Re: [boost] News about proposed Boost.Application library

On Tue, Jul 29, 2014 at 12:19 PM, Joseph Van Riper < fleeb.fantastique@gmail.com> wrote:

On Tue, Jul 29, 2014 at 5:43 AM, Rob Stewart wrote:

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On July 29, 2014 3:39:16 AM EDT, "Klaim - Joël Lamotte" < mjklaim@gmail.com> wrote:

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I disagree. In my experience it would be more useful if it contained a weak_ptr to the library, with a centralised system keeping all loaded libraries alive and allowing the used to unload the library explicitely. The get_symbol return type would then be testable. That way it is easier to find issues related to plugins and its still possible for the user to prevent these issues at runtime.

Please explain. Using a weak_ptr means each access must be tested, with synchronized access, to boot, versus assurance of validity with the shared_ptr.

I think there are two scenarios. In one, you're dynamically loading the library, and you want it to remain in memory for the duration of the application's run (you never intend to deallocate it until the application is finished). You might consider this scenario for situations where you're licensing different facets of your application, and licensing determines available features.

However, there's another scenario where you dynamically load the library, then deallocate it as soon as you're done with it (and where it is important to do so). For those scenarios, this centralized system of keeping all loaded libraries alive is an unnecessary and unwanted burden. You might consider this scenario in cases where you have a long-running service that you cannot lightly take down, but you want to be capable of making updates to features the service use. At least on Windows, you cannot replace a DLL if it is loaded, so if it's only loaded when in use, you'd be able to swap out the library without stopping the service.

I think both scenarios can be supported by having an object that acts as this centralized system which one may use if desired, while also having the current system.

- Trey

Thanks this is exactly what I meant. I understand the first case but when using dynamically loaded libraries/modules, I more often endup in the second case (in particular when it's "plugins" of a tool or game or simulation engine, that need to be activated and deactivated when necessary). In the first case, you indeed want to make sure that the necessary libraries are always loaded. However in the second, you really need to have a way to be notified when calling something from an unloaded library. In this case, you might just trigger an error, or just do some logging or something. In my own cases I would not test the access returned object, because I assume that that object must not be accessible anymore once the related library is unloaded. Therefore, I consider this case a bug and want the application to crash early if that happen. I was suggesting weak_ptr (wrapped inside something that looks like a std::function<>) so that the user decides if or not he wants to check if the library is still loaded before using the function. But personally I wouldn't. I agree with Trey's conclusion.

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