Re: append uninitialized elements to array

[Ali Çehreli via Digitalmars-d-learn]

On 07/30/2018 10:40 AM, realhet wrote:

Hello,

I've already found out how to create an array with uninitialized elements, but what I'm looking for is a way to append 16 uninitialized ushorts to it and after I will it directly from 2 SSE registers.

The approximate array length is known at the start so I could be able to do this by making an uninitializedArray and then doing the appending manually, but I wonder if there is a way to do this with array.reserve().

Basically it would be a thing that when this special uninitialized append is happening and when the reserved array size is big enough, it only increments the internal array length effectively.

Thanks

Knowing that the length of a slice is its first member:

void appendUninitialized(T)(ref T[] arr, size_t N = 1) {
    arr.reserve(arr.length + N);
    auto length_p = cast(size_t*)(&arr);
    *length_p += N;
}

unittest {
    ushort[] arr;
    arr.appendUninitialized(2);
    assert(arr.length == 2);
    arr[0] = 1;
    arr[1] = 2;
    assert(arr == [ 1, 2 ]);
}

void main() {
    int[] arr;
    arr.appendUninitialized(100);
    import std.stdio : writeln;
    writeln(arr);
}

Ali