Question

I’ve come to a problem using the new c++11 std::thread interface.
I can’t figure out how to pass a reference to a std::ostream to the function that the thread will execute.

Here’s an example with passing an integer(compile and work as expected under gcc 4.6) :

void foo(int &i) {
    /** do something with i **/
    std::cout << i << std::endl;
}

int k = 10;
std::thread t(foo, k);

But when I try passing an ostream it does not compile :

void foo(std::ostream &os) {
    /** do something with os **/
    os << "This should be printed to os" << std::endl;
}

std::thread t(foo, std::cout);

Is there a way to do just that, or is it not possible at all ??

NB: from the compile error it seems to come from a deleted constructor…

Answer 1

Threads copy their arguments (think about it, that’s The Right Thing). If you want a reference explicitly, you have to wrap it with std::ref (or std::cref for constant references):

std::thread t(foo, std::ref(std::cout));

(The reference wrapper is a wrapper with value semantics around a reference. That is, you can copy the wrapper, and all copies will contain the same reference.)

As usual, this code is only correct as long as the object to which you refer remains alive. Caveat emptor.