std::async([](){ std::cout<< "Hello "; }) build error

Question

CppCon 2015: Detlef Vollmann “Executors for C++ - A Long Story ..." starts off with this example:

std::async([](){ std::cout << "Hello "; });
std::async([](){ std::cout << "World!\n"; });

C++ reference shows std::async is in <future> and std::cout is in <iostream>. What is missing to make the build work?

$ cat >hw.cpp <<EOF
> #include <iostream>
> int main(){
>     std::cout << "Hello World!\n";
> }
> EOF
$ clang++ -std=c++14 hw.cpp
$ ./a.out
Hello World!
$ cat >cppcon15.cpp <<EOF
> #include <future>
> #include <iostream>
> int main(){
>     std::async([](){ std::cout << "Hello "; });
>     std::async([](){ std::cout << "World!\n"; });
> }
> EOF
$ clang++ -std=c++14 cppcon15.cpp
/tmp/cppcon15-4f0a58.o: In function `std::thread::thread<std::__future_base::_Async_state_impl<std::_Bind_simple<main::$_1 ()>, void>::_Async_state_impl(std::_Bind_simple<main::$_1 ()>&&)::{lambda()#1}>(std::__future_base::_Async_state_impl<std::_Bind_simple<main::$_1 ()>, void>::_Async_state_impl(std::_Bind_simple<main::$_1 ()>&&)::{lambda()#1}&&)':
cppcon15.cpp:(.text+0x2cf6): undefined reference to `pthread_create'
/tmp/cppcon15-4f0a58.o: In function `std::thread::thread<std::__future_base::_Async_state_impl<std::_Bind_simple<main::$_0 ()>, void>::_Async_state_impl(std::_Bind_simple<main::$_0 ()>&&)::{lambda()#1}>(std::__future_base::_Async_state_impl<std::_Bind_simple<main::$_0 ()>, void>::_Async_state_impl(std::_Bind_simple<main::$_0 ()>&&)::{lambda()#1}&&)':
cppcon15.cpp:(.text+0x6bb6): undefined reference to `pthread_create'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Answer 1

You need to compile with -pthread so that the linker lets you make use of async/future/thread functionality.