Re: C++0x, thread, promise and future

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

On 16 July 2011 08:00, Claudio La Rosa wrote:
>
> Hello!
>
> I am making a simple template class for communication with two thread in
> C++0x, using the promise and future.
> My class make a channel to communicate thread with the main program and
> another channel to allow the child thread to initiate communication with the
> parent thread (or main program).
>
> This is the code:
>
>
> #include <iostream>
> #include <future>
> #include <thread>
>
> using namespace std;
>
> template<typename T = std::string,typename T1 = std::string>
> class ThreadChannel
> {
> ? ?private:
> ? ? std::promise<T> ch1_Promise;
> ? ? std::future<T> ?ch1_Future;
>
> ? ? std::promise<T1> ch2_Promise;
> ? ? std::future<T1> ?ch2_Future;
>
> ? ?public:
> ? ? ? ?ThreadChannel();
> ? ? ? ?virtual ~ThreadChannel();
>
> ? ? ? ? T1 recvFromFather(void);
> ? ? ? ? void sendToChild(T1);
> ? ? ? ? //canale 1
> ? ? ? ? void sendToFather(T);
> ? ? ? ? T recvFromChild(void);
>
> };
>
>
> //
> template<typename T,typename T1>
> ThreadChannel<T,T1>::ThreadChannel()
> {
> ?ch1_Future = ch1_Promise.get_future();
> ?ch2_Future = ch2_Promise.get_future();
> }
>
> template<typename T,typename T1>
> ThreadChannel<T,T1>::~ThreadChannel()
> {
> ?//cout<<"Sono nel distruttore!!"<<endl;
> }
>
> template<typename T,typename T1>
> T ThreadChannel<T,T1>::recvFromChild()
> {
> ?return ch1_Future.get();
> }
>
> template<typename T,typename T1>
> void ThreadChannel<T,T1>::sendToFather(T data)
> {
> ?return ch1_Promise.set_value(data);
> }
>
> template<typename T,typename T1>
> void ThreadChannel<T,T1>::sendToChild(T1 data)
> {
> ?return ch2_Promise.set_value(data);
> }
>
> template<typename T,typename T1>
> T1 ThreadChannel<T,T1>::recvFromFather()
> {
> ?return ch2_Future.get();
> }
>
>
> void myAsyncFun(ThreadChannel<int,int>* ch)
> {
> ? ?while(1)
> ? ?{
> ? ? int r = ch->recvFromFather();
> ? ? ch->sendToFather(++r);
> ? ?}
> }
>
> int main()
> {
> ? ?ThreadChannel<int,int>* myChannel=new ThreadChannel<int,int>();
> ? ?// thread
> ? ?std::thread* t = new std::thread(myAsyncFun,myChannel);
> ? ?int myVar = 33;
> ? ?myChannel->sendToChild(myVar);
> ? ?int resp = myChannel->recvFromChild();
> ? ?cout<<"Response from thread: "<<resp<<endl;
>
>
> ? ?t->join();
> ? ?delete myChannel;
>
> ? ?return 0;
> }
>
> The first time the while loop executes (in myAsyncFun), it works well.
> but the second time, when for the second time the statement is executed:
>
> int r = ch-> recvFromFather ();
>
> I have a segmentation fault.

Are you sure you get a segfault?  You should get an exception in the
myAsyncFun thread, which is not caught so the program calls
std::teminate

You can't use a promise more than once, so your loop will always fail
on the second iteration.

Also, as matters of style, you don't need to create the std::thread
using 'new' but if you do then you should delete it.