Passing class object as argument to 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".

copy constructor is implicitly generated if you don't block it — Bartek Banachewicz

CrazyCasta CrazyCasta · Accepted Answer · 2012-09-25T07:59:36

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;
}

Passing class object as argument to boost::thread

1 Answers