Re: proposal for new assignment operators

[Hyman Rosen <hyrosen@mail.com>]

Richard Riehle wrote:
> Limited types are not defined as part of the C++ standard.  One can, of
> course, do as you  have with your example.   I rather doubt it is as common
> as one would find in Ada.

Well, for one thing, the standard input and output streams are
of such a limited type. There are many objects for which assignment
would be meaningless or destructive, and this idiom appears in most
every moderately advanced book about C++.

> Container types, perhaps not those you have created, are often designed
> as limited types.   There are different points-of-view on this.   I recall
> a PhD dissertation from OSU on this subject where the dissertation
> recommended that all types for a container be limited, even the generic
> formal parameters.

But this is not how Ada arrays work, so why should other containers
work that way? In C++, the standard containers all support assignment,
with the semantics of making the target of the assignment compare
equal to the source after the assignment happens (which means that the
two containers will have the same size, and the elements will compare
equal).

> Exactly.  This is one reason for not providing assignment over a limited
> type.   Instead, we typically create a copy procedure and declare the
> name of that copy procedure as specifically as possible.

In C++, that procedure is just called "operator=". We can program
assignment from arbitrary source types if we choose, by using templates.