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Learn/Dynamic ABI and nested layouts
In this track: Dynamic ABI and nested layouts
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Nested dynamic array walkthrough

Each dynamic array layer has a count and its own element region.

LET’S PICTURE IT

A list of lists needs directions to each inner list and a count for each one.

SEE IT FOR YOURSELF
Follow a dynamic string from head to tailStep 1 / 4
Follow a dynamic string from head to tailArgument headbyte 0 · offset 32hex word ends in 20String tailstarts at byte 32length + contentPositions are measured from argument byte zero, excluding any selectorOffset destinationargument byte 32read a length wordLength = 5hello is five UTF-8 bytescontent starts at byte 64A length word and an offset word have different jobsContent word · argument byte 6468 65 6c 6c 6f5 payload bytes27 padding bytesBlocks are schematic; their drawn widths do not represent the exact byte ratioHeadoffset: 32argument byte 0Tail lengthlength: 5argument byte 32Tail contenthello + paddingargument byte 6432 + 32 + 32 = 96 bytesWith a four-byte selector, total calldata length is 100 bytes

This example encodes one string parameter with value hello. A dynamic argument uses an offset in its head, even when it is the only argument.

Animated local teaching model. No wallet, RPC or live blockchain data. Some labels and hashes are abbreviated for readability.

Start with the outer array

For uint256[][] holding [1,2] and [3], the top-level head points to an outer length of two. Its two elements are themselves dynamic arrays.

Read outer element pointers

The outer element tuple begins after the outer count. Its two pointer words contain 64 and 160: the first inner array consumes ninety-six bytes.

Read each inner list

At the first destination, length two is followed by integer words 1 and 2. At the second destination, length one is followed by the integer word 3.

Keep counts and offsets distinct

An inner length is a number of elements, while a pointer is a number of bytes from a local base. Identical-looking integer words can have very different structural roles.

Trace a local example

types: uint256[][]
values: [[["1","2"],["3"]]]
no function selector prefix

word 0 · argument offset 0 bytes
0x0000000000000000000000000000000000000000000000000000000000000020

word 1 · argument offset 32 bytes
0x0000000000000000000000000000000000000000000000000000000000000002

word 2 · argument offset 64 bytes
0x0000000000000000000000000000000000000000000000000000000000000040

word 3 · argument offset 96 bytes
0x00000000000000000000000000000000000000000000000000000000000000a0

word 4 · argument offset 128 bytes
0x0000000000000000000000000000000000000000000000000000000000000002

word 5 · argument offset 160 bytes
0x0000000000000000000000000000000000000000000000000000000000000001

word 6 · argument offset 192 bytes
0x0000000000000000000000000000000000000000000000000000000000000002

word 7 · argument offset 224 bytes
0x0000000000000000000000000000000000000000000000000000000000000001

word 8 · argument offset 256 bytes
0x0000000000000000000000000000000000000000000000000000000000000003

The outer element pointers are 64 and 160.

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Why is the second inner offset 160?

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