accessing lua class functions in C++

Question

I have come upon a problem with binding C++ and Lua. I have implemented a simple class system in Lua that makes me able to create an "instance" of a lua class from another lua file using

require 'classname'
m_newObj = classname() --"classname() creates a new instance

then I can access functions in m_newObj using

m_newObj:functionname(parameter)

This works perfectly fine, but I want to be able to access an instance of a lua class from C++ code.

Normally you can create access to lua functions in C++ using

lua_State* pL = luaL_newState();
...
lua_getglobal(pL, "functionName");
lua_call(pL,0,0);

But this only calls a function in a luafile, it does not call that specific function on a specific instance of the "class".

So basically what I want to do is

get access to an instance of a lua class in C++
call functions on specific instance

The reason why I want to do this is because I've discovered that in performance it requires a lot more to use C++ functions in lua than using lua functions in C++, So to be able to use lua to extend entities without having the lua code call a lot of C++ functions I need to get access to lua classes in C++ instead of getting access to C++ classes in lua.

"in performance it requires a lot more to use C++ functions in lua than using lua functions in C++" What profiling did you use to determine this? As I understand, marshaling between the two languages works the same way either way. Depending of course on how you attach your C functions to Lua. — Nicol Bolas
– Nicol Bolas, Commented Jan 17, 2013 at 17:55

furq · Accepted Answer · 2013-01-17 19:29:05Z

2

Push your class onto the stack, lua_getfield() the function from it, and then copy your class back onto the top of the stack before calling the function. Something like this:

int nresults = 1;                  // number of results from your Lua function

lua_getglobal(L, "classname");
lua_getfield(L, -1, "funcname");
lua_pushvalue(L, -2);              // push a copy of the class to the top of the stack
lua_call(L, 1, nresults);          // equivalent to classname.funcname(classname)

answered Jan 17, 2013 at 19:29

furq

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1 Comment

leafblight Over a year ago

Sorry for disappearing, this is the answer I was looking for! thanks!

Nicol Bolas · Accepted Answer · 2013-01-17 17:56:47Z

2

m_newObj:functionname(parameter)

This is syntactic sugar for this:

m_newObj.functionname(m_newObj, parameter)

So just do the equivalent of that from your C++ code.

answered Jan 17, 2013 at 17:56

Nicol Bolas

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