Template Collections of Different Types

[BBasile via Digitalmars-d-learn]

On Sunday, 16 August 2015 at 01:39:54 UTC, DarthCthulhu wrote:
> Say I want to do something like:
>
> Propertery!int pi = 42;
> PropertyCollection pc;
>
> pc.attach("Life_and_Everything", pi);
>
> assert(pc.Life_and_Everything == 42);
>
> Property!string ps = "Hello World";
>
> pc.attach("text", ps);
>
> assert(pc.text == "Hello World");
>
> How would one store the Property objects in the 
> PropertyCollection? You can't use something like an array 
> because the two Properties are of different types. Do you 
> really need to do something like make a member of 
> PropertyCollection for every type of Property you are 
> interested in storing and using static ifs to determine which 
> variable it goes into?
>
> I feel like there is an obvious solution here that I'm missing.

You would need a kind of type info system:

---
enum Ti {tibyte, tiubyte, ...}
Ti getTypeInfo(T)(T t){statif is(t == ubyte) return Ti.tibyte; 
else...}

pc.attach("Life_and_Everything", pi, getTypeInfo(pi));
pc.attach("BlaBlaBla", pi, getTypeInfo(pi));
pc.attach("zkfozekf", 42, getTypeInfo(42));
---

internally the container would store 3 infos (string, pointer to 
data and a Ti) and when you query a property from its identifier, 
it looks for the attached type info so that you can cast the 
result (let's say it would return a pointer).

---
TypeInfo getTypeInfo(string identifier);
void* getValue(string identifier);
---

These kind of mechanism are really common in languages that have 
a poor/none compile time reflection features, but in D you would 
have to build your own run time type info system. My example is a 
bit shitty but you should get the idea.