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[DOC] More details about packed encoding.

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chriseth 2018-12-19 17:34:39 +01:00
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docs/abi-spec.rst
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@ -609,7 +609,9 @@ Through ``abi.encodePacked()``, Solidity supports a non-standard packed mode whe
- types shorter than 32 bytes are neither zero padded nor sign extended and
- dynamic types are encoded in-place and without the length.
As an example encoding ``int8, bytes1, uint16, string`` with values ``-1, 0x42, 0x2424, "Hello, world!"`` results in:
This packed mode is mainly used for indexed event parameters.
As an example, the encoding of ``int8(-1), bytes1(0x42), uint16(0x2424), string("Hello, world!")`` results in:
.. code-block:: none
@ -619,12 +621,18 @@ As an example encoding ``int8, bytes1, uint16, string`` with values ``-1, 0x42,
^^^^ uint16(0x2424)
^^^^^^^^^^^^^^^^^^^^^^^^^^ string("Hello, world!") without a length field
More specifically, each statically-sized type takes as many bytes as its range has
and dynamically-sized types like ``string``, ``bytes`` or ``uint[]`` are encoded without
their length field. This means that the encoding is ambiguous as soon as there are two
dynamically-sized elements.
More specifically:
- Each value type takes as many bytes as its range has.
- The encoding of a struct or fixed-size array is the concatenation of the
encoding of its members/elements without any separator or padding.
- Mapping members of structs are ignored as usual.
- Dynamically-sized types like ``string``, ``bytes`` or ``uint[]`` are encoded without
their length field.
In general, the encoding is ambiguous as soon as there are two dynamically-sized elements,
because of the missing length field.
If padding is needed, explicit type conversions can be used: ``abi.encodePacked(uint16(0x12)) == hex"0012"``.
Since packed encoding is not used when calling functions, there is no special support
for prepending a function selector.
for prepending a function selector. Since the encoding is ambiguous, there is no decoding function.