将类对象作为参数传递给boost :: thread

Question

I have a function called "producer" that takes a class object as argument. I am trying to create thread for producer using boost::thread. However it is causing error because of the class object that I am passing as argument. I cannot figure out why is it giving error. If I remove function arguments and let it pass in as global variable, it works fine. Below is my code.

BlockingQueue.h

#ifndef BLOCKINGQUEUE_H_
#define BLOCKINGQUEUE_H_

#include <queue>
#include <iostream>
#include <boost/thread.hpp>

template<class T>
class BlockingQueue
{
private:
    std::queue<T>               m_queBlockingQueue;
    boost::condition_variable   m_cvSignal;
    boost::mutex                m_mtxSync;

public:
    BlockingQueue();
    bool isEmpty();
    T& popElement();
    void pushElement(T nElement);
    virtual ~BlockingQueue();
};

template<class T>
BlockingQueue<T>::BlockingQueue()
{
}

template<class T>
bool BlockingQueue<T>::isEmpty()
{
    bool bEmpty;
    boost::mutex::scoped_lock lock(m_mtxSync);
    m_cvSignal.wait(lock);
    bEmpty = m_queBlockingQueue.empty();
    return bEmpty;
}

template<class T>
T& BlockingQueue<T>::popElement()
{
    boost::mutex::scoped_lock lock(m_mtxSync);
    while (m_queBlockingQueue.empty())
    {
        m_cvSignal.wait(lock);
    }
    T& nElement = m_queBlockingQueue.front();
    m_queBlockingQueue.pop();
    return nElement;
}

template<class T>
void BlockingQueue<T>::pushElement(T nElement)
{
    boost::mutex::scoped_lock lock(m_mtxSync);
    m_queBlockingQueue.push(nElement);
    m_cvSignal.notify_one();
}

template<class T>
BlockingQueue<T>::~BlockingQueue()
{
}

#endif /* BLOCKINGQUEUE_H_ */

Main.cpp

#include "BlockingQueue.h"

#include <iostream>

using namespace std;

void producer (BlockingQueue<int>& blockingQueue)
{
    for (int i=0; i<100; i++)
    {
        cout<<"Producer about to push("<<i<<")..."<<endl;
        blockingQueue.pushElement(i);
        sleep(1);
    }
}

void consumer (BlockingQueue<int>& blockingQueue)
{
    for (int i=0; i<100; i++)
    {
        cout<<"Consumer received: "<<blockingQueue.popElement()<<endl;
        sleep(3);
    }
}

int main ()
{
    BlockingQueue<int> blockingQueue;
    cout<<"Program started..."<<endl;
    cout.flush();

    boost::thread tConsumer(consumer, blockingQueue);
    boost::thread tProducer(producer, blockingQueue);

    tProducer.join();
    tConsumer.join();

    return 0;
}

The errors that I am getting are something like:

1) no known conversion for argument 1 from 'const BlockingQueue' to 'BlockingQueue&' 2) BlockingQueue::BlockingQueue(BlockingQueue&) within this context 3) no matching function for call to 'BlockingQueue::BlockingQueue(const BlockingQueue&)'

I have other errors like: thread.hpp:148:47: error: initializing argument 1 of 'static boost::detail::thread_data_ptr boost::thread::make_thread_info(F) [with F = boost::_bi::bind_t&), boost::_bi::list1 > > >, boost::detail::thread_data_ptr = boost::shared_ptr]'

Is there some function like copy constructor etc in my class which is missing or what. If I use my code to pass primitive data types like int, double it works fine. It was some issue with my class "BlockingQueue".

Answer 1

If you want to pass by reference you need to use boost::ref, or just use a pointer. boost::thread copies by value, and therefore cannot use your pass by reference function. For instance you could do:

#include "BlockingQueue.h"

#include <iostream>

using namespace std;

void producer (boost::reference_wrapper<BlockingQueue<int>> blockingQueue)
{
    for (int i=0; i<100; i++)
    {
        cout<<"Producer about to push("<<i<<")..."<<endl;
        blockingQueue.get_pointer()->pushElement(i);
        sleep(1);
    }
}

void consumer (boost::reference_wrapper<BlockingQueue<int>> blockingQueue)
{
    for (int i=0; i<100; i++)
    {
        cout<<"Consumer received: "<<blockingQueue.get_pointer()->popElement()<<endl;
        sleep(3);
    }
}

int main ()
{
    BlockingQueue<int> blockingQueue;
    cout<<"Program started..."<<endl;
    cout.flush();

    boost::thread tConsumer(consumer, ref(blockingQueue));
    boost::thread tProducer(producer, ref(blockingQueue));

    tProducer.join();
    tConsumer.join();

    return 0;
}