ethertools.
Learn/Dynamic ABI and nested layouts
In this track: Dynamic ABI and nested layouts
advanced · 5 MIN READ

Nested ABI challenge set

Resolve a nested layout and justify every offset base.

LET’S PICTURE IT

A good route explanation says where each direction begins, not just how far to travel.

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.

Challenge one: classify the shape

Consider (uint256,string[]) as one parameter. The array member is dynamic, making the entire tuple dynamic. Predict an outer tuple pointer before inspecting the example.

Challenge two: find the local bases

The tuple body has an integer and an array offset. The array body begins with a count; its string element offsets use the region immediately after that count as their base.

Challenge three: handle an empty member

In the example, the array contains an empty string and x. The empty string tail consumes its length word only. Its neighbor therefore follows after thirty-two bytes, not sixty-four.

Check the reasoning, not just the result

Decode the bytes back to the intended values, then explain each pointer destination in words. If you cannot name a pointer’s base, revisit that layer before moving on to event and error layouts.

Trace a local example

types: (uint256,string[])
values: [["7",["","x"]]]
no function selector prefix

word 0 · argument offset 0 bytes
0x0000000000000000000000000000000000000000000000000000000000000020

word 1 · argument offset 32 bytes
0x0000000000000000000000000000000000000000000000000000000000000007

word 2 · argument offset 64 bytes
0x0000000000000000000000000000000000000000000000000000000000000040

word 3 · argument offset 96 bytes
0x0000000000000000000000000000000000000000000000000000000000000002

word 4 · argument offset 128 bytes
0x0000000000000000000000000000000000000000000000000000000000000040

word 5 · argument offset 160 bytes
0x0000000000000000000000000000000000000000000000000000000000000060

word 6 · argument offset 192 bytes
0x0000000000000000000000000000000000000000000000000000000000000000

word 7 · argument offset 224 bytes
0x0000000000000000000000000000000000000000000000000000000000000001

word 8 · argument offset 256 bytes
0x7800000000000000000000000000000000000000000000000000000000000000

Track the tuple base, array element base, and each string tail separately.

TURN UNDERSTANDING INTO PRACTICEOpen the ABI encoder →
QUICK CHECK · NO PRESSURE

How much tail space does an empty string consume here?

Your progress stays in this browser.
Keep exploring the original referenceSolidity documentation ↗

Original explanations by EtherTools. Examples are educational and source-linked; this site is independent of the Ethereum Foundation. Our approach.