Template type reduction

[Engine Machine via Digitalmars-d-learn]

On Tuesday, 16 August 2016 at 17:39:14 UTC, Lodovico Giaretta 
wrote:
> On Monday, 15 August 2016 at 19:31:14 UTC, Engine Machine wrote:
>> Suppose I have a templated type like
>>
>> struct S(T) { int x; static if (T is Y) int y; }
>>
>> I would like to be able to create a reference to S(T) for any 
>> T,
>>
>> struct Q
>> {
>>   S!* s; // Can hold any type of S.
>> }
>>
>> and be able to access s.x, since it is common to all S.
>>
>> Can D do anything like this? It is sort of like runtime 
>> inheritance, but at the compile time level.
>>
>> I do not want to have to cast to S!T every time just to access 
>> x, e.g.,
>>
>> struct Q
>> {
>>    Object s;
>> }
>>
>> which is too general as s can be things that are not of type 
>> S!*.
>>
>>
>> Is this possible?
>
> I don't know if this is exactly what you want:
>
> =====================================
> import std.traits: hasMember;
>
> struct Q(T)
>     if (hasMember!(T, "x"))
> {
>     T s;
>
>     @property auto x() { return s.x; }
> }
> auto makeQ(T)(auto ref T val)
> {
>     return Q!T(val);
> }
>
> auto s = myTypeWithFieldX();
> auto q = makeQ(s);
> assert(q.x == s.x);
> =====================================
>
> `Q` can store any type with an `x` field and gives access to 
> it. The auxiliary function `makeQ` acts as a constructor for 
> `Q` with template parameter deduction.

The only problem is Q is still defined by T. Your makeQ then 
requires the type implicitly, which I don't necessarily have. 
You've simply added complexity to the problem but the issue is 
still there.

I realize now that D simply cannot do what I ask directly because 
It's type parameters are part of the type. It can't be told that 
in some cases they are not. Any use of the parameters, as I am 
using them, will result in this issue.

I'm now trying a different method which builds the based types 
using partial oop and partial CT code. The CT code is only for 
performance and convenience anyways.