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clarify multiple inheritance in solidity and its differences against to python

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Simon Chen 2018-08-14 15:20:37 +02:00
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docs/contracts.rst
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@ -842,13 +842,14 @@ Solidity supports multiple inheritance by copying code including polymorphism.
All function calls are virtual, which means that the most derived function All function calls are virtual, which means that the most derived function
is called, except when the contract name is explicitly given. is called, except when the contract name is explicitly given.
When a contract inherits from multiple contracts, only a single When a contract inherits from other contracts, only a single
contract is created on the blockchain, and the code from all the base contracts contract is created on the blockchain, and the code from all the base contracts
is copied into the created contract. is copied into the created contract.
The general inheritance system is very similar to The general inheritance system is very similar to
`Python's <https://docs.python.org/3/tutorial/classes.html#inheritance>`_, `Python's <https://docs.python.org/3/tutorial/classes.html#inheritance>`_,
especially concerning multiple inheritance. especially concerning multiple inheritance, but there are also
some :ref:`differences <multi-inheritance>`.
Details are given in the following example. Details are given in the following example.
@ -1062,6 +1063,8 @@ contracts' constructors, it will be abstract.
.. index:: ! inheritance;multiple, ! linearization, ! C3 linearization .. index:: ! inheritance;multiple, ! linearization, ! C3 linearization
.. _multi-inheritance:
Multiple Inheritance and Linearization Multiple Inheritance and Linearization
====================================== ======================================
@ -1074,7 +1077,13 @@ disallows some inheritance graphs. Especially, the order in
which the base classes are given in the ``is`` directive is which the base classes are given in the ``is`` directive is
important: You have to list the direct base contracts important: You have to list the direct base contracts
in the order from "most base-like" to "most derived". in the order from "most base-like" to "most derived".
Note that this order is different from the one used in Python. Note that this order is the reverse of the one used in Python.
Another simplifying way to explain this is that when a function is called that
is defined multiple times in different contracts, the given bases
are searched from right to left (left to right in Python) in a depth-first manner,
stopping at the first match. If a base contract has already been searched, it is skipped.
In the following code, Solidity will give the In the following code, Solidity will give the
error "Linearization of inheritance graph impossible". error "Linearization of inheritance graph impossible".
@ -1093,6 +1102,8 @@ The reason for this is that ``C`` requests ``X`` to override ``A``
requests to override ``X``, which is a contradiction that requests to override ``X``, which is a contradiction that
cannot be resolved. cannot be resolved.
Inheriting Different Kinds of Members of the Same Name Inheriting Different Kinds of Members of the Same Name
====================================================== ======================================================