How are sizes and reserve volumes of slices stored in programming language memory?

In Go, a slice is a data structure that contains three main fields:

1. Pointer to array — address of the first element of the slice in memory.
2. Length (len) — the number of elements available for reading.
3. Capacity (cap) — the maximum number of elements the slice can hold without allocating new memory.

In memory, a slice is roughly represented as:

type SliceHeader struct {
    Data uintptr // pointer to array
    Len  int     // length of the slice
    Cap  int     // capacity of the slice
}

When creating a slice, an array with a certain capacity is allocated. If adding elements exceeds the capacity, a new larger array is allocated, and data is copied there.

The reserve volume (capacity) allows efficient memory management, reducing allocations during growth.

Example:

s := make([]int, 3, 5) // len=3, cap=5
s = append(s, 4)        // len=4, cap=5
s = append(s, 5)        // len=5, cap=5
s = append(s, 6)        // len=6, cap=10 (new memory allocated)

Thus, slice sizes and capacities are stored in the slice structure and managed automatically during runtime.