How do I get return values from a boost thread?

Question

I want to do something simple like

void returnVal(int a, int &b)
{
    b = a;
}

int main()
{
    int b = 0;
    boost::thread t(returnVal,1,b);
    t.join();
}

This throws errors. Cannot convert int& to int. There must be a simple way to get return values in boost, and if there is not does anyone have a decent explanation as to why?

Answer 1

boost::thread constructor uses boost::bind . boost::bind takes its arguments by copy default, so

b = a;

modifies copy of b from main. You need to use boost::ref() to pass reference to b into thread :

boost::thread t(returnVal,1,boost::ref(b));