Using aout
– A Concurrency-safe Wrapper for cout
¶
When using cout
from multiple actors, output often appears
interleaved. Moreover, using cout
from multiple actors – and thus
from multiple threads – in parallel should be avoided regardless, since the
standard does not guarantee a thread-safe implementation.
By replacing std::cout
with caf::aout
, actors can achieve a
concurrency-safe text output. The header caf/all.hpp
also defines overloads
for std::endl
and std::flush
for aout
, but does not support the full
range of ostream operations (yet). Each write operation to aout
sends a
message to a “hidden” actor. This actor only prints lines, unless output is
forced using flush
. The example below illustrates printing of lines of text
from multiple actors (in random order).
#include <random>
#include <chrono>
#include <cstdlib>
#include <iostream>
#include "caf/all.hpp"
using namespace caf;
using std::endl;
void caf_main(actor_system& system) {
for (int i = 1; i <= 50; ++i) {
system.spawn([i](blocking_actor* self) {
aout(self) << "Hi there! This is actor nr. "
<< i << "!" << endl;
std::random_device rd;
std::default_random_engine re(rd());
std::chrono::milliseconds tout{re() % 10};
self->delayed_send(self, tout, 42);
self->receive(
[i, self](int) {
aout(self) << "Actor nr. "
<< i << " says goodbye!" << endl;
}
);
});
}
}
CAF_MAIN()