How can I get a value back from a boost::thread?

Question

Trying this with boost::thread:

void MyClass::Func(int a, int b, int c, int &r) { r = a + b + c; } void MyClass::Func2(int a, int b, int c) { memberVar = a + b + c; } void MyClass::Work() { int a = 1, b = 2, c = 3; int r; boost::thread_group tg; for(int i = 0; i < 10; ++j) { boost::thread *th = new boost::thread(Func, a, b, c, r); //* error tg.add_thread(th); } tg.join_all(); }

1) I get this error on line //* of which I cannot find the reason:

error: expected primary-expression before ',' token

2) Is a reference parameter (r) a good way to get a value back from a thread? Or should I do like in Func2(), setting a member variable? (taking care of who wrote what)

3) Once I put a thread in a thread_group, how can I get values back from it? I cannot use the original pointer any more...

Thank you.

You cannot. You should use a packaged_task; there's an example in this long-winded answer. — Kerrek SB
– Kerrek SB, Commented Jul 19, 2013 at 18:18
You cannot use a bare member function name to get a pointer-to-member. You have to use &MyClass::Func. Both the class name and the ampersand are mandatory. — n. m. could be an AI
– n. m. could be an AI, Commented Jul 19, 2013 at 18:43

thomas · Accepted Answer · 2013-07-20 11:10:10Z

#include <boost/thread/thread.hpp> #include <boost/bind.hpp> using namespace std; class MyClass{ public: void Func(int a, int b, int c, int &r) { r = a + b + c; } void Work() { using namespace boost; int a = 1, b = 2, c = 3; int r=0; thread_group tg; for(int i = 0; i < 10; ++i){ thread *th = new thread(bind(&MyClass::Func,this,a, b, c, r)); tg.add_thread(th); } tg.join_all(); } }; void main(){ MyClass a; a.Work(); }

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