Why I cannot get result from boost::compute::function make_function_from_source

`using namespace boost;

using compute::int_;
// get default device and setup context
compute::device device = compute::system::default_device();
compute::context context(device);
compute::command_queue queue(context, device);

// point coordinates
int a[]={2};

// create vector for five points
compute::vector vector(1, context);

// copy point data to the device
compute::copy(
reinterpret_cast(a),
reinterpret_cast(a) + 1,
vector.begin(),
queue
);

compute::vector out(1, context);
boost::compute::function add_four =
boost::compute::make_function_from_source(
"add_four",
"int add_four(int x) { return x + 4; }"
);

boost::compute::transform(vector.begin(), vector.end(), out.begin(), add_four, queue);
cout<