Return the SV from the GV.

Prior to Perl v5.9.3, this would add a scalar if none existed. Nowadays, use "GvSVn" for that, or compile perl with -DPERL_CREATE_GVSV. See perl5100delta.

Return the AV from the GV.

Return the HV from the GV.

Return the CV from the GV.

Make sure there is a slot of the given type (AV, HV, IO, SV) in the GV gv.