This member function does not affect the logical state of the class.
Making std::condition_variable and std::mutex data members mutable is
idiomatic as these classes are thread-safe synchronization primitives.
The C++ standard specifies `std::condition_variable::wait(lock, pred)`
as equivalent to `while (!pred()) wait(lock);`. It is implemented
exactly so in libstdc++, libc++ and MSVC.
This function is called only from ~ConcurrentQueue(). joinAll() is not
thread-safe and it cannot be called earlier without introducing a null
state. Moving the function's implementation into the definition of
~ConcurrentQueue() makes the code clearer. Removing joinAll() also
allows to establish and document invariants for two data members.
Assert consistency between jobsLeft and _queue in ~ConcurrentQueue().
ConcurrentQueue is currently used only by two classes and a test, but
modifying concurrent_queue.h requires recompiling 30 source files. None
of the member functions is so lightweight as to make it worth inlining.
An alternative to `@note ConcurrentQueue is unable to execute jobs if
@p threadCount == 0.` is `assert(threadCount != 0);`. But this would
force classes that contain a ConcurrentQueue data member to always start
a thread, even if they detect at runtime that they are never going to
enqueue a job.
Add Job type alias to avoid repeating the type.
Use default member initializers instead of the member initializer list
to make it clear [to the reader of the header] that no data member is
left uninitialized.
