TITLE: aggregates, PODs, and constructors

(Newsgroup: comp.std.c++, 3 Feb 99)


ELLIS: sellis@geocities.com (Sean Ellis)

>I've had a poke around in the FAQ, and used Deja news to go through a
>bunch of old posts, and I haven't found the answer to this question
>anywhere:
>
>* Why are constructors prohibited on aggregate types 
> (and therefore on POD types)?


CLAMAGE: Steve Clamage, stephen.clamage@sun.com

You've got it backwards. If a type has a user-defined constructor
or destructor, it is not an aggregate, by definition.

The term aggregate is defined in order specify certain kinds of C
compatibility. The term POD ("Plain Old Data") is defined in order
to specify stricter kinds of C compatibility.

An aggregate is a type that has properties that allow certain
operations to proceed exactly as they do in C. In particular,
you can use brace-initialization.

But if a class has a user-defined constructor, it must be assumed
that the constructor is necessary for initialization, and the compiler
cannot just assign values to data members. Once you reach that
point, the brace-initialization rules break down. In simple cases
they might still apply, but in typical cases the brace-initialization
rules don't work.

Since we are concerned only about C compatibility for aggregates
and PODs, we allow only those C++ features that have no interaction
with C semantics. Constructors don't pass that test.