Re: implementing a thread-safe queue

Gert-Jan de Vos <>
Sat, 14 May 2011 19:38:19 CST
On May 14, 10:29 am, itcecsa <> wrote:

this is my implementation. feel free to give me commands.

My first question is: What is the purpose of this class? A
multi threaded queue usually decouples producer and consumer
threads. In that case you should reconsider your design for
these conditions:
1) significantly higher rate of pushes over pops
2) significantly higher rate of pops over pushes

In your design 1) eventually triggers a bad_alloc and
2) eventually triggers a ReadEmptyQueue exception.

Is this really what you want? Threads are fully asynchronous
and there is nothing to prevent 1 or 2 from happening.

Consider to block the call when either:
1) push() when size() > limit
2) pop() when empty()
By blocking the call that runs at a too high rate you prevent
the problem from happening. The queue will synchronize the
producer and consumer rates between some boundary limits.

Have a look at condition variables for how to implement
blocking calls based on some condition.

#include <pthread.h>
#include <deque>
#include <exception>

/* exception class for pop() with empty queue */
class ReadEmptyQueue : public std::exception {
        virtual const char* what() const throw() {
                return "Queue is empty!";


template <class T>
class Queue {
        std::deque<T> *container; // container for the elements

Why a pointer to deque<T>? container could just be a value member,
this saves you explicit initialization and destruction.

        pthread_mutex_t mutex; // mutex for sync the queue
        Queue(const Queue& q); // disable copy constructor
        Queue& operator= (const Queue& q); // disable assign operator


        // to make it simple, this is the only constructor for Queue instance
        explicit Queue():mutex(PTHREAD_MUTEX_INITIALIZER), container(new

explicit only has a meaning for single argument constructors. It
should not be here.

        // destructor
        virtual ~Queue(){
                delete container;

virtual indicates that your class is designed as a base class.
Is that really your intention?

        // number of elements
        typename std::deque<T>::size_type size() const {
                return container->size();

This is not thread safe. A simultaneous push or pop call could be
modifying the containers size while you read it.

        //is queue empty?
        bool empty() const {
                return container->empty();

This is not thread safe. A simultaneous push or pop call could be
modifying the containers size while you read its empty state.

        // insert element into the queue
        void push (const T& elem) {
                pthread_mutex_lock( &mutex );
                pthread_mutex_unlock( &mutex );

This is not exception safe. The push_back() or T's copy constructor
could throw and the lock will not be released. Make a RAII class to
represent the locked state of the mutex object, its destructor will
take care of the unlock no matter how push() ends. Better yet, use
boosts or C++0x's mutex object. It is portable and provides lock

        // pop element from the top of the queue
        T pop () {
                pthread_mutex_lock( &mutex );

                if (container->empty()) {
                        throw ReadEmptyQueue();

                T elem(container->front());
                pthread_mutex_unlock( &mutex );

                return elem;

This is not exception safe for the same reasons as for push()
but also:

Your throw ReadEmptyQueue() does not release the lock.

The return statement might invoke a copy constructor
which might throw. In that case you have lost the
element you just popped of the queue. This is a direct
example from Sutter's Exceptional C++ books. His advice
is to design your queue as the STL does: separate the
front() and pop_front() primitives. Either can be made
exception safe, combined they can not. More
pragmatically I think a void pop(T& result) member could
work or else just accept the problem. Copies don't throw
for POD types.



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