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Specman E
Interview Questions

WHEN AND LIKE

There are two ways to implement object-oriented inheritance in e: like inheritance and when inheritance

Like

Like inheritance is the classical, single inheritance familiar to users of all object-oriented languages and is specified with the like clause in new struct definitions.

EXAMPLE:
<'

struct exam {
from_exam() is {
  out ( "This is BASE" );
};

};

struct e_exam like exam{
from_exam() is only {
out( "This is EXTENDED" );

};

};

extend sys {
exam : exam;
e_exam : e_exam;
run() is also {
exam.from_exam();
e_exam.from_exam();
};

};
'>

RESULT:

This is BASE
This is EXTENDED

When

When inheritance is a concept unique to e and is specified by defining subtypes with when struct members. When inheritance provides the following advantages compared    to like inheritance:
-Ability to have explicit reference to the when fields
-Ability to have multiple, orthogonal subtypes
-Ability to extend the struct later

EXAMPLE:when
<'
type pack_kind :[long, short];
struct packet {
kind: pack_kind;
when long packet {
a: int;
};
check_my_type() is {
if me is a long packet (l) {
print l;
};
if me is not a long packet {
print kind;
};
};
};
extend sys {
p:packet;
run() is also {
p.check_my_type();
}
};
'>
RESULT

  l = packet-@0: packet
--------------------------------------------------@test
0       kind:                           long
1       long'a:                         2129590818

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