Arena allocation in Zig

Five allocations, zero frees: the arena returns everything in one deinit.

const std = @import("std");

pub fn main() !void {
    // An arena frees everything at once instead of piecemeal.
    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
    defer arena.deinit();
    const alloc = arena.allocator();

    var total: usize = 0;
    for (0..5) |i| {
        const chunk = try alloc.alloc(u8, (i + 1) * 10);
        total += chunk.len;
    }
    std.debug.print("five chunks, {d} bytes, one deinit\n", .{total});
}

How it works

1. ArenaAllocator.init wraps a backing allocator, here the page allocator.
2. The loop allocates five growing chunks and frees none of them.
3. defer arena.deinit() releases the lot at once — the arena's whole trade.

Keywords and builtins used here

• const
• defer
• fn
• for
• pub
• try
• u8
• usize
• var
• void

The run, in numbers

Lines

15

Characters to type

411

Tokens

116

Three-star pace

65 tpm

At the three-star pace of 65 tokens a minute, this run takes about 107 seconds.

Type this snippet

Step 3 of 3 in Memory, step 21 of 27 in Language basics.

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