现代C ++。 从工作线程返回数据结构，继续执行

Question

I need to launch working thread, perform some initialization, return data structure as initialization result and continue thread execution. What is the best (or possible) code to achieve this using modern c++ features only? Note, launched thread should continue its execution (thread does not terminated as usual). Unfortunately, most solutions assume worker thread termination.

Pseudo code:

// Executes in WorkerThread context 
void SomeClass::Worker_treadfun_with_init()
{
    // 1. Initialization calls...
    // 2. Pass/signal initialization results to caller
    // 3. Continue execution of WorkerThread    
}

// Executes in CallerThread context
void SomeClass::Caller()  
{
    // 1. Create WorkerThread with SomeClass::Worker_treadfun_with_init()" thread function
    // 2. Sleep thread for some initialization results 
    // 3. Grab results
    // 3. Continue execution of CallerThread
}

Answer 1

Try using a pointer or reference to the data structure with the answer in it, and std::condition_variable to let you know when the answer has been computed:

#include <iostream>           
#include <thread>             
#include <mutex>              
#include <condition_variable> 

#include <chrono>
#include <vector>

std::vector<double> g_my_answer;
std::mutex g_mtx;
std::condition_variable g_cv;
bool g_ready = false;


void Worker_treadfun_with_init()
{
  //Do your initialization here
  {
     std::unique_lock<std::mutex> lck( g_mtx );
     for( double val = 0; val < 10; val += 0.3 )
       g_my_answer.push_back( val );

     g_ready = true;
     lck.unlock();
     g_cv.notify_one();
  }

  //Keep doing your other work..., here we'll just sleep
  for( int i = 0; i < 100; ++i )
  {
    std::this_thread::sleep_for( std::chrono::seconds(1) );
  }
}

void Caller()  
{
  std::unique_lock<std::mutex> lck(g_mtx);

  std::thread worker_thread = std::thread( Worker_treadfun_with_init );

  //Calling wait will cause current thread to sleep until g_cv.notify_one() is called.
  g_cv.wait( lck, [&g_ready](){ return g_ready; } ); 

  //Print out the answer as the worker thread continues doing its work
  for( auto val : g_my_answer )
    std::cout << val << std::endl;

  //Unlock mutex (or better yet have unique_lock go out of scope)
  // incase worker thread needs to lock again to finish
  lck.unlock();  

  //...

  //Make sure to join the worker thread some time later on.
  worker_thread.join();
}

Of course in actual code you wouldnt use global variables, and instead pass them by pointer or reference (or as member variables of SomeClass ) to the worker function, but you get the point.

Answer 2

I think std::future meets your requirements.

// Executes in WorkerThread context 
void SomeClass::Worker_treadfun_with_init(std::promise<Result> &pro)
{
    // 1. Initialization calls...
    // 2. Pass/signal initialization results to caller
    pro.set_value(yourInitResult);
    // 3. Continue execution of WorkerThread    
}

// Executes in CallerThread context
void SomeClass::Caller()  
{
    // 1. Create WorkerThread with SomeClass::Worker_treadfun_with_init()" thread function
    std::promise<Result> pro;
    auto f=pro.get_future();
    std::thread([this,&pro](){Worker_treadfun_with_init(pro);}).detach();
    auto result=f.get();
    // 3. Grab results
    // 3. Continue execution of CallerThread
}