# Cross-Compiler Compatiblity of Plugins

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Hi all, I'm planning on making a game with a component object model. To be able to add objects later on, I want to make all objects plugins. Now I read a really detailed article about plugins: Building Your Own Plugin Framework, by Gigy Sayfan My question is this now: The Binary Compatibility Problem and the fact that there is no standard C++ ABI, no standard how compilers must translate C++ to machine code, creates the problem, that different compilers (such as on Linux or Windows, or even on the same platform), even different compiler versions can create different vtables (virtual function tables) for virtual functions of derived classes (which I need). And this in turn can make plugins compiled with a different compiler than the main program not work (when vtables are created differently). Gigy Sayfan suggests sticking to pure C in plugins to get around this. In fact, one really can "simulate" C++ objects with straight C (read "Pure C"). One simply creates structs with function pointers, functions which accept their structs as first arguments, inheritance with base structs as first data member, etc. Now I've figured out a way how to implement my object model in straight C, however, I would still like to use STL containers such as maps, etc. My question is this: If I create pure C "objects", but these objects contain C++ STL containers such as "map", "vector", etc. will that still create a vtable that can break when the plugin is compiled on a different compiler? Programming my own container types, such as "map", "vector", "list", etc. in straight C seems an awful lot of work. Or should I just go with pure C++ (perhaps that's just easier in the long run) and forget about cross-compiler and cross-platform compatibility? (But then, that seems to negate a big part of what's so cool about plugins, namely that EVERYBODY, no matter what compiler he has, can create and program an add-on...) Mark

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Most people doing something similar to what you are trying to do would write the base application in C++ and then use something like Boost Python to allow third party developers to add their own Python scripts (could be Ruby, Perl or whatever the principle is the same) in order to extend the application.

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Try:

extern "C" {void* CreateSomeObject(){return new SomeObject;}void DeleteSomeObject(void object){delete (SomeObject*)object;}int CallSomeFunction(void* object,int arg1){return ((SomeObject*)object)->SomeFunction(arg1);}// etc...}

In the Plugin:
void* object = CreateSomeObject();CallSomeFunction(object,10);DeleteSomeObject(object);

You'll have to keep all your containers on their end of the application but as long as the object is obscured in this way, you should be able to have anything you like in it... except having one of your extern "C" functions return anything without C linkage.

you can typedef the void pointer to something like SomeObjectHandle to make it more meaningful.

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Quote:
 My question is this: If I create pure C "objects", but these objects contain C++ STL containers such as "map", "vector", etc. will that still create a vtable that can break when the plugin is compiled on a different compiler?

No, just having more fields won't touch the vtable for that object. So you could safely access the other fields in that object, but the foreign binary probably won't be able to safely access the contents of the map/vector/etc.

I think the best strategy (which the Building Your Own Plugin Framework article mentions at the end), is to pass around opaque pointers to C++ objects, and provide pure C accessors for those objects. You can even go further and write a shim C++ class that wraps around the C accessors, if you want to.

Edit: Yeah, just like Kwizatz said. Then if you wanted to put a C++ shim on top of that, it would look like this:

class MyShimClass{public:    MyShimClass() {        _data = CreateSomeObject();    }    ~MyShimClass() {        DeleteSomeObject(_data);    }    void callSomeFunction(int i) {        CallSomeFunction(_data, i);    }private:    void* _data;};

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Quote:
 Original post by MarkieOr should I just go with pure C++ (perhaps that's just easier in the long run) and forget about cross-compiler and cross-platform compatibility?

This.

Quote:
 (But then, that seems to negate a big part of what's so cool about plugins, namely that EVERYBODY, no matter what compiler he has, can create and program an add-on...)

Who cares, be pragmatic. Don't make your life miserable ("C objects"...) just because of a few who don't use the main compiler for a given platform. Just tell them to use MSVC on Windows and gcc on Linux or OSX. Problem solved. A lot of professional software does it this way.

Quote:
 Original post by CromulentMost people doing something similar to what you are trying to do would write the base application in C++ and then use something like Boost Python to allow third party developers to add their own Python scripts (could be Ruby, Perl or whatever the principle is the same) in order to extend the application.

That is not a plugin system then, but a scripting system. A plugin based application relies on a multi-component concept, where each component fully interfaces with the system (and possibly with each other) at a low level, while being compiled in the same language as the main system to ensure maximum performance. Think 3DSMax, for example, which is a very heavily plugin based system (the entire application is built by plugins around a basic skeleton framework). Imagine a plugin such as VRay (global illumination renderer) written in Python...

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Quote:
Original post by Yann L
Quote:
 Original post by MarkieOr should I just go with pure C++ (perhaps that's just easier in the long run) and forget about cross-compiler and cross-platform compatibility?

This.

Quote:
 (But then, that seems to negate a big part of what's so cool about plugins, namely that EVERYBODY, no matter what compiler he has, can create and program an add-on...)

Who cares, be pragmatic. Don't make your life miserable ("C objects"...) just because of a few who don't use the main compiler for a given platform. Just tell them to use MSVC on Windows and gcc on Linux or OSX. Problem solved. A lot of professional software does it this way.

Well, the problem can go deeper than that. For example, a standard class, such as std::string has been different between versions of the same compiler and different for debug and release builds, which causes problems. Namely, it can blow up if created in a module that "sees" it one way and then if used or deleted in the other. It's very easy to violate the one-definition rule across DLL boundaries. Of course, there's solutions for this if you're always using a version built with the same settings and libraries.

I wonder if there's a library that does kind of what SWIG does, except using a C interface to bridge C++ DLLs instead of using the C interface to bridge between languages.

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 Original post by RydinareWell, the problem can go deeper than that. For example, a standard class, such as std::string has been different between versions of the same compiler and different for debug and release builds, which causes problems. Namely, it can blow up if created in a module that "sees" it one way and then if used or deleted in the other. It's very easy to violate the one-definition rule across DLL boundaries. Of course, there's solutions for this if you're always using a version built with the same settings and libraries.

Well, it depends. The main problem is DLL-boundary memory and resource allocation. But this can be solved in many different ways. Also, under Windows at least, each DLL is a separately compiled unit. Internal symbols are resolved at compile time, and only symbols that are explicitly exported are runtime linked. This is different under Linux, btw, which will always runtime bind everything, even internal dependencies. This is a major pain in the ass when developing plugin systems for Linux or OSX.

Anyway, if you're careful, then the CRT versions don't have to match 100%. You can, for example, debug a plugin (compiled as debug, linked with the debug CRT) while it is running as a host on a release application (compiled as release, linked to the release CRT).

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It's always a pain having to remember which version of VS (including service pack!) I need to use to develop for each version of Max/Maya/Gamebryo/whatever.

But it is TEN TIMES as much pain to deal with certain middleware packages which have, for ABI-agnostic shits and giggles, reimplemented virtual function binding and discarded traditional RAII and standard library classes.

ABI problems are annoying, but then you get them right and they stay right. The workarounds are infinitely worse.

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Quote:
Original post by Yann L
Quote:
 Original post by RydinareWell, the problem can go deeper than that. For example, a standard class, such as std::string has been different between versions of the same compiler and different for debug and release builds, which causes problems. Namely, it can blow up if created in a module that "sees" it one way and then if used or deleted in the other. It's very easy to violate the one-definition rule across DLL boundaries. Of course, there's solutions for this if you're always using a version built with the same settings and libraries.

Well, it depends. The main problem is DLL-boundary memory and resource allocation. But this can be solved in many different ways. Also, under Windows at least, each DLL is a separately compiled unit. Internal symbols are resolved at compile time, and only symbols that are explicitly exported are runtime linked. This is different under Linux, btw, which will always runtime bind everything, even internal dependencies. This is a major pain in the ass when developing plugin systems for Linux or OSX.

Anyway, if you're careful, then the CRT versions don't have to match 100%. You can, for example, debug a plugin (compiled as debug, linked with the debug CRT) while it is running as a host on a release application (compiled as release, linked to the release CRT).

Good points. I was referring to a specific case, in the case of STL containers, starting with VS 2005, there is extra storage used for debugging purposes, so if you're using STL, you can no longer mix debug and release builds even from the same compiler version, which is obnoxious.

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Quote:
 Original post by SneftelIt's always a pain having to remember which version of VS (including service pack!) I need to use to develop for each version of Max/Maya/Gamebryo/whatever.But it is TEN TIMES as much pain to deal with certain middleware packages which have, for ABI-agnostic shits and giggles, reimplemented virtual function binding and discarded traditional RAII and standard library classes.ABI problems are annoying, but then you get them right and they stay right. The workarounds are infinitely worse.

No arguments here. It's why I dislike the use of COM in C++, unless it's utterly necessary, because most C++ paradigms are broken in COM.

But, that being said, COM had a broader purpose. If you simply want to wrap custom classes with a C-API underneath and serialize the rest, I think it can be ok.

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