Question

I’m having trouble adding a copy constructor to this class:
http://www.justsoftwaresolutions.co.uk/threading/implementing-a-thread-safe-queue-using-condition-variables.html

I need to add it so that I can add the concurrent_queue in an stl vector container.
Tried the below, and it almost compiles. In fact, if I remove the_mutex and the_condition_variable from the copy constructor it compiles. When I add it back in it doesn’t. The assignment operator seems to be OK with regards to compilation.

concurrent_queue(concurrent_queue<Data> const& rhs):
    the_queue(rhs.the_queue),
    the_mutex(rhs.the_mutex),
    the_condition_variable(rhs.the_condition_variable)
{
}

concurrent_queue<Data>& operator = (concurrent_queue<Data> const& rhs)
{
    if (this == &rhs) return *this; // check for self assignment

    the_queue = rhs.the_queue;
    the_mutex(rhs.the_mutex);
    the_condition_variable(rhs.the_condition_variable);
}

The error I get is the following:

concurrentqueue.h: In copy constructor ‘concurrent_queue<Data>::concurrent_queue(const concurrent_queue<Data>&) [with Data = Packet*]’:
/usr/include/c++/4.4/bits/stl_construct.h:74:   instantiated from ‘void std::_Construct(_T1*, const _T2&) [with _T1 = concurrent_queue<Packet*>, _T2 = concurrent_queue<Packet*>]’
/usr/include/c++/4.4/bits/stl_uninitialized.h:187:   instantiated from ‘static void std::__uninitialized_fill_n<<anonymous> >::uninitialized_fill_n(_ForwardIterator, _Size, const _Tp&) [with _ForwardIterator = concurrent_queue<Packet*>*, _Size = long unsigned int, _Tp = concurrent_queue<Packet*>, bool <anonymous> = false]’
/usr/include/c++/4.4/bits/stl_uninitialized.h:223:   instantiated from ‘void std::uninitialized_fill_n(_ForwardIterator, _Size, const _Tp&) [with _ForwardIterator = concurrent_queue<Packet*>*, _Size = long unsigned int, _Tp = concurrent_queue<Packet*>]’
/usr/include/c++/4.4/bits/stl_uninitialized.h:318:   instantiated from ‘void std::__uninitialized_fill_n_a(_ForwardIterator, _Size, const _Tp&, std::allocator<_Tp2>&) [with _ForwardIterator = concurrent_queue<Packet*>*, _Size = long unsigned int, _Tp = concurrent_queue<Packet*>, _Tp2 = concurrent_queue<Packet*>]’
/usr/include/c++/4.4/bits/stl_vector.h:1035:   instantiated from ‘void std::vector<_Tp, _Alloc>::_M_fill_initialize(size_t, const _Tp&) [with _Tp = concurrent_queue<Packet*>, _Alloc = std::allocator<concurrent_queue<Packet*> >]’
/usr/include/c++/4.4/bits/stl_vector.h:230:   instantiated from ‘std::vector<_Tp, _Alloc>::vector(size_t, const _Tp&, const _Alloc&) [with _Tp = concurrent_queue<Packet*>, _Alloc = std::allocator<concurrent_queue<Packet*> >]’
test.cpp:18:   instantiated from here
concurrentqueue.h:24: error: no match for call to ‘(boost::mutex) (boost::mutex&)’

EDIT:
It seems that boost mutex inherits non-copyable and condition variable I think is the same.
The interesting thing is that in the assignment I’m able to copy it. Why would that even compile? Do I need to worry about the fact that it’s non-copyable in practice for the situation I’m using it for?

Answer 1

Both boost::mutex and boost::condition_variable only have default constructors. Aside from that, you don’t want their (potentially locked) state to be copied – just use the default constructors.

Also you shouldn’t copy the_queue directly as this is not thread-safe.