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C++11 Threads

Posted by Lee A. Stripp, 07 March 2013 · 796 views

I was just brushing up on some C++11 concepts the last few nights (looking for parts of the engine to improve) and noticed quite a few people struggling with threads inside classes, as in using a class object as a thread. Some of the responses were so confusing to say the least.

Also they were struggling with std::cout while threads were running. So here's some example code of these problems all in one. I hope it points someone in the right direction.

Files:
lsThread2.h / .cpp
lsThread.h / .cpp
core.h / .cpp
main.cpp
// file : core.h

#ifndef cppThreads_core_h
#define cppThreads_core_h

#include <iostream>
#include <vector>
#include <thread>
#include <mutex>

using namespace std;

/*! Base class
 Used to start some lsThread objects.
 */
class core
{
public:
    core();
    virtual ~core();
    
    void run();
    void logText( const string& val, int tid  );
    
private:
    int nr_threads;
    mutex m;
};


#endif

// file : core.cpp
​
#include "core.h"
#include "lsThread.h"

core::core()
{
    nr_threads = 10;
}

core::~core()
{
}

void core::run()
{
    cout << "########## Start test ##########" << endl;
    
    vector<thread> th;
    lsThread *mt = new lsThread[nr_threads];
    
    //Launch a group of threads
    for( int i = 0; i < nr_threads; ++i )
    {
        mt[i].setTid( i );
        // Shows how to pass args to the start function in lsThread
        th.push_back( thread( &lsThread::start, mt[i], this, 5 ) );
    }
    
    //Join the threads with the main thread which core is running
    for( auto &t : th )
    {
        t.join();
    }
    
    // delete our array of lsThread objects
    delete [] mt;
    cout << "########## End test ##########" << endl;
}

/*! Logging
 This function is here so its in a common thread for all other threads to call.
 Remember in a real world App you would make a better logger.
 */
void core::logText( const string& val, int tid )
{
    // Lock for output
    m.lock();
    cout << "Thread : " << tid << " - " << val << endl;
    m.unlock();
}




Above: a base class that runs in the main thread.
// file : lsThread.h
​
#ifndef cppThreads_lsThread_h
#define cppThreads_lsThread_h

#include <iostream>
#include <thread>
#include <vector>

class core;

using namespace std;

/*! Thread class
 Nothing special about this class, you could thread any class.
 */
class lsThread
{
public:
    lsThread();
    virtual ~lsThread();
    
    virtual void start( core *c, int val );
    
    void setTid( int val )
    {
        tid = val;
    }
    
private:
    int tid;
    
};


#endif

// File : lsThread.cpp

#include "core.h"
#include "lsThread.h"
#include "lsThread2.h"

lsThread::lsThread()
{
    tid = 0;
}

lsThread::~lsThread()
{
}

void lsThread::start( core *c, int val )
{
    c->logText( "lsThread start", tid );
    
    // Here we create some more threads with a new class, based on the passed value.
    vector<thread> th;
    int nr_threads = val;
    lsThread2 *mt = new lsThread2[nr_threads];
    
    //Launch a group of threads
    for( int i = 0; i < nr_threads; ++i )
    {
        mt[i].setTid( i );
        th.push_back( thread( &lsThread2::start, mt[i], c ) );
    }
    
    // Join the threads with the current thread
    // This could be any of the threads created by the core class.
    for( auto &t : th )
    {
        t.join();
    }
    
    // delete our array of lsThread2 objects.
    delete [] mt;
}




Above: This class show how to launch more threads inside the current thread.
// File : lsThread2.h

#ifndef cppThreads_lsThread2_h
#define cppThreads_lsThread2_h

#include <iostream>
#include <thread>
#include <vector>

class core;

using namespace std;

class lsThread2
{
public:
    lsThread2();
    virtual ~lsThread2();
    
    virtual void start( core *c );
    
    void setTid( int val )
    {
        tid = val;
    }
    
private:
    int tid;
};


#endif

// File : lsThread2.cpp

#include "core.h"
#include "lsThread2.h"


lsThread2::lsThread2()
{
    tid = 0;
}

lsThread2::~lsThread2()
{
}

void lsThread2::start( core *c )
{
    // In this thread we just call the core class logging function
    c->logText( "lsThread2 start", tid );
}




Above: Simple thread that will just output a message to stdout.
// File : main.cpp

#include "core.h"


int main(int argc, const char * argv[])
{
    // Thread demo
    core *c = new core();
    c->run();
    delete c;
    
    return 0;
}


Above: And last but not least the main()

I've kept this example as simple as possible but still trying to show as much as possible. Hope it helps someone.

Cheers
Lee




Is this straight c/c++? What does

for( auto &t : th )

do?

What about

&lsThread::start

How can you use that as an argument?

 

[edit] Oh, C++11

Yes its C++11, I might add some more comments to the code soon if anyone wants me too.

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