Re: Providing C __eq method via metatable for userdata

[Oliver <ogtifs@...>]

I found the answer in the source code for lua:

function int luaV_equalval (lua_State *L, const TValue *t1, const TValue *t2) in lvm.c

only tries to find the metatable __eq function if the data is FULL userdata. If it is lightuserdata, only the pvalues are compared.

I felt the manual was rather unclear on this.

2011/3/10 Oliver <ogtifs@gmail.com>

Hi all,

I am trying to make Lua a == b call a C function. I have provided a metatable from C following the example described here:

http://lua-users.org/wiki/SimplerCppBinding

To debug I have tried to follow the logic steps described in the manual

http://www.lua.org/manual/5.1/manual.html#2.8

Yet the following script (invoked from lua_pcall after pushing instances onto the stack and setting their metatables):

function testfunc(a, b)

print("Type:")

print(type(a))

print("Metatables equal?")

print(type(a) == type(b))

print(getmetatable(a) == getmetatable(b))

print(getmetatable(a)["__eq"] == getmetatable(b)["__eq"])

eqfunc = getmetatable(a)["__eq"]

print("Calling __eq directly")

print(eqfunc(a, b))

print("Calling via ==")

print(a == b)

end

produces

Type:

userdata

Metatables equal?

true

true

true

Calling __eq directly

*** INSIDE C FUNCTION ***

false

Calling via ==

false

i.e. the == operator does not call the C function

Could anyone please shed some light on this issue?

Thanks

Oliver