Re: [boost] utility class: lazy data type

Subject: Re: [boost] utility class: lazy data type
From: Zachary Turner (divisortheory_at_[hidden])
Date: 2009-01-19 09:38:47

On Mon, Jan 19, 2009 at 8:19 AM, Olaf Peter <ope-devel_at_[hidden]> wrote:

> > template<class T>
> > class lazy
> > {
> > public:
> >     explicit lazy(function<T ()> fetch)
> >         : _fetch(fetch)
> >     {
> >     }
> >
> >     T get()
> >     {
> >         if (!_cached)
> >             _cached = _fetch();
> >         return _cached.get();
> >     }
> >
> > private:
> >     optional<T> _cached;
> >     function<T ()> _fetch;
> > };
>
> I miss a set function; e.g. from parser you get a string which is
> interpreted as a number and used, later changed/rewritten and saved
> (e.g. by use of spririt's Qi & Karma) as string again. Maybe another
> pattern for this use case is appropriate?
>

Hmm, yes I never considered the case of writing the value back out to the
original location it was fetched from.

I think it would be easy to modify the class to support this.  But you would
still want to allow the case where there is no update function.  So probably
add a second constructor:

template<typename T>
explicit lazy(function<T ()> fetch)
//empty_1ary_function is undefined here, but purpose is clear from context.

//simple function returns void, 1 arg of type T, and does nothing
    : _fetch(fetch), _update(empty_1ary_function<void, T>())
{
}

template<typename T>
explicit lazy(function<T ()> fetch, function<void (T)> update)
    : _fetch(fetch), _update(update)
{
}

then add member function update (is the boolean arg here useful?)

void update(T val, bool update_source=true)
{
    if (update_source)
        _update(val);
    _cached = val;
}

Additionally I think a few utility methods method would also be useful.

T force()
{
    _cached = _fetch();
    return _cached.get();
}

bool is_cached() const
{
    return _cached.is_initialized();
}

Also slight bug in the original get method.  It should use
!_cached.is_initialized() instead of !cached.  In case of optional<bool>

Haven't compiled this code to see if it works,b ut I think it should.