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Example Use Case

As an example is often better than lengthy explanations, let's consider the case of a function

int numel(THDoubleTensor *t);

which returns the number of elements of t. Writing a complete wrapper of this function would look like:

static int wrapper_numel(lua_State *L)
{
  THDoubleTensor *t;

  /* always good to check the number of arguments */
  if(lua_gettop(L) != 1)
    error("invalid number of arguments: <tensor> expected");

  /* check if we have a tensor on the stack */
  /* we use the luaT library, which deals with Torch objects */
  /* we assume the torch_DoubleTensor_id has been already initialized */
  t = luaT_checkudata(L, 1, torch_DoubleTensor_id);

  /* push result on stack */
  lua_pushnumber(L, numel(t));

  /* the number of returned variables */
  return 1;
}

For anybody familiar with the Lua C API, this should look very simple (and it is simple, Lua has been designed for that!). Nevertheless, the wrapper contains about 7 lines of C code, for a quite simple function. Writing wrappers for C functions with multiple arguments, where some of them might be optional, can become very quickly a tedious task. The wrap package is here to help the process. Remember however that even though you might be able to treat most complex cases with wrap, sometimes it is also good to do everything by hand yourself!