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Gmane
From: yuri kilochek <yuri.kilochek <at> gmail.com>
Subject: Re: Weak functor: Interested? (How to boost::bind to a weak_ptr)
Newsgroups: gmane.comp.lib.boost.devel
Date: Tuesday 10th January 2012 17:19:56 UTC (over 4 years ago)
>2012/1/10 Vladimír Třebický 
>
> Hi!
>
> I saw people asking how to do boost::bind using a weak pointer.
> Generally it's not possible without first specifying what should
> happen if the weak_ptr cannot be converted to shared_ptr. The
> "weak_fn" I wrote is a small class holding a member pointer, a weak
> pointer and a "policy":
>
> * It is callable so you can insert it to boost::bind.
> * You can construct it using free function templates.
> * The policy says what should happen when the object is called yet the
> weak_ptr cannot be converted to shared_ptr.
> * It's implemented both using variadic templates and Boost.Preprocessor.
>
> Copy of the help comment:
>
> /** Returns a callback that can be used eg. in boost::bind. When called,
it
>  * tries to lock weak_ptr to get a shared_ptr. If successful, it calls
>  * given member function with given arguments. If not successful, it
calls given
>  * policy functor. Built-in policies are:
>  *
>  * "ignore_if_invalid" - does nothing
>  * "throw_if_invalid" - throws "bad_weak_ptr"
>  * "return_default_if_invalid" - returns given value
>  *
>  * Example:
>  *
>  * struct Foo {
>  *     void bar(int i) {
>  *         std::cout << i << std::endl;
>  *     }
>  * };
>  *
>  * struct do_something {
>  *     void operator()() {
>  *         std::cout << "outdated reference" << std::endl;
>  *     }
>  * };
>  *
>  * int main()
>  * {
>  *     boost::shared_ptr sp(new Foo());
>  *     boost::weak_ptr wp(sp);
>  *
>  *     boost::bind(boost::weak_fn(&Foo::bar, wp), _1)(1);
>  *     sp.reset();
>  *     boost::bind(boost::weak_fn(&Foo::bar, wp), 1)();
>  *     boost::bind(boost::weak_fn(&Foo::bar, wp, do_something()),
1)();
>  * }
>  */
>
> What's missing:
>
> * Cannot point to a free function. Only member functions.
> * ?
>
> Wdys?
>
> Vladimir.

I beleive people want to use std::weak_ptr with std::bind when they
want to store a callback, not just passing a functor to an algorithm,
and when people do that, that usually means storingin in an
std::function instance. We already have similar pattern for
shared/weak pointers, so wouldn't it be better to adopt an existing
desing and make weak_fn noncallable, but with the ability to obtain an
std::function instance from it, that would store a strong reference to
the object via shared_ptr?

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