Stackoverflow has been a tremendous help to me and I'd to give something back to the community. I have been implementing a simple threadpool using the tinythread C++ portable thread library, using what I have learnt from Stackoverflow. I am new to thread programming, so not that comfortable with mutexes, etc. I have a question best asked after presenting the code (which runs quite well under Linux):

// ThreadPool.h

class ThreadPool
{
 public:

 ThreadPool();
~ThreadPool();

// Creates a pool of threads and gets them ready to be used
void CreateThreads(int numOfThreads);

// Assigns a job to a thread in the pool, but doesn't start the job
// Each SubmitJob call will use up one thread of the pool.
// This operation can only be undone by calling StartJobs and
// then waiting for the jobs to complete. On completion,
// new jobs may be submitted.
void SubmitJob( void (*workFunc)(void *), void *workData );

// Begins execution of all the jobs in the pool.
void StartJobs();

// Waits until all jobs have completed.
// The wait will block the caller.
// On completion, new jobs may be submitted.
void WaitForJobsToComplete();

private:

enum typeOfWorkEnum { e_work, e_quit };

 class ThreadData
 {
   public:

    bool ready;  // thread has been created and is ready for work  
    bool haveWorkToDo;
    typeOfWorkEnum  typeOfWork;

    // Pointer to the work function each thread has to call.
    void (*workFunc)(void *);

    // Pointer to work data
    void *workData;

    ThreadData() : ready(false), haveWorkToDo(false) {  };
 };

struct ThreadArgStruct
{
    ThreadPool *threadPoolInstance;
    int         threadId;
};

// Data for each thread
ThreadData  *m_ThreadData;

ThreadPool(ThreadPool const&); // copy ctor hidden
ThreadPool& operator=(ThreadPool const&); // assign op. hidden

// Static function that provides the function pointer that a thread can call
// By including the ThreadPool instance in the void * parameter,
// we can use it to access other data and methods in the ThreadPool instance.
static void ThreadFuncWrapper(void *arg)
{
    ThreadArgStruct *threadArg = static_cast<ThreadArgStruct *>(arg);
    threadArg->threadPoolInstance->ThreadFunc(threadArg->threadId);
}

// The function each thread calls    
void ThreadFunc( int threadId );

// Called by the thread pool destructor
void DestroyThreadPool();

// Total number of threads available
// (fixed on creation of thread pool)
int m_numOfThreads;
int m_NumOfThreadsDoingWork;
int m_NumOfThreadsGivenJobs;

// List of threads
std::vector<tthread::thread *> m_ThreadList;

// Condition variable to signal each thread has been created and executing
tthread::mutex              m_ThreadReady_mutex;
tthread::condition_variable m_ThreadReady_condvar;

 // Condition variable to signal each thread to start work
tthread::mutex              m_WorkToDo_mutex;
tthread::condition_variable m_WorkToDo_condvar;

// Condition variable to signal the main thread that 
// all threads in the pool have completed their work
tthread::mutex              m_WorkCompleted_mutex;
tthread::condition_variable m_WorkCompleted_condvar;
};

cpp file:

//
//  ThreadPool.cpp
//

#include "ThreadPool.h"    

// This is the thread function for each thread.
// All threads remain in this function until
// they are asked to quit, which only happens
// when terminating the thread pool.
void ThreadPool::ThreadFunc( int threadId )
{
 ThreadData *myThreadData = &m_ThreadData[threadId]; 
 std::cout << "Hello world: Thread " << threadId << std::endl;

 // Signal that this thread is ready
 m_ThreadReady_mutex.lock();
       myThreadData->ready = true;
       m_ThreadReady_condvar.notify_one(); // notify the main thread
 m_ThreadReady_mutex.unlock();       

 while(true)
 {
    //tthread::lock_guard<tthread::mutex> guard(m);
    m_WorkToDo_mutex.lock();

    while(!myThreadData->haveWorkToDo) // check for work to do
         m_WorkToDo_condvar.wait(m_WorkToDo_mutex); // if no work, wait here 
    myThreadData->haveWorkToDo = false; // need to do this before unlocking the mutex

    m_WorkToDo_mutex.unlock();

    // Do the work
    switch(myThreadData->typeOfWork)
    {
        case e_work:
            std::cout << "Thread " << threadId << ": Woken with work to do\n";

            // Do work
            myThreadData->workFunc(myThreadData->workData);

            std::cout << "#Thread " << threadId  << ": Work is completed\n";
            break;

         case e_quit:
             std::cout << "Thread " << threadId << ": Asked to quit\n";
             return; // ends the thread
    }

    // Now to signal the main thread that my work is completed
    m_WorkCompleted_mutex.lock();
       m_NumOfThreadsDoingWork--;

      // Unsure if this 'if' would make the program more efficient
      // if(NumOfThreadsDoingWork == 0)
           m_WorkCompleted_condvar.notify_one(); // notify the main thread
    m_WorkCompleted_mutex.unlock();       
  }

}


ThreadPool::ThreadPool() 
{ 
   m_numOfThreads = 0;  m_NumOfThreadsDoingWork = 0; m_NumOfThreadsGivenJobs = 0;
}


ThreadPool::~ThreadPool()
{
    if(m_numOfThreads)
    {
    DestroyThreadPool(); 
    delete [] m_ThreadData;
    }
}


void ThreadPool::CreateThreads(int numOfThreads)
{
// Check a thread pool has already been created
if(m_numOfThreads > 0) 
   return;

m_NumOfThreadsGivenJobs = 0;
m_NumOfThreadsDoingWork = 0;
m_numOfThreads = numOfThreads;
m_ThreadData = new ThreadData[m_numOfThreads];
ThreadArgStruct threadArg;

for(int i=0; i<m_numOfThreads; ++i)
 {   
    threadArg.threadId = i;
    threadArg.threadPoolInstance = this;

    // Creates the thread and save in a list so we can destroy it later
    m_ThreadList.push_back( new tthread::thread( ThreadFuncWrapper, (void *)&threadArg  ) ); 

    // It takes a little time for a thread to get established.
    // Best wait until it gets established before creating the next thread.
    m_ThreadReady_mutex.lock();
    while(!m_ThreadData[i].ready)  // Check if thread is ready
        m_ThreadReady_condvar.wait(m_ThreadReady_mutex); // If not, wait here
    m_ThreadReady_mutex.unlock();    
 } 
} 


// Adds a job to the batch, but doesn't start the job
void ThreadPool::SubmitJob(void (*workFunc)(void *), void *workData)
{
 // Check that the thread pool has been created
 if(!m_numOfThreads) 
    return;

 if(m_NumOfThreadsGivenJobs >= m_numOfThreads)
    return;

 m_ThreadData[m_NumOfThreadsGivenJobs].workFunc = workFunc;
 m_ThreadData[m_NumOfThreadsGivenJobs].workData = workData;  

 std::cout << "Submitted job " << m_NumOfThreadsGivenJobs << std::endl;

 m_NumOfThreadsGivenJobs++;  
}

void ThreadPool::StartJobs()
{
// Check that the thread pool has been created
// and some jobs have been assigned
if(!m_numOfThreads || !m_NumOfThreadsGivenJobs) 
   return;

// Set 'haveworkToDo' flag for all threads 
m_WorkToDo_mutex.lock();
   for(int i=0; i<m_NumOfThreadsGivenJobs; ++i)
       m_ThreadData[i].haveWorkToDo = true;
   m_NumOfThreadsDoingWork = m_NumOfThreadsGivenJobs;

   // Reset this counter so we can resubmit jobs later
   m_NumOfThreadsGivenJobs = 0;

   // Notify all threads they have work to do
   m_WorkToDo_condvar.notify_all();
   m_WorkToDo_mutex.unlock();
}


void ThreadPool::WaitForJobsToComplete()
{
  // Check that a thread pool has been created
  if(!m_numOfThreads) 
   return;

 m_WorkCompleted_mutex.lock();
 while(m_NumOfThreadsDoingWork > 0)  // Check if all threads have completed their work
   m_WorkCompleted_condvar.wait(m_WorkCompleted_mutex); // If not, wait here
 m_WorkCompleted_mutex.unlock();    
}


void ThreadPool::DestroyThreadPool()
{
std::cout << "Ask threads to quit\n";
m_WorkToDo_mutex.lock();
   for(int i=0; i<m_numOfThreads; ++i)
   {
     m_ThreadData[i].haveWorkToDo = true;
     m_ThreadData[i].typeOfWork = e_quit;
   }
   m_WorkToDo_condvar.notify_all();
m_WorkToDo_mutex.unlock();

// As each thread terminates, catch them here
for(int i=0; i<m_numOfThreads; ++i)
 {
     tthread::thread *t = m_ThreadList[i];

     // Wait for thread to complete
     t->join();
 }
 m_numOfThreads = 0;
}

Example of usage: (this calculates pi-squared/6)

struct CalculationDataStruct
{
int inputVal;
double outputVal;
};

void LongCalculation( void *theSums )
{
CalculationDataStruct *sums = (CalculationDataStruct *)theSums;

int terms = sums->inputVal;
double sum;
for(int i=1; i<terms; i++)
    sum += 1.0/( double(i)*double(i) );
sums->outputVal = sum;
}


int main(int argc, char** argv)
{ 
int numThreads = 10;

// Create pool
ThreadPool threadPool;
threadPool.CreateThreads(numThreads);

// Create thread workspace
CalculationDataStruct sums[numThreads];

// Set up jobs
for(int i=0; i<numThreads; i++)
{
    sums[i].inputVal = 3000*(i+1);
    threadPool.SubmitJob(LongCalculation, &sums[i]);
}

// Run the jobs
threadPool.StartJobs();
threadPool.WaitForJobsToComplete();

// Print results
for(int i=0; i<numThreads; i++)
   std::cout << "Sum of " << sums[i].inputVal << " terms is " << sums[i].outputVal << std::endl;

 return 0;
}

Question: In the ThreadPool::ThreadFunc method, would better performance be obtained if the following if statement

if(NumOfThreadsDoingWork == 0)

was included? Also, I'd be grateful of criticisms and ways to improve the code. At the same time, I hope the code is of use to others.