Re: User-defined assignment

[mcsun!sunic!lth.se!newsuser@uunet.uu.net (Dag Bruck)]

In <comp.lang.ada> stt@spock.camb.inmet.com (Tucker Taft) writes:
>
>Assignment is used implicitly in subprogram calls for by-copy parameters,
>in creating aggregates, in explicit and implicit initialization, on
>function return, in assignment for composite types,
>etc.

C++ makes a very clear distinction between creation and assignment:

	- constructors create an object in a piece of storage
	  with no existing content (except random bits)

	- assignment changes the content of existing well-defined
	  object

This distinction seems to be missing from the current discussion in
comp.lang.ada.  In my view it is a fundamental distinction that also
has practical ramifications for the programmer.  For example, an ADT
which uses shared representation in its implementation may have to
create a copy of the underlying data when a new value is assigned.  To
create a new object with same value, the underlying data may be shared.

Argument passing in C++ is not done by the assignment operator; it is
done by the "copy constructor" which creates a copy of an existing
object.


>Of course, this is not as friendly or flexible as being able
>to define ":=" for any type, and having it used automatically
>everywhere it is appropriate.  However, it is significantly
>easier to implement, .....

Hmmm.  I see this as a syntactic issue.  Why would user-defined ":="
be harder to implement that user-defined "Assign(....)"?  Would it be
hard for the compiler to know what ":" to use?  Or do you mean that it
would be hard for the user to write a ":=" that meets the semantic
demands of all compiler-generated uses of ":="?

In any case, in my view bit-wise ":=" is almost always wrong for
non-trivial data types. 

				-- Dag