Re: implementing a thread-safe queue

Chris Vine <>
Mon, 16 May 2011 16:54:33 CST
On Sat, 14 May 2011 02:29:05 CST
itcecsa <> wrote:

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

#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 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 std::deque<T>())

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

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

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

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

    // 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;

Others have pointed out that your mutex locking is not thread safe
(you should use a lock class which releases the mutex in its

But there is another issue concerning your pop() method. You were
right to combine the non-thread safe front() and pop() methods of
std::queue so that items are removed from the queue under a single
atomic lock by a pop() method which also provides the value.

However, your pop() is not strongly exception safe because the copy
operation required by your 'return elem' call might throw. You can
retain strong exception safety and thread safety by providing the
popped value by means of a non-const reference argument instead of a
return value. Most thread-safe queue implementations do it that way.


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