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One seemingly stupid question to which I've been unable to find
definitive answer in the docs.

In lua_tostring() docs last paragrah reads "Because Lua has garbage
collection, there is no guarantee that the pointer returned by
lua_tolstring will be valid after the corresponding Lua value is
removed from the stack. ".

Must it be kept in the stack or is it enoguh if I keep the original
value referenced. I.e., I'm inside a method of one of my classes ( so
I have a this pointer ) and I've arrived there from a C function which
has the relevant userdata as first argument, index 1, and I have a
string at the TOS, if I do:

const char * s = lua_tostring(L,-1,0);

and then take it off the stack but keep it by doing

lua_setuservalue(L, 1);

or alternatively:

lua_rawsetp(L, LUA_REGISTRYINDEX, this);

Can I still keep using s safely?

And also,  can I keep using s if I guarantee nobody has messed with
the uservalue/registry?

It seems likely, and it is true on userdata, but the wording on
lua_tostring make me fear the string can me moved around by the
collector if it is not pinned especifically by a stack entry.

Francisco Olarte.