[boost] event, Was: future library (N2561/Williams version)

From: Peter Dimov (pdimov_at_[hidden])
Date: 2008-05-25 14:24:08

Johan Torp:

> I'm doing a thesis related to C++0x and threading. Peter Dimov suggested I
> could try and implement an event-like waiting primitive, similar to 
> Windows'
> WaitForMultipleObjects. If we had such a primitive, we might get better 
> ways
> to solve this problem. I think part of the reason this is so tricky is
> because condition_variable's aren't meant to be used for multiple waiting.
> Maybe Dimov or somebody else has sketches of such an interface.

I believe that futures only need the basic "manual reset event" in Windows 
terms:

class event
{
private:

    bool is_set_;

public:

    event(); // post: !is_set_

    void set(); // post: is_set_
    void reset(); // post: !is_set_

    void wait(); // block until is_set_
    bool try_wait();
    bool try_wait_for(...);
    bool try_wait_until(...);
};

The advantage of this primitive is that it maps to Windows directly. The 
disadvantage is that it's not robust when reset() is used because this 
introduces a race. This is not a problem for futures since they are one-shot 
and their underlying ready_ event is never reset.

A more robust primitive is the so-called "eventcount" which doesn't need to 
be reset and avoids the race:

class event
{
private:

    unsigned k_;

public:

    event(); // post: k_ == 0

    void signal(); // effects: ++k_

    unsigned get(); // returns: k_
    void wait( unsigned k ); // block until k_ != k

    // ... try_wait ...
};

Typical use is

// waiter

    for( ;; )
    {
        unsigned k = event_.get();
        if( predicate ) return;
        event_.wait( k );
    }

// setter

    predicate = true;
    event_.signal();