Old problem with performance

[Walter Bright]

Derek Parnell wrote:
> On Sun, 08 Feb 2009 11:13:00 -0800, Walter Bright wrote:
> 
> 
>> In particular, classes are *meant* to be used as reference types, but 
>> the program is trying to treat them as value types. Virtual functions 
>> are orthogonal to what value types are - a continuing problem C++ 
>> programs have is conflating value types with reference types.
> 
> In D, what is the recommend technique to derive one user-defined value type
> from another?

There isn't one. Deriving types only makes sense when expecting 
polymorphic behavior, which is completely orthogonal to the whole idea 
of a value type. A polymorphic type should be a reference type.

A value type can have *aggregation*, however, as in:


struct Sbase
{
     ... base members ...
}

struct S
{
     Sbase base;
     ... more members ...
}

You can also embed interfaces in a struct:

interface I { ... }
struct S
{
     I i;
}