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POReturn already existing objects from C++ to Lua
 

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    <p>In C++, say I have a class that creates a binary tree like structure and I use it something like this:</p> <pre><code>CTreeRoot* root = new CTreeRoot(/* whatever */); CNode* leftNode = root-&gt;getLeftNode(); CNode* rightNode = root-&gt;getRightNOde(); leftNode-&gt;doSomething(); rightNode-&gt;doSomething(); // etc </code></pre> <p>And assume that the left and right nodes have their own left and right nodes (hence, a binary tree). Now I want to expose this to Lua (<strong>not</strong> using luabind) so I can do the same kind of thing:</p> <pre><code>local root = treeroot.new(/* whatever */) local left = root:getLeftNode() local right = root:getRightNode() left:doSomething(); right:doSomething(); </code></pre> <p>I've gotten most of it to work. However, for the getLeftNode() and getRightNode() methods, I'm pretty sure I'm doing it "wrong". Here is how I'm implementing getLeftNode() in C++ for example:</p> <pre><code>int MyLua::TreeRootGetLeftNode(luaState* L) { CTreeRoot* root = (CTreeRoot*)luaL_checkudata(L, 1, "MyLua.treeroot"); CNode* leftNode = root-&gt;getLeftNode(); if (leftNode != NULL) { int size = sizeof(CNode); // create a new copy of the CNode object inplace of the memory Lua // allocated for us new ((CNode*)lua_newuserdata(L,size)) CNode((const CNode&amp;)*leftNode); lua_setmetatable(L, "MyLua.treenode"); } else { lua_pushnil(L); } return 1; } </code></pre> <p>I cast the userdata back to a CTreeRoot object, call getLeftNode(), make sure it exists and then (here's the "wrong part") I create another userdata data object with a copy constructor copying the object I want to return. </p> <p>Is this "standard practice" for this type of scenario? It seems like you would want to avoid creating another copy of the object since what you really want is just a reference to an already existing object. </p> <p>It sounds like this would be the perfect place for lightuserdata since I don't want to create a new object, I would be happy returning the object that already exists. The problem, though, is that lightuserdata has no meta table so the objects would be useless to me once I got them back. Basically, I want to do something like:</p> <pre><code>int MyLua::TreeRootGetLeftNode(luaState* L) { CTreeRoot* root = (CTreeRoot*)luaL_checkudata(L, 1, "MyLua.treeroot"); CNode* leftNode = root-&gt;getLeftNode(); if (leftNode != NULL) { // "WRONG" CODE BUT SHOWS WHAT I WISH I COULD DO lua_pushlightuserdata(L, (void*)leftNode); lua_setmetatable(L, "MyLua.treenode"); } else { lua_pushnil(L); } return 1; } </code></pre> <p>Can someone please tell me how I can have my <code>MyLua::TreeRootGetLeftNode</code> method return to Lua a copy of the object that already exists in such a way that I can use that object as an 'object' in Lua?</p>
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