lua metatable registration from c question -

hello have following bit of code seems work, i'm not sure why - i've built testclass follows

class testclass {     int ivalue; public:     int getivalue();     void setivalue(int &v); }; 

and registered testclass (bits left out actual functions they're pretty basic). it's registration of metatables i'm not following. (etivalue , setivalue c functions call class functions of same name)

static const struct lual_reg arraylib_f [] = {     {"new", new_testclass},     {null, null} };  static const struct lual_reg arraylib_m [] = {     {"set", setivalue},     {"get", getivalue},     {null, null} };  int luaopen_testclass (lua_state *l) {     lual_newmetatable(l, "luabook.testclass");     lua_pushvalue(l, -1); /* duplicates metatable */     lua_setfield(l, -2, "__index");     lual_register(l, null, arraylib_m);     lual_register(l, "testclass", arraylib_f);     return 1; } 

the bit don't understand i'm adding functions __index metatable when run

a = testclass.new() a:set(10) print(a:get()) 

then works expected. bit don't understand why set being called when think i've loaded in __index metatable? i've done or else?

tia

int luaopen_testclass (lua_state *l) {     lual_newmetatable(l, "luabook.testclass"); //leaves new metatable on stack     lua_pushvalue(l, -1); // there 2 'copies' of metatable on stack     lua_setfield(l, -2, "__index"); // pop 1 of copies , assign                                     // __index field od 1st metatable     lual_register(l, null, arraylib_m); // register functions in metatable     lual_register(l, "testclass", arraylib_f);     return 1; } 

that code equivalent example lua code:

metatable = {} metatable.__index = metatable metatable.set = function() --[[ stuff --]] end metatable.get = function() --[[ stuff --]] end 

i assume 'new_testclass' c function sets metatable "luabook.testclass" returned table.

in code dont add functions metatable __index field. assign pointer metatable metatable's field named __index, , register set , functions it.

now, if set metatable value returned 'new_testclass' function (which assume do) - lets call value 'foo', , call foo:set(10), lua:

1. checks there no such field 'set' in 'foo'
2. sees 'foo' has metatable
3. looks @ metatable's __index field - sees it's table
4. checks if table assigned __index field has field 'set' , it's value function
5. calls 'set' method passing 'foo' self parameter

i hope figure out whats going on here.