Slices

Slices are objects from type T[] for any given type T. Slices provide a view on a subset of an array of T values - or just point to the whole array. Slices and dynamic arrays are the same.

A slice consists of two members - a pointer to the starting element and the length of the slice:

T* ptr;
size_t length; // unsigned 32 bit on 32bit, unsigned 64 bit on 64bit

Getting a slice via new allocation

If a new dynamic array is created, a slice to this freshly allocated memory is returned:

auto arr = new int[5];
assert(arr.length == 5); // memory referenced in arr.ptr

Actual allocated memory in this case is completely managed by the garbage collector. The returned slice acts as a "view" on underlying elements.

Getting a slice to existing memory

Using a slicing operator one can also get a slice pointing to some already existing memory. The slicing operator can be applied to another slice, static arrays, structs/classes implementing opSlice and a few other entities.

In an example expression origin[start .. end] the slicing operator is used to get a slice of all elements of origin from start to the element before end:

auto newArr = arr[1 .. 4]; // index 4 is NOT included
assert(newArr.length == 3);
newArr[0] = 10; // changes newArr[0] aka arr[1]

Such slices generate a new view on existing memory. They don't create a new copy. If no slice holds a reference to that memory anymore - or a sliced part of it - it will be freed by the garbage collector.

Using slices, it's possible to write very efficient code for things (like parsers, for example) that only operate on one memory block, and slice only the parts they really need to work on. In this way, there's no need to allocate new memory blocks.

As seen in the previous section, the [$] expression is a shorthand form for arr.length. Hence arr[$] indexes the element one past the slice's end, and thus would generate a RangeError (if bounds-checking hasn't been disabled).

In-depth

rdmd playground.d

{{programOutput}}