Chaining opIndex

deed via Digitalmars-d-learn digitalmars-d-learn at puremagic.com
Mon May 9 13:14:25 PDT 2016


struct Foo {
     Bars bars;
     ...
}

struct Foos {
     Foo[] arr;
     Foo opIndex (size_t idx) { return arr[idx]; }
     ...
}

struct Bar {
     // No Car[] cars;
     ...
}

struct Bars {
     Bar[] arr;
     Bar opIndex (size_t idx) { return arr[idx]; }
     ...
}

struct Car {
     ...
}

Foos foos;
Foo foo = foos[1];                  // Works
Bar bar = foos[1].bars[2];          // Works
Car car = foos[1].bars[2].cars[3];  // Desired abstraction.

For any Bar there are some Cars, but Bar doesn't hold any Cars. 
In other words, there could be a function Car cars (Bar bar, 
size_t idx) { ... }, but that would be called with parens;

Car car = foos[i].bars[j].cars(k);

which would be inconsistent and confusing. Defining

struct Cars {
     Car opIndex (Bar bar, size_t idx) {}
}

and

struct Bar {
     Cars cars;
     ...
}

doesn't enable chaining and then would have to be used like this, 
AFAIK:

Car car = cars[foos[i].bars[j], k];

Which is out of the question. Any suggestions to achieve the 
desired abstraction in a clean manner?



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