mirror of
https://github.com/ethereum/solidity
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300 lines
10 KiB
ReStructuredText
300 lines
10 KiB
ReStructuredText
.. index:: ! inheritance, ! base class, ! contract;base, ! deriving
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***********
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Inheritance
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***********
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Solidity supports multiple inheritance including polymorphism.
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All function calls are virtual, which means that the most derived function
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is called, except when the contract name is explicitly given or the
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``super`` keyword is used.
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When a contract inherits from other contracts, only a single
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contract is created on the blockchain, and the code from all the base contracts
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is compiled into the created contract.
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The general inheritance system is very similar to
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`Python's <https://docs.python.org/3/tutorial/classes.html#inheritance>`_,
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especially concerning multiple inheritance, but there are also
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some :ref:`differences <multi-inheritance>`.
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Details are given in the following example.
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::
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pragma solidity ^0.5.0;
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contract owned {
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constructor() public { owner = msg.sender; }
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address payable owner;
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}
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// Use `is` to derive from another contract. Derived
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// contracts can access all non-private members including
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// internal functions and state variables. These cannot be
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// accessed externally via `this`, though.
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contract mortal is owned {
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function kill() public {
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if (msg.sender == owner) selfdestruct(owner);
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}
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}
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// These abstract contracts are only provided to make the
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// interface known to the compiler. Note the function
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// without body. If a contract does not implement all
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// functions it can only be used as an interface.
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contract Config {
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function lookup(uint id) public returns (address adr);
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}
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contract NameReg {
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function register(bytes32 name) public;
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function unregister() public;
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}
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// Multiple inheritance is possible. Note that `owned` is
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// also a base class of `mortal`, yet there is only a single
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// instance of `owned` (as for virtual inheritance in C++).
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contract named is owned, mortal {
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constructor(bytes32 name) public {
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Config config = Config(0xD5f9D8D94886E70b06E474c3fB14Fd43E2f23970);
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NameReg(config.lookup(1)).register(name);
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}
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// Functions can be overridden by another function with the same name and
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// the same number/types of inputs. If the overriding function has different
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// types of output parameters, that causes an error.
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// Both local and message-based function calls take these overrides
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// into account.
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function kill() public {
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if (msg.sender == owner) {
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Config config = Config(0xD5f9D8D94886E70b06E474c3fB14Fd43E2f23970);
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NameReg(config.lookup(1)).unregister();
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// It is still possible to call a specific
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// overridden function.
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mortal.kill();
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}
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}
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}
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// If a constructor takes an argument, it needs to be
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// provided in the header (or modifier-invocation-style at
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// the constructor of the derived contract (see below)).
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contract PriceFeed is owned, mortal, named("GoldFeed") {
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function updateInfo(uint newInfo) public {
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if (msg.sender == owner) info = newInfo;
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}
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function get() public view returns(uint r) { return info; }
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uint info;
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}
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Note that above, we call ``mortal.kill()`` to "forward" the
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destruction request. The way this is done is problematic, as
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seen in the following example::
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pragma solidity >=0.4.22 <0.6.0;
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contract owned {
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constructor() public { owner = msg.sender; }
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address payable owner;
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}
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contract mortal is owned {
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function kill() public {
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if (msg.sender == owner) selfdestruct(owner);
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}
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}
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contract Base1 is mortal {
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function kill() public { /* do cleanup 1 */ mortal.kill(); }
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}
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contract Base2 is mortal {
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function kill() public { /* do cleanup 2 */ mortal.kill(); }
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}
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contract Final is Base1, Base2 {
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}
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A call to ``Final.kill()`` will call ``Base2.kill`` as the most
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derived override, but this function will bypass
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``Base1.kill``, basically because it does not even know about
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``Base1``. The way around this is to use ``super``::
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pragma solidity >=0.4.22 <0.6.0;
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contract owned {
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constructor() public { owner = msg.sender; }
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address payable owner;
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}
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contract mortal is owned {
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function kill() public {
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if (msg.sender == owner) selfdestruct(owner);
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}
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}
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contract Base1 is mortal {
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function kill() public { /* do cleanup 1 */ super.kill(); }
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}
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contract Base2 is mortal {
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function kill() public { /* do cleanup 2 */ super.kill(); }
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}
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contract Final is Base1, Base2 {
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}
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If ``Base2`` calls a function of ``super``, it does not simply
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call this function on one of its base contracts. Rather, it
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calls this function on the next base contract in the final
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inheritance graph, so it will call ``Base1.kill()`` (note that
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the final inheritance sequence is -- starting with the most
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derived contract: Final, Base2, Base1, mortal, owned).
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The actual function that is called when using super is
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not known in the context of the class where it is used,
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although its type is known. This is similar for ordinary
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virtual method lookup.
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.. index:: ! constructor
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.. _constructor:
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Constructors
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============
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A constructor is an optional function declared with the ``constructor`` keyword
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which is executed upon contract creation, and where you can run contract
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initialisation code.
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Before the constructor code is executed, state variables are initialised to
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their specified value if you initialise them inline, or zero if you do not.
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After the constructor has run, the final code of the contract is deployed
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to the blockchain. The deployment of
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the code costs additional gas linear to the length of the code.
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This code includes all functions that are part of the public interface
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and all functions that are reachable from there through function calls.
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It does not include the constructor code or internal functions that are
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only called from the constructor.
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Constructor functions can be either ``public`` or ``internal``. If there is no
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constructor, the contract will assume the default constructor, which is
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equivalent to ``constructor() public {}``. For example:
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::
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pragma solidity ^0.5.0;
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contract A {
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uint public a;
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constructor(uint _a) internal {
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a = _a;
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}
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}
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contract B is A(1) {
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constructor() public {}
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}
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A constructor set as ``internal`` causes the contract to be marked as :ref:`abstract <abstract-contract>`.
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.. warning ::
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Prior to version 0.4.22, constructors were defined as functions with the same name as the contract.
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This syntax was deprecated and is not allowed anymore in version 0.5.0.
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.. index:: ! base;constructor
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Arguments for Base Constructors
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===============================
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The constructors of all the base contracts will be called following the
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linearization rules explained below. If the base constructors have arguments,
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derived contracts need to specify all of them. This can be done in two ways::
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pragma solidity >=0.4.22 <0.6.0;
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contract Base {
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uint x;
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constructor(uint _x) public { x = _x; }
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}
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// Either directly specify in the inheritance list...
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contract Derived1 is Base(7) {
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constructor() public {}
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}
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// or through a "modifier" of the derived constructor.
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contract Derived2 is Base {
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constructor(uint _y) Base(_y * _y) public {}
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}
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One way is directly in the inheritance list (``is Base(7)``). The other is in
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the way a modifier is invoked as part of
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the derived constructor (``Base(_y * _y)``). The first way to
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do it is more convenient if the constructor argument is a
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constant and defines the behaviour of the contract or
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describes it. The second way has to be used if the
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constructor arguments of the base depend on those of the
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derived contract. Arguments have to be given either in the
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inheritance list or in modifier-style in the derived constructor.
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Specifying arguments in both places is an error.
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If a derived contract does not specify the arguments to all of its base
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contracts' constructors, it will be abstract.
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.. index:: ! inheritance;multiple, ! linearization, ! C3 linearization
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.. _multi-inheritance:
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Multiple Inheritance and Linearization
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======================================
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Languages that allow multiple inheritance have to deal with
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several problems. One is the `Diamond Problem <https://en.wikipedia.org/wiki/Multiple_inheritance#The_diamond_problem>`_.
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Solidity is similar to Python in that it uses "`C3 Linearization <https://en.wikipedia.org/wiki/C3_linearization>`_"
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to force a specific order in the directed acyclic graph (DAG) of base classes. This
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results in the desirable property of monotonicity but
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disallows some inheritance graphs. Especially, the order in
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which the base classes are given in the ``is`` directive is
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important: You have to list the direct base contracts
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in the order from "most base-like" to "most derived".
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Note that this order is the reverse of the one used in Python.
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Another simplifying way to explain this is that when a function is called that
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is defined multiple times in different contracts, the given bases
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are searched from right to left (left to right in Python) in a depth-first manner,
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stopping at the first match. If a base contract has already been searched, it is skipped.
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In the following code, Solidity will give the
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error "Linearization of inheritance graph impossible".
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::
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pragma solidity >=0.4.0 <0.6.0;
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contract X {}
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contract A is X {}
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// This will not compile
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contract C is A, X {}
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The reason for this is that ``C`` requests ``X`` to override ``A``
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(by specifying ``A, X`` in this order), but ``A`` itself
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requests to override ``X``, which is a contradiction that
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cannot be resolved.
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Inheriting Different Kinds of Members of the Same Name
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======================================================
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When the inheritance results in a contract with a function and a modifier of the same name, it is considered as an error.
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This error is produced also by an event and a modifier of the same name, and a function and an event of the same name.
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As an exception, a state variable getter can override a public function.
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