delphi - same record helper inheritance from two different sub-types -

having type declaration 2 sub-types deriving same internal type, how can have different record helper each sub-type?

example:

type   singlea = single;   singleb = single;    _singlea = record helper singlea     procedure dothingstoa;     end;    _singleb = record helper singleb     procedure dothingstob;     end; 

if declare var of type singlea, helper type singleb, know normal behaviour if override same internal type, why happen different types?

any appreciated...

thanks in advance.

you not declaring sub-types here. code:

type   singlea = single;   singleb = single; 

is merely declaring aliases, not types. such, singlea , singleb same type single.

this explained here:

type compatibility , identity (delphi)

when 1 type identifier declared using type identifier, without qualification, denote same type. thus, given declarations:

type   t1 = integer;   t2 = t1;   t3 = integer;   t4 = t2; 

t1, t2, t3, t4, , integer denote same type. create distinct types, repeat word type in declaration. example:

type tmyinteger = type integer; 

creates new type called tmyinteger not identical integer.

in effect, = type x construct creates new type info type that
typeinfo(singlea) <> typeinfo(singleb).

in original code, declaring 2 aliases same type single.

for given type (and aliases), can only have 1 type helper in scope, in original code record helper singleb hides record helper singlea.

by upgrading aliases own types, avoid issue:

type   singlea = type single;       singleb = type single;  <<-- type keyword makes singleb distinct singlea 

now have 2 distinct types, , record helpers work expected.