Old problem with performance

[Christopher Wright]

Rainer Deyke wrote:
>> What mechanism for sharing state is available to by-reference objects
>> but not by-value objects?
> 
> struct C {
>   T t;
> }
> 
> C c1;
> C c2 = c1;
> 
> If T was a reference type, 'c1' and 'c2' now share state, and it's up to
> the programmer to write code to prevent this.  Moreover, the D language
> doesn't even give a hint as to whether T or a reference type or a value
> type.  If T is a template parameter, it could be both.

Okay, reference types don't have value semantics. That's exactly what I 
would expect. Why is it a problem? Because there is no explicit builtin 
copying mechanism for classes? If there were such, would it be a problem 
if copying were not the default?