In our company we're using a plugin system that I've written using QT plugin system.
Each plugin has "needed functions" and "own functions" so, if a plugin needs a function called *func_1*, there must be a plugin that has a function called *func_1*.
This works great, but now I have a problem: a plugin needs to know the pointer to a certain class object that another plugin uses but that is external. I cannot put any kind of information about this class in the plugins interface as it has nothing to do with it.
The only solution I have, is to write a interface function with which I can pass void pointers, but I'd like to know if there's other solutions maybe less C style.
Thanks in advance.
The modern C++ "replacement" for a void-pointer is something like Boost.Any. It'll allow you to pass anything through the interface, and still gives you a certain type-safety.
Related
How can I find a class to call it's methods by knowing class name and method names?
Details:
I'm trying to write library to replace some functions from another program which is also written using C++ by LD_PRELOAD functionality.
I need to have an ability to call functions from program in which this library gonna be integrated.
C++ loses all class naming information during the compile process!
You can`t do something like:
Class.forName("MyClass");
Like you know it from Java.
http://en.cppreference.com/w/cpp/language/class
This technique is called reflection and its not suported by C++
You can use a Framework (e.g. Boost) to help you out there, but the methods you want to call have to be declared and defined with this Framework.
C++ does not support reflection so you cannot search for functions/classes by name after compile time.
It sounds like if possible the design needs to be reworked. Ideally you should update the library to include what it needs from the programs. Either pull out the common logic from the programs into a third library - or put the functions that needs to be called by the programs into the current library and just pass the relevant data that will be manipulated into the library.
If this is not possible you can pass a function pointer from your programs into your library - this allows the library to have access to the functions it needs without any real knowledge of where it is coming from.
ie
void library_function1(std::function<void(int)> func)
{
func(1);
}
I have looked at multiple sources and I just do not understand them. Mostly all of them are either using a library like luaBind (Which I do not want to use because it relies on Boost) or they are not already instanced objects in C++ but rather created in Lua directly.
I have a Player class, which is wrapped in another class to handle Lua calling. I have created a std::vector list of both these objects in my initialization of the application.
So basically I do not want the Lua script to be creating these player objects I would just like to create a function getPlayer() which then returns the Lua wrapped object. I have no idea where to start with defining the Lua wrapped object for Lua to call nor do I know how to return an instance of the object to Lua so I can use calls from it.
Here's an example of what I would like my Lua script to look like:
player = getPlayer(1) -- Returns the Lua wrapped object from C++ where 1 is the object's index in the std::vector list
print("Player's name: ", player:name()) -- Print's the player's name
Could someone point me to a decent tutorial that explains how to do this. I am pretty sure that this isn't some sort of "Never been done and why would you want to" case so surely there must be at least one tutorial out there. I have been searching for about 2 days now with no positive results.
This isn't a Never been done task, but companies which they use Lua have been already done lots of changes and improvements on the raw source of Lua to make it fit in the place that they want. Since all of us programmers are not that cool to do such stuffs on third-parties sources, we have to use whatever exists out there. You think Crytek is using the exact same version of Lua which is resided on its website's repository? No sir.
You can make this happen by using userdata. Or you can use meta tables. There is also another piece of code named Luna which it helps you to achieve faster.
Otherwise, it's all Luabind, and believe me you're going to end up using it one way or another. Besides, its use of Boost is not that broad and there is going to be just 200kb added to the final executable. Check this address if you want to use Luabind sometime, it's really a good article on the web. http://blog.nuclex-games.com/tutorials/cxx/luabind-introduction/
If you're too bored with Lua, I may suggest you to use squirrel as an alternative to Lua. Personally, I prefer squirrel more than Lua because of its C like syntax and its abilities which they are the ones that we want. Besides, you can even declare constants and use real classes in its context which Lua is pretty failed at this. The funny thing about squirrel is that the designer is a guy that he has been working on FarCry on developing Lua for its engine.
I've been developing for some time. And these beasts appear from time to time in MFC, wxWidgets code, and yet I can't find any info on what they do exactly.
As I understand, they appeared before dynamic_cast was integrated into core C++. And the purpose, is to allow for object creation on the fly, and runtime dynamic casts.
But this is where all the information I found, ends.
I've run into some sample code that uses DECLARE_DYNAMIC_CLASS and IMPLEMENT_DYNAMIC_CLASS within a DLL, and that is used for exported classes. And this structure confuses me.
Why is it done this way? Is that a plugin based approach, where you call LoadLibrary and then call the CreateDynamicClass to get a pointer which can be casted to the needed type?
Does the DECLARE/IMPLEMENT_DYNAMIC work over DLL boundaries? Since even class is not so safe to DLLEXPORT, and here we have a custom RTTI table in addition to existing problems.
Is it possible to derive my class from a DYNAMIC_CLASS from another DLL, how would it work?
Can anyone please explain me what these things are for, or where I can find more than a two sentences on a topic?
This stuff appends addional type information to your class, which allows to RTTI in runtime-independent manner, possibility of having factories to create your classes and many other things. You can find similar approach at COM, QMetaObject, etc
Have you looked at the definitions of DECLARE/IMPLEMENT_DYNAMIC?
In the MS world, all uppercase usually denotes a macro, so you can just look up the definition and try to work out what it's doing from there. If you're in Visual Studio, there's a key you can hit to jump to the definition - see what it says, and look that up and try to work from there.
I was wondering if it is possible to change the logic of an application at runtime? Meybe we could replace the implementation of an abstract class with another implementation? Or maybe we could replace a shared library at runtime...
update: Suppose that I've got two implementations of function foo(x, y) and can use any of them based on strategy pattern. Now I want to know if it's possible to add a third implementation of foo(x, y) without restarting the application.
You can use a plugin (a library that you will load at runtime) that expose a new foo function.
I remember we implemented something similar at school, a calculator in which we could add new operations at runtime, without having to restart the program. See dlsym and dlopen.
Addenda
Be very careful when dlclose-ing a plugin that it is not still used in some active call stack frame. On Linux you can call many thousands of times dlopen (so you could accept not dlclose-ing plugins, with some address space leak).
Exactly, as you said "replace the implementation of an abstract class with another implementation" if by it you mean, you can use runtime polymorphism and change the instances of concrete classes with instances of another set of concrete classes.
More specifically, there is a well-known pattern called Strategy pattern exactly for this purpose. Have a look at the wiki page, as it explains this very nicely, even with a code example along with diagram.
C++ mechanism of virtual functions does not allow you to change the implementation at run-time.
However, you can implement whatever implementation change at runtime with function pointers.
Here is an article on self-modifying code that I read recently: http://mainisusuallyafunction.blogspot.com/2011/11/self-modifying-code-for-debug-tracing.html
I'm writing an NPAPI plugin in C++ on Windows. When my plugin's instantiated, I want to pass it some private data from my main application (specifically, I want to pass it a pointer to a C++ object). There doesn't seem to be a mechanism to do this. Am I missing something? I can't simply create my object in the plugin instance, since it's meant to exist outside of the scope of the plugin instance and persists even when the plugin instance is destroyed.
Edit:
I'm using an embedded plugin in C++ via CEF. This means that my code is essentially the browser and the plugin. Obviously, this isn't the way standard NPAPI plugins behave, so this is probably not something that's supported by NPAPI itself.
You can't pass a C++ object to javascript; what you can do is pass an NPObject that is also a C++ object and exposes things through the NPRuntime interface.
See http://npapi.com/tutorial3 for more information.
You may also want to look at the FireBreath framework, which greatly simplifies things like this.
Edit: it seems I misunderstood your question. What you want is to be able to store data linked to a plugin instance. What you need is the NPP that is given to you when your plugin is created; the NPP has two members, ndata (netscape data) and pdata (plugin data). The pdata pointer is yours to control -- you can set it to point to any arbitrary value that you want, and then cast it back to the real type whenever you want to use it. Be sure to cast it back and delete it on NPP_Destroy, of course. I usually create a struct to keep a few pieces of information in it. FireBreath uses this and sends all plugin calls into a Plugin object instance so that you can act as though it were a normal object.
Relevant code example from FireBreath:
https://github.com/firebreath/FireBreath/blob/master/src/NpapiCore/NpapiPluginModule_NPP.cpp#L145
Pay particular attention to NPP_New and NPP_Destroy; also pay particular attention to how the pdata member of the NPP is used.
This is also discussed in http://npapi.com/tutorial2
There is no way to do this via NPAPI, since the concept doesn't make sense in NPAPI terms. Even if you hack something up that passes a raw pointer around, that assumes everything is running in one process, so if CEF switches to the multi-process approach Chromium is designed around, the hack would break.
You would be better off pretending they are different processes, and using some non-NPAPI method of sharing what you need to between the main application and the plugin.