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POBinding C++ methods and variables to a table, side by side
 

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    <p>I've been attempting to create a "host" application that exposes a C++ API to Lua code, and have been fairly successful so far, but have run into snags when attempting to bind "variables" in with the methods.</p> <p>The pattern I have developed to bind "classes" to lua involves each class having a <code>_new</code> and <code>_gc</code> function, as well as a static <code>luaL_Reg RegData[]</code> for each class. I can then assign the desired functions into the RegData array, and call a helper method to bind them to lua. The following is some code illustrating my method:</p> <pre class="lang-c prettyprint-override"><code>int Host::_new(lua_State * ls) { Host ** udata = (Host **)lua_newuserdata(ls, sizeof(Host *); *udata = new Host(); luaL_getmetatable(ls, "luaL_Host); lua_setmetatable(ls, -2); return 1; } int Host::_gc(lua_State * ls) { Host * host = *(Host **)luaL_checkudata(ls, 1, "luaL_Host"); delete host; return 0; } const luaL_Reg Host::RegistrationData[] = { { "new" , Host::_new }, { "__gc" , Host::_gc }, { 0 , 0 } }; </code></pre> <p>And elsewhere:</p> <pre class="lang-c prettyprint-override"><code>void LuaState::registerObject(const char * name, const luaL_Reg data[]) { int len = strlen(name) + 6; char * luaLname = new char[len]; snprintf(luaLname, len, "luaL_%s", name); // create the metatable with the proper functions luaL_newmetatable(_state, luaLname); luaL_setfuncs(_state, data, 0); // copy the metatable on the stack lua_pushvalue(_state, -1); // assign the index to the copy lua_setfield(_state, -1, "__index"); // expose the table as the global "Host" lua_setglobal(_state, name); delete luaLname; } </code></pre> <p>Lets say that I want my Lua code to also be able to view keyboard state as well, and would like the Lua code to access it to look like:</p> <pre class="lang-lua prettyprint-override"><code>host = Host.new() pressed = host.Keyboard.getKeyPressed(1) </code></pre> <p>It becomes easy enough to copy the exact same pattern into the Keyboard class to setup the keyboard table, but I can't seem to come up with a good way to add that Keyboard table to my Host table. Is there a simple way to do so without messing up my pattern? Is there a better pattern that I should be using?</p> <p>I've found plenty of resources on how to create various different aspects of C-Lua APIs, but haven't really found much in terms of patterns/best practices to use while doing so. Also, I know there are libraries, such as LuaBind, that can help with this, but prefer to make my own (at least the first time).</p>
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    1. USChris Covert
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    1. COYou should probably use something like luabind - it doesn't require anything lua-specific for C++ classes, and you'd probably actually end up with less code. The one unfortunate thing about it though is that it requires boost (and that I can't find a maintained version of it).
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    2. COI "should" use one maybe, but as I said above, I'm trying not to. This is supposed to be a bit of a learning experience if nothing else. I don't intend to reinvent the wheel forever, but at least until I know how it works. :)
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      1. USChris Covert
    3. COwell, you don't actually have to use third party libraries if you don't want to, but you could at least look at their approach. If you have a decent subset of C++11 features available, you can write something similar (in a basic way, they do seem to do some more complicated things that I didn't really understand) on your own with relatively little effort, which seems more manageable than what you're doing right now to me.
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