with_capacity_by is not expected

[xingzhicn]

I want to create a Binary Heap of fixed length，but the length is not expected.

    // custom-sort heap
    let mut heap1 = BinaryHeap::with_capacity_by(2,|a: &(i32,i32), b: &(i32,i32)| b.1.cmp(&a.1));
    heap1.push((1,5));
    heap1.push((3,2));
    heap1.push((2,1));
    heap1.push((1,1));
    heap1.push((1,0));
    assert_eq!(heap1.pop(), Some((1,0))); // min value
    assert_eq!(heap1.pop(), Some((2,1))); // min value
    assert_eq!(heap1.pop(), Some((1,1))); // min value
    assert_eq!(heap1.pop(), Some((3,2))); // min value

[sekineh]

Hi,

BinaryHeap::with_capacity_by() does not limit the actual length of the heap. It behaves exactly like Vec::with_capacity() which allocate the capacity beforehand to avoid later reallocation.

For more explanation, please read the docs below:

• https://doc.rust-lang.org/std/vec/struct.Vec.html#method.with_capacity
• https://doc.rust-lang.org/std/vec/struct.Vec.html#capacity-and-reallocation
• https://docs.rs/binary-heap-plus/0.2.0/binary_heap_plus/struct.BinaryHeap.html#method.with_capacity_by

  This preallocates enough memory for capacity elements, so that the BinaryHeap does not have to be reallocated until it contains at least that many values.

If you want to limit the actual length of any BinaryHeap or Vec, you need to check .len() before pushing element onto it. I believe you can write a thin wrapper that perform this operation automatically.