Using LuaBridge with pointers to built-in types

Question

Let's consider the following C function :

void returnMultipleValuesByPointer(int* ret0, int* ret1, int* ret2)
{
    *ret0 = 0;
    *ret1 = 1;
    *ret2 = 2;
}

How to I expose it in Lua using LuaBridge ? All the documentation and examples illustrate complex cases where the pointed objects are classes but I can't find how to deal with such a simple case...

The trivial approach (ie luabridge::getGlobalNamespace(L).addFunction("test", &returnMultipleValuesByPointer)) compiles but does not understand the values are to be passed by address.

score 2 · Accepted Answer · answered Jan 10 '13 at 19:52

You can't do this directly. You have to write a wrapper method that will convert the parameters into return values. This requires writing such a method. For example, if these are pure return values (ie: they don't take parameters):

int lua_ReturnMultipleValuesByPointer(lua_State *L)
{
  int ret0;
  int ret1;
  int ret2;
  returnMultipleValuesByPointer(&ret0, &ret1, &ret2);
  lua_pushinteger(L, ret0);
  lua_pushinteger(L, ret1);
  lua_pushinteger(L, ret2);
  return 3;
}

You will need to register this with "addCFunction", since it's a standard Lua function.

Thanks, that seems to work fine and it's easy enough to wrap in macros. — docdocdoc9, Jan 15 '13 at 00:26

score 0 · Answer 2 · answered Jan 10 '13 at 09:46

0

If what you want to do is something like:

a=0
b=0
c=0
test(a,b,c)
-- NOW a is 0, b is 1, c is 2

I dont think it's possible. You should revert to a lua call like this:

a,b,c=test()

Now, test can be declared this way:

luabridge::getGlobalNamespace(L).addCFunction("test", &returnMultipleValues)

int returnMultipleValues (lua_State* L) {
    push(L,0);
    push(L,1);
    push(L,2);
    return 3;
}

answered Jan 10 '13 at 09:46

Alar

760
4
11

Where does it call the C function in this case? – Nicol Bolas Jan 10 '13 at 19:30
As sai: a,b,c=test() In LuaBRidge, a CFunction is a function wich implements the low level stack management. SO you push your return value on the stack and return how many value you pushed The "push" function used is a templlatized function you find inside luabridge – Alar Jan 11 '13 at 20:30
I meant where does `returnMultipleValues` call the OP's original function `returnMultipleValuesByPointer`? – Nicol Bolas Jan 11 '13 at 20:50

Using LuaBridge with pointers to built-in types

2 Answers2