Lua Metatables for function call trapping

Question

I'm trying to use Lua Metatables to make a prettier interface to some internal C++ functions.

Here's my code that's working so far. (my.get and my.set are implemented in C++)

function setDefault(t)
  local mt = {
  __index = function(tab,k) return my.get(t,k) end,
  __newindex = function(tab,k,v) return my.set(t,k,v) end
  }
  _G[t] = {}
  setmetatable(_G[t],mt)
end

setDefault("LABEL")

LABEL.Text = "wibble" -- evaluates as my.set("LABEL","Text","wibble")
foo = LABEL.Text -- foo = my.get("LABEL","Text")

Fine so far. The next bit I want to make work is function calls on the table, like this:

LABEL.Flash(500) --must evaluate my.execute("LABEL","Flash", 500)

I know this invokes my.get("LABEL","Flash") — and I can make that return a C++ function (using lua_pushcfunction), but when the C++ function is called, it's missing the LABEL and Flash parameters.

Here's the C++ snippet of my.get.

static int myGet(lua_State * l)
{
  std::string p1(luaGetString(l, 1)); // table
  std::string p2(luaGetString(l, 2)); // 'method'

  if (isMethod(p1,p2))
  {
    lua_pushcfunction(l, myExec);
    lua_pushvalue(l, 1); // re-push table
    lua_pushvalue(l, 2); // re-push method
    return 3;
  }
  else
  {
  // do my.get stuff here.
  }
}

score 2 · Answer 1 · edited Aug 12 '15 at 19:47

With a small change, I've got something that works: Push a C closure instead of a C function.

  if (isMethod(p1,p2))
  {
    lua_pushvalue(l, 1); // re-push table
    lua_pushvalue(l, 2); // re-push method
    lua_pushcclosure(l, myExecClosure,2);
    return 1;
  }

myExecClosure is like myExec, but it reads the first two parameters via upvalues (e.g. luaupvaluindex(1)) rather than from stack indexes 1 and 2.

Lua Metatables for function call trapping

1 Answers1