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[DOC] Some changes to functions.

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chriseth 2019-12-05 16:36:01 +01:00
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docs/contracts/functions.rst
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@ -286,7 +286,8 @@ Below you can see an example of a Sink contract that uses function ``receive``.
Fallback Function Fallback Function
================= =================
A contract can have at most one ``fallback`` function, declared using ``fallback () external [payable]``. A contract can have at most one ``fallback`` function, declared using ``fallback () external [payable]``
(without the ``function`` keyword).
This function cannot have arguments, cannot return anything and must have ``external`` visibility. This function cannot have arguments, cannot return anything and must have ``external`` visibility.
It is executed on a call to the contract if none of the other It is executed on a call to the contract if none of the other
functions match the given function signature, or if no data was supplied at functions match the given function signature, or if no data was supplied at
@ -294,19 +295,31 @@ all and there is no :ref:`receive Ether function <receive-ether-function>`.
The fallback function always receives data, but in order to also receive Ether The fallback function always receives data, but in order to also receive Ether
it must be marked ``payable``. it must be marked ``payable``.
In the worst case, if a payable fallback function is also used in place of a receive function, it can only rely on 2300 gas being In the worst case, if a payable fallback function is also used in
available (see :ref:`receive Ether function <receive-ether-function>` for a brief description of the implications of this). place of a receive function, it can only rely on 2300 gas being
available (see :ref:`receive Ether function <receive-ether-function>`
for a brief description of the implications of this).
Like any function, the fallback function can execute complex operations as long as there is enough gas passed on to it. Like any function, the fallback function can execute complex
operations as long as there is enough gas passed on to it.
.. warning:: .. warning::
A ``payable`` fallback function is also executed for plain Ether transfers, if no :ref:`receive Ether function <receive-ether-function>` A ``payable`` fallback function is also executed for
is present. It is recommended to always define a receive Ether function as well, if you define a payable fallback function plain Ether transfers, if no :ref:`receive Ether function <receive-ether-function>`
is present. It is recommended to always define a receive Ether
function as well, if you define a payable fallback function
to distinguish Ether transfers from interface confusions. to distinguish Ether transfers from interface confusions.
.. note:: .. note::
Even though the fallback function cannot have arguments, one can still use ``msg.data`` to retrieve Even though the fallback function cannot have arguments, one can still use ``msg.data`` to retrieve
any payload supplied with the call. any payload supplied with the call.
After having checked the first four bytes of ``msg.data``,
you can use ``abi.decode`` together with the array slice syntax to
decode ABI-encoded data:
``(c, d) = abi.decode(msg.data[4:], (uint256, uint256));``
Note that this should only be used as a last resort and
proper functions should be used instead.
:: ::