Jeff Schwab
10/25/2008 9:00:00 AM
Joe, G.I. wrote:
> I want to store 3 events onto a priority_queue and have them execute
> n seconds from the time they were created
See below.
> priority_queue<boost::function<void (Event)> > pq;
>
> // create 3 events and place in priority_queue
> boost::bind(destroy_target, placeholders::_1, 'pq');
Each of those calls binds the first argument of destroy_target to the
multi-character character constant 'pq'. That is probably not what you
want. Use the priority_queue's "push" method to insert the functions.
Note that the new C++ draft standard includes std::function and
std::bind (already available in recent compilers), so you probably don't
need boost for this at all.
> I don't know how
> to pull the Event object off the queue and check f it's timestamp is
> expired
You can use the std::priority_queue's top() method to access the object
(and pop() to remove it from the queue), but where did you store the
timestamp?
> // this is the function I eventually want my event to execute
> void destroy_target() { ... }
That function has no arguments; to what were you binding that placeholder?
Assuming the callback function takes no parameters, here is some naive,
POSIX-specific code that shows how you could implement such a timed
callback mechanism using boost::function:
/* POSIX */
#include <unistd.h>
/* Boost */
#include <boost/function.hpp>
/* C++ Standard */
#include <ctime>
#include <functional>
#include <iostream>
#include <queue>
namespace {
typedef std::time_t time_type;
void sleep(unsigned seconds =1) {
/* POSIX ::sleep(unsigned int) is declared in unistd.h. */
::sleep(seconds);
}
time_type current_time() {
using std::time;
return time(0);
}
struct scheduler {
typedef boost::function<void ()> function;
private:
struct scheduled_function {
time_type when;
function what;
scheduled_function(
time_type time,
function const& func ):
when( time ),
what( func ) { }
};
friend bool operator<(
scheduled_function const& a,
scheduled_function const& b) {
return a.when > b.when;
}
std::priority_queue<scheduled_function> queue_;
public:
void at(time_type when, function const& what ) {
queue_.push(scheduled_function( when, what ));
}
void run() {
for (; !queue_.empty(); sleep()) {
time_type const now = current_time();
while (!queue_.empty() &&
queue_.top().when <= now) {
queue_.top().what();
queue_.pop();
}
}
}
};
}
#include <iostream>
template<int I>
void print() {
std::cout << I << '\n';
}
int main(int argc, char const** argv) {
scheduler sched;
time_type const now = current_time();
sched.at(now + 1, print<1>);
sched.at(now + 2, print<2>);
sched.at(now + 3, print<3>);
sched.run();
return 0;
}