Re: tostring userdata

[Daurnimator <quae@...>]

On Mon, 1 Jul 2019 at 02:42, Andrew Gierth <andrew@tao11.riddles.org.uk> wrote:
>
> >>>>> "Tom" == Tom Sutcliffe <tomsci@me.com> writes:
>
>  Tom> I've tracked this down to the fact that the test - not the
>  Tom> production code :) - was relying on the tostring representation of
>  Tom> a full userdata 0xABCD being the same as the lightuserdata
>  Tom> representation of the same pointer - they were both "userdata:
>  Tom> 0x1234" on 5.3
>
> er... no?
>
> In 5.3, the tostring representation of something that's not a number,
> string, boolean or nil and which has no __tostring metamethod is:
>
>     lua_pushfstring(L, "%s: %p", kind, lua_topointer(L, idx));
>
> where "kind" is the value of the __name metafield if it exists and is a
> string, and is the luaL_typename of the original value otherwise.
>
> I checked my own code and I definitely get "somename: 0xblah" when
> printing full userdatas that have __name metafields.
>
>  Tom> I couldn't find anywhere in the manual that this was mentioned -
>  Tom> in particular __name is mentioned solely in the docs for
>  Tom> luaL_newmetatable which states "The entry __name is used by some
>  Tom> error-reporting functions" - and this change has broadened the
>  Tom> scope of what __name is used for. I realise that the exact
>  Tom> formatting of tostring is not defined in the API but previously
>  Tom> changes like that of tostring(2.0) in 5.3 were at least mentioned
>  Tom> in the "Changes in the Language" section - could we do the same
>  Tom> here?
>
> It's presumably not documented as a change because it didn't actually
> change.
>
> Most likely, what you're seeing is resulting from some other change that
> is causing either the metatable to be (still?) set when it previously
> wasn't, or __name to be set in the metatable when it previously wasn't.
>
> --
> Andrew.
>

IIRC __name in tostring output was introduced between 5.3.3 and 5.3.4.
It was allowed as the output of tostring on objects without a
__tostring has always been undefined.