(&m...)

lock_guard(mutex_type&MutexTypes&... m, adopt_lock_t);

Requires: The calling thread owns all the mutexes in m.
Postcondition: &pm == &mmake_tuple(&m...)
Throws: Nothing.

~lock_guard();

Effects: pm.unlock() For each 0 ≤ i < sizeof...(MutexTypes), get<i>(pm)->unlock()

Appendix: Example implementation

We interrupt this proposal for a message from our sponsor (N4064)

We now return to our regularly scheduled proposal

Feature test macro

For the purpose of SG10, we recommend the feature test macro __cpp_lib_variadic_lock_guard