Slices and Pointers

A slice's header exposes its data pointer and length directly, which is what makes low-level and foreign-code access possible without copying.

Reaching the Backing Memory

A slice's data field is a regular *T pointer. You can pass it to extern functions or perform pointer arithmetic on it when low-level access is needed:

extern func memcpy(dst: *opaque, src: *const opaque, n: uint) -> *opaque;

let src = [1, 2, 3, 4];
let dst: uint8[4];
memcpy(dst.data, src.data, src.length);

Passing data hands the callee the address of the first element; passing length tells it how many elements are valid. The two are always used together — a pointer without its length cannot be bounded safely.

sizeof and Element Size

sizeof(T) returns the size of a single element in bytes. Multiply by length to get the total byte size of the backing buffer — for example, when telling a C API how many bytes to read or copy:

let buf: float64[8];
let byteSize = sizeof(float64) * buf.length;   // 64

Note that length counts elements, not bytes; only after multiplying by sizeof(T) do you get a byte count.

See Also

• Slices — the slice overview and memory layout
• Pointer Arithmetic — advancing a pointer by element size
• Pointers and extern — exchanging memory with native code