How to do "inheritance" in D structs

[lobo via Digitalmars-d-learn]

On Wednesday, 12 October 2016 at 02:18:47 UTC, TheFlyingFiddle 
wrote:
> On Wednesday, 12 October 2016 at 01:22:04 UTC, lobo wrote:
>> Hi,
>>
>> I'm coming from C++ and wondered if the pattern below has an 
>> equivalent in D using structs. I could just use classes and 
>> leave it up to the caller to use scoped! as well but I'm not 
>> sure how that will play out when others start using my lib.
>>
>> Thanks,
>> lobo
>>
>>
>> module A;
>>
>> class Base1 {
>>     int ival = 42;
>> }
>> class Base2 {
>>     int ival = 84;
>> }
>>
>> module B;
>>
>> class S(ABase) : ABase {
>>     string sval = "hello";
>> }
>>
>> module C;
>>
>> import A;
>> import B;
>>
>> void main() {
>>     auto s= scoped!(S!Base1); // scoped!(S!Base2)
>> }
>
> You could use "alias this" to simulate that type of inheritence.
>
> module A;
> struct Base1
> {
>     int ival = 42;
> }
>
> module B;
>
> struct Base2
> {
>     int ival = 84;
> }
>
> module C;
> import A, B;
>
> struct S(Base) if(is(Base == struct))
> {
>     Base base;
>     alias base this;
>     string sval = "Hello ";
> }
>
> void foo(ref ABase base)
> {
>     base.ival = 32;
> }
>
> void main()
> {
>     S!Base1 a;
>     S!Base2 b;
>     writeln(a.sval, a.ival);
>     writeln(b.sval, b.ival);
>     foo(a);
>     writeln(a.sval, a.ival);
> }

This approach works nicely although it feels clumsy but that's 
probably just because I'm so used to C++. It also handles private 
members as I'd expect, i.e. they're not accessible outside module 
scope through the alias struct instance, but there is no 
protected. Protected appears to behave the same way as private.

I think I can live with that because I usually try to avoid 
protected anyway.

Thanks,
lobo