Can someone post a simple example of starting two (Object Oriented) threads in C++.
I'm looking for actual C++ thread objects that I can extend run methods on (or something similar) as opposed to calling a C-style thread library.
I left out any OS specific requests in the hopes that whoever replied would reply with cross platform libraries to use. I'm just making that explicit now.
This question is related to
c++
multithreading
It largely depends on the library you decide to use. For instance, if you use the wxWidgets library, the creation of a thread would look like this:
class RThread : public wxThread {
public:
RThread()
: wxThread(wxTHREAD_JOINABLE){
}
private:
RThread(const RThread ©);
public:
void *Entry(void){
//Do...
return 0;
}
};
wxThread *CreateThread() {
//Create thread
wxThread *_hThread = new RThread();
//Start thread
_hThread->Create();
_hThread->Run();
return _hThread;
}
If your main thread calls the CreateThread method, you'll create a new thread that will start executing the code in your "Entry" method. You'll have to keep a reference to the thread in most cases to join or stop it. More info here: wxThread documentation
#include <thread>
#include <iostream>
#include <vector>
using namespace std;
void doSomething(int id) {
cout << id << "\n";
}
/**
* Spawns n threads
*/
void spawnThreads(int n)
{
std::vector<thread> threads(n);
// spawn n threads:
for (int i = 0; i < n; i++) {
threads[i] = thread(doSomething, i + 1);
}
for (auto& th : threads) {
th.join();
}
}
int main()
{
spawnThreads(10);
}
There is also a POSIX library for POSIX operating systems. Check for compatability
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <iostream>
void *task(void *argument){
char* msg;
msg = (char*)argument;
std::cout<<msg<<std::endl;
}
int main(){
pthread_t thread1, thread2;
int i1,i2;
i1 = pthread_create( &thread1, NULL, task, (void*) "thread 1");
i2 = pthread_create( &thread2, NULL, task, (void*) "thread 2");
pthread_join(thread1,NULL);
pthread_join(thread2,NULL);
return 0;
}
compile with -lpthread
It largely depends on the library you decide to use. For instance, if you use the wxWidgets library, the creation of a thread would look like this:
class RThread : public wxThread {
public:
RThread()
: wxThread(wxTHREAD_JOINABLE){
}
private:
RThread(const RThread ©);
public:
void *Entry(void){
//Do...
return 0;
}
};
wxThread *CreateThread() {
//Create thread
wxThread *_hThread = new RThread();
//Start thread
_hThread->Create();
_hThread->Run();
return _hThread;
}
If your main thread calls the CreateThread method, you'll create a new thread that will start executing the code in your "Entry" method. You'll have to keep a reference to the thread in most cases to join or stop it. More info here: wxThread documentation
#include <thread>
#include <iostream>
#include <vector>
using namespace std;
void doSomething(int id) {
cout << id << "\n";
}
/**
* Spawns n threads
*/
void spawnThreads(int n)
{
std::vector<thread> threads(n);
// spawn n threads:
for (int i = 0; i < n; i++) {
threads[i] = thread(doSomething, i + 1);
}
for (auto& th : threads) {
th.join();
}
}
int main()
{
spawnThreads(10);
}
It largely depends on the library you decide to use. For instance, if you use the wxWidgets library, the creation of a thread would look like this:
class RThread : public wxThread {
public:
RThread()
: wxThread(wxTHREAD_JOINABLE){
}
private:
RThread(const RThread ©);
public:
void *Entry(void){
//Do...
return 0;
}
};
wxThread *CreateThread() {
//Create thread
wxThread *_hThread = new RThread();
//Start thread
_hThread->Create();
_hThread->Run();
return _hThread;
}
If your main thread calls the CreateThread method, you'll create a new thread that will start executing the code in your "Entry" method. You'll have to keep a reference to the thread in most cases to join or stop it. More info here: wxThread documentation
Unless one want a separate function in global namespacs, we can use lambda functions for creating threads.
One of the major advantage of creating thread using lambda is that we don't need to pass local parameters as an argument list. We can use capture list for the same and the closure property of lambda will take care of the lifecycle.
Here is a sample code
int main() {
int localVariable = 100;
thread th { [=](){
cout<<"The Value of local variable => "<<localVariable<<endl;
}};
th.join();
return 0;
}
By far, I've found C++ lambdas to be the best way of creating threads especially for simpler thread functions
It largely depends on the library you decide to use. For instance, if you use the wxWidgets library, the creation of a thread would look like this:
class RThread : public wxThread {
public:
RThread()
: wxThread(wxTHREAD_JOINABLE){
}
private:
RThread(const RThread ©);
public:
void *Entry(void){
//Do...
return 0;
}
};
wxThread *CreateThread() {
//Create thread
wxThread *_hThread = new RThread();
//Start thread
_hThread->Create();
_hThread->Run();
return _hThread;
}
If your main thread calls the CreateThread method, you'll create a new thread that will start executing the code in your "Entry" method. You'll have to keep a reference to the thread in most cases to join or stop it. More info here: wxThread documentation
When searching for an example of a C++ class that calls one of its own instance methods in a new thread, this question comes up, but we were not able to use any of these answers that way. Here's an example that does that:
Class.h
class DataManager
{
public:
bool hasData;
void getData();
bool dataAvailable();
};
Class.cpp
#include "DataManager.h"
void DataManager::getData()
{
// perform background data munging
hasData = true;
// be sure to notify on the main thread
}
bool DataManager::dataAvailable()
{
if (hasData)
{
return true;
}
else
{
std::thread t(&DataManager::getData, this);
t.detach(); // as opposed to .join, which runs on the current thread
}
}
Note that this example doesn't get into mutex or locking.
Well, technically any such object will wind up being built over a C-style thread library because C++ only just specified a stock std::thread model in c++0x, which was just nailed down and hasn't yet been implemented. The problem is somewhat systemic, technically the existing c++ memory model isn't strict enough to allow for well defined semantics for all of the 'happens before' cases. Hans Boehm wrote an paper on the topic a while back and was instrumental in hammering out the c++0x standard on the topic.
http://www.hpl.hp.com/techreports/2004/HPL-2004-209.html
That said there are several cross-platform thread C++ libraries that work just fine in practice. Intel thread building blocks contains a tbb::thread object that closely approximates the c++0x standard and Boost has a boost::thread library that does the same.
http://www.threadingbuildingblocks.org/
http://www.boost.org/doc/libs/1_37_0/doc/html/thread.html
Using boost::thread you'd get something like:
#include <boost/thread.hpp>
void task1() {
// do stuff
}
void task2() {
// do stuff
}
int main (int argc, char ** argv) {
using namespace boost;
thread thread_1 = thread(task1);
thread thread_2 = thread(task2);
// do other stuff
thread_2.join();
thread_1.join();
return 0;
}
When searching for an example of a C++ class that calls one of its own instance methods in a new thread, this question comes up, but we were not able to use any of these answers that way. Here's an example that does that:
Class.h
class DataManager
{
public:
bool hasData;
void getData();
bool dataAvailable();
};
Class.cpp
#include "DataManager.h"
void DataManager::getData()
{
// perform background data munging
hasData = true;
// be sure to notify on the main thread
}
bool DataManager::dataAvailable()
{
if (hasData)
{
return true;
}
else
{
std::thread t(&DataManager::getData, this);
t.detach(); // as opposed to .join, which runs on the current thread
}
}
Note that this example doesn't get into mutex or locking.
Well, technically any such object will wind up being built over a C-style thread library because C++ only just specified a stock std::thread model in c++0x, which was just nailed down and hasn't yet been implemented. The problem is somewhat systemic, technically the existing c++ memory model isn't strict enough to allow for well defined semantics for all of the 'happens before' cases. Hans Boehm wrote an paper on the topic a while back and was instrumental in hammering out the c++0x standard on the topic.
http://www.hpl.hp.com/techreports/2004/HPL-2004-209.html
That said there are several cross-platform thread C++ libraries that work just fine in practice. Intel thread building blocks contains a tbb::thread object that closely approximates the c++0x standard and Boost has a boost::thread library that does the same.
http://www.threadingbuildingblocks.org/
http://www.boost.org/doc/libs/1_37_0/doc/html/thread.html
Using boost::thread you'd get something like:
#include <boost/thread.hpp>
void task1() {
// do stuff
}
void task2() {
// do stuff
}
int main (int argc, char ** argv) {
using namespace boost;
thread thread_1 = thread(task1);
thread thread_2 = thread(task2);
// do other stuff
thread_2.join();
thread_1.join();
return 0;
}
Source: Stackoverflow.com