TITLE: what is a POD

[ DWP means C++ Draft Working Paper, the pending language 
  standard. You can find it online from the "C++ Working Draft"
  link at http://www.ses.com/~clarke -adc ]


PROBLEM: jhenderson@shivasys.com (Jerry Henderson)

The (4/28/95) DWP is full of references to "POD" and "non-POD" data
types. Finally, on page 121, I found a footnote that explained that
"POD" meant "plain ol' data." I presume this is some sort of geek
humor, the meaning of which eludes me. Can someone please explain what
constitutes "plain ol' data" and what would be the alternative?


RESPONSE: Gregory Bond (gnb@bby.com.au)

From the DWP, sect 9.0.4:

A POD-struct (1) is an aggregate class that has no members of type
reference, pointer to member, non-POD- struct or non-POD-union.

(plus stuff in 9.2.13ff about "layout-compatible POD-structures").


RESPONSE: thp@cs.ucr.edu (Tom Payne)

Conjecture: the distinction between POD and non-POD classes (and
structs and unions) is a hack to the standard to accomodate the view
that references (and member pointers) aren't objects.


RESPONSE: clamage@Eng.Sun.COM (Steve Clamage), 19 Jan 96

Not at all. A POD-struct (or POD-union) is intended to have exactly
the same layout guarantees as the corresponding C struct (or union).
That means it has no features affecting class layout that are not
in C: no virtual functions, no base classes, no access specifiers,
and so on. C doesn't have references or pointers to members, so
neither does a POD-struct (or union).

The reason for bothering to introduce PODs at all is so that a C++
program can access C code containing structs and the programmer
is assured that the C declaration compiled as C++ has the same
meaning.

In addition, C provides a guarantee that different structs with the
same definition have the same layout even if they don't have the
same name. C++ does not provide such a guarantee, but so that C
programs can continue to work, the guarantee is made for PODs.