FFI methods for userdata objects
- From: Simon Cooke <sjcfwd@xxxxxxxxx>
- To: luajit@xxxxxxxxxxxxx
- Date: Fri, 24 Aug 2012 14:01:33 -0400
I have an existing application that uses the standard Lua API to wrap
some simple C++ classes as full userdata objects. Converting entirely
to an FFI-based interface would be significant work so I'd like
instead to improve the performance of a few critical methods &
metamethods by replacing them with FFI functions, having the userdata
as the first (void*) argument. It seems that this works for a simple
method but not for a metamethod. The following example demonstrates
the problem using a userdata that simply holds a number:
/* test.cpp */
#include "lua.hpp"
extern "C" {
// returns a userdata with an empty metatable
int luaopen_testmodule(lua_State * L)
{
void * ud = lua_newuserdata(L,8);
* reinterpret_cast<double*>(ud) = 123.456;
lua_newtable(L);
lua_setmetatable(L,-2);
return 1;
}
double get_double(void * ud) { return * reinterpret_cast<double*>(ud); }
}
------ test.lua
local ffi = require'ffi'
ffi.cdef[[
double get_double(void *);
]]
clib = ffi.load'test'
ud = require'testmodule'
getmetatable(ud).__index = { get = clib.get_double }
print(ud:get()) -- ok
getmetatable(ud).__call = clib.get_double
print(ud()) -- Error
------ outputs:
123.456
...test.lua:16: attempt to call global 'ud' (a userdata value)
(Tested using latest git version on Linux x64 & v2.0.0-beta10 on Windows x64)
The simple FFI method call to ud:get() works as expected, but the
metamethod __call fails. I encountered similar behaviour with other
metamethods, but with different errors.
I understand that changing metamethods on cdata objects is disallowed,
but that is not the case here.
Is there a way to get this to work for metamethods, or is this
behaviour not supported?
Thanks,
Simon
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