c - Creating referenced table element -

i created more or less complex table in c. want create reference on lower level of tree. possible?

idea:

elem000 +--> elem010         +--> elem020 +--> elem120         |            +--> **elem121**         |            +--> elem122         +--> elem030 +--> elem130         |            +--> elem131         |            +--> elem132         +--> **elem121** 

the elem121 should visible 1 level above, i.e. reference

i added example of wanted to..

void pushl(lua_state *l, const char * str) { char s[255]; strcpy(s, "elem"); strcat(s, str);   lua_pushstring(l, s);  // key strcpy(s, "value"); strcat(s, str);  lua_pushstring(l, s);  // value lua_settable(l, -3); } void maketable( lua_state *l ) { lua_pushstring(l, "tbl0");  // name of sub-table  lua_createtable(l, 0, 0); lua_checkstack(l, 3); {  pushl(l, "000");  lua_pushstring(l, "tbl1");  lua_createtable(l, 0, 0); lua_checkstack(l, 3); {   pushl(l, "010");   pushl(l, "020");    lua_pushstring(l, "tbl2");    lua_createtable(l, 0, 0);   lua_checkstack(l, 3);   {     pushl(l, "120");     pushl(l, "121");     pushl(l, "122");     lua_settable(l, -3);   }   pushl(l, "030");    lua_pushstring(l, "tbl3");    lua_createtable(l, 0, 0);   lua_checkstack(l, 3);   {     pushl(l, "130");     pushl(l, "131");     pushl(l, "132");     lua_settable(l, -3);   }    lua_settable(l, -3); }  lua_pushstring(l, "elem121"); lua_pushstring(l, "should reference elem121"); lua_settable(l, -3); } lua_setglobal(l,"____mytable"); } 

bottom line: in lua, field variables cannot referenced there ways of doing want.

here comparison between c data structures , lua data structures.

in c, either:

1. have copy of value in 2 places

   you can create copies in lua, too.

2. or, have pointer in 1 place points other place.

   in c, means you'd have access them differently, 1 pointer deference, other without.

   in lua, have function in 1 place returns value in other place. means you'd have access them differently, 1 function call, other without.

the following equivalent read-only pointer:

local elem000 = {    elem010 = "elem010 value",   elem020 = {      elem120 = "elem120 value",     elem121 = "elem121 value",     elem122 = "elem122 value" },   elem030 = {      elem130 = "elem130 value",     elem131 = "elem131 value",     elem132 = "elem132 value" },   elem121 = function(self) return self.elem020.elem121 end }  print(elem000.elem020.elem121) print(elem000:elem121()) elem000.elem020.elem121 = elem000.elem020.elem121 .. " updated"  print(elem000.elem020.elem121) print(elem000:elem121()) 

if need writable pointer approach needed.

---

update

a simple way writable pointer add optional value parameter. commonly used in javascript apis javascript has advantage of undefined data type. in lua, we'll have use nil, means can't write nil value.

elem121 = function(self, value)      if (value ~= nil) self.elem020.elem121 = value end      return self.elem020.elem121      end 

---

for true read-write field access, use __index , __newindex metatmethods. requires field not have key in table. metamethods invoked when indexing non-existing field reading (__index) or writing (__newindex).

local elem000 = {    elem010 = "elem010 value",   elem020 = {      elem120 = "elem120 value",     elem121 = "elem121 value", -- captured initial value     elem122 = "elem122 value"},    elem030 = {      elem130 = "elem130 value",     elem131 = "elem131 value",     elem132 = "elem132 value" },   elem121 = nil -- ignored  }  setmetatable(elem000, {    __index = function(base, key)        if (key=="elem121") return base.elem020.elem121        else return nil end end,   __newindex = function (base, key, value)        if (key=="elem121") base.elem020.elem121 = value        else rawset(base, key, value) end end })  setmetatable(elem000.elem020, {    elem121 = elem000.elem020.elem121, --[[ backing storage field,                                            initialized existing value]]    __index = function(base, key)        if (key=="elem121") return getmetatable(base).elem121        else return nil end end,   __newindex = function (base, key, value)        if (key=="elem121") getmetatable(base).elem121 = value        else rawset(base, key, value) end end })  -- make sure metamethods invoked on these fields   rawset(elem000, "elem121", nil)      rawset(elem000.elem020, "elem121", nil) 

usage:

print(elem000.elem020.elem121) print(elem000.elem121)  elem000.elem020.elem121 = elem000.elem020.elem121 .. " updated"  print(elem000.elem020.elem121) print(elem000.elem121)  elem000.elem121 = elem000.elem121 .. " again"  print(elem000.elem020.elem121) print(elem000.elem121) 

there various places store backing field , different styles of coding metamethods. code perhaps concise. and, leave code in lua c api, if wish.