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std.ArrayList (Zig 0.16.0)

Dynamic array (vector) that grows as needed.

Note: std.ArrayListUnmanaged is deprecated - use std.ArrayList (unmanaged-style API with allocator passed to methods).

Initialization

// Use the supported explicit empty initializer.
var list: std.ArrayList(u32) = .empty;
defer list.deinit(allocator);

// With pre-allocated capacity
var reserved = try std.ArrayList(u32).initCapacity(allocator, 100);
defer reserved.deinit(allocator);

// From existing slice (takes ownership)
var list = std.ArrayList(u32).fromOwnedSlice(existing_slice);

// Fixed buffer: use bounded/no-allocator operations only. Passing an allocator
// to a method on an initBuffer list is illegal.
var buffer: [8]i32 = undefined;
var stack = std.ArrayList(i32).initBuffer(&buffer);

Basic Operations

// Append
try list.append(allocator, 42);
try list.appendSlice(allocator, &[_]u32{1, 2, 3});

// Append without allocation (asserts capacity exists)
list.appendAssumeCapacity(42);
list.appendSliceAssumeCapacity(&[_]u32{1, 2, 3});

// Access items
const items = list.items;  // []T slice
const first = list.items[0];
const last = list.getLast();          // returns T; asserts if empty
const maybe_last = list.getLastOrNull(); // returns ?T
const popped = list.pop();    // returns ?T, removes last

// Insert at index
try list.insert(allocator, 2, value);
try list.insertSlice(allocator, 2, slice);

// Remove
const removed_ordered = list.orderedRemove(index); // O(n), preserves order
const removed_swapped = list.swapRemove(index);    // O(1), changes order

Capacity Management

// Ensure space for N more items
try list.ensureUnusedCapacity(allocator, 10);

// Ensure total capacity is at least N
try list.ensureTotalCapacity(allocator, 100);

// Attempt to shrink capacity. If resizing runs out of memory, the list keeps
// excess capacity while still reducing its logical length.
list.shrinkAndFree(allocator, list.items.len);

// Clear
list.clearRetainingCapacity();  // keeps memory
list.clearAndFree(allocator);   // frees memory

Ownership Transfer

// Get owned slice (empties list, caller owns memory)
const owned = try list.toOwnedSlice(allocator);
defer allocator.free(owned);

// Get null-terminated slice
const z_str = try list.toOwnedSliceSentinel(allocator, 0);
defer allocator.free(z_str);

Iteration

for (list.items) |item| {
    // read-only
}

for (list.items) |*item| {
    item.* += 1;  // modify in place
}

for (list.items, 0..) |item, i| {
    // with index
}

Common Patterns

// Collect from iterator
var list: std.ArrayList(u8) = .empty;
for (some_iterator) |item| {
    try list.append(allocator, item);
}

// Build string
var buf: std.ArrayList(u8) = .empty;
try buf.appendSlice(allocator, "Hello ");
try buf.appendSlice(allocator, name);
const result = try buf.toOwnedSlice(allocator);

// Remove while iterating (iterate backwards)
var i: usize = list.items.len;
while (i > 0) {
    i -= 1;
    if (shouldRemove(list.items[i])) {
        _ = list.swapRemove(i);
    }
}

Reserve-First Pattern (Exception Safety)

When inserting into multiple containers or when partial mutation would corrupt state, use reserve-first: separate fallible reservation from infallible mutation.

// BAD - partial failure leaves invalid state
fn addItem(list: *std.ArrayList(u32), map: *std.AutoHashMapUnmanaged(u32, usize), gpa: Allocator, value: u32) !void {
    try list.append(gpa, value);              // Can fail
    try map.put(gpa, value, list.items.len - 1); // If this fails, list has orphan entry!
}

// GOOD - reserve first, then mutate
fn addItem(list: *std.ArrayList(u32), map: *std.AutoHashMapUnmanaged(u32, usize), gpa: Allocator, value: u32) !void {
    if (map.contains(value)) return error.DuplicateItem;
    const index = list.items.len;

    // Phase 1: Reserve. Logical contents are unchanged, but capacity may grow
    // and existing element pointers may be invalidated.
    try list.ensureUnusedCapacity(gpa, 1);
    try map.ensureUnusedCapacity(gpa, 1);

    errdefer comptime unreachable;  // Phase 2: No errors after this point

    // Phase 3: Mutate (infallible)
    list.appendAssumeCapacity(value);
    map.putAssumeCapacityNoClobber(value, index);
}

Key methods:

ensureUnusedCapacity(gpa, n) - Reserve space for n more items. It can fail and can reallocate, but it does not append or remove logical elements.
appendAssumeCapacity(item) - Append without allocation (cannot fail, asserts capacity)
appendSliceAssumeCapacity(items) - Append slice without allocation

See Reserve-First Exception Safety for detailed explanation and real-world examples.

BoundedArray Replacement

std.BoundedArray was REMOVED in 0.15.x. Use initBuffer instead:

// OLD (removed)
var arr = std.BoundedArray(u8, 64){};

// NEW
var buffer: [64]u8 = undefined;
var arr = std.ArrayList(u8).initBuffer(&buffer);
// Bounded operations report capacity exhaustion.
try arr.appendBounded(value);  // error.OutOfMemory if full
// Assume-capacity operations assert; allocator-taking operations are illegal
// for an initBuffer list.

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