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https://github.com/ethereum/solidity
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49 lines
2.2 KiB
ReStructuredText
49 lines
2.2 KiB
ReStructuredText
.. index:: ! contract;abstract, ! abstract contract
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.. _abstract-contract:
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******************
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Abstract Contracts
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******************
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Contracts need to be marked as abstract when at least one of their functions was not implemented.
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You can also mark contracts as abstract even though all functions are implemented.
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This can be done by using the ``abstract`` keyword as shown in the following example. Note, that this contract need to be
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defined as abstract, because the function ``utterance()`` was defined, but no implementation was
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provided (no implementation body ``{ }`` was given).::
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pragma solidity >=0.4.0 <0.7.0;
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abstract contract Feline {
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function utterance() public returns (bytes32);
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}
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Such abstract contracts can not be instantiated directly. This is also true, if an abstract contract itself does implement
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all defined functions. The usage of an abstract contract as a base class is shown in the following example::
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pragma solidity >=0.4.0 <0.7.0;
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abstract contract Feline {
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function utterance() public virtual returns (bytes32);
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}
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contract Cat is Feline {
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function utterance() public override returns (bytes32) { return "miaow"; }
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}
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If a contract inherits from an abstract contract and does not implement all non-implemented functions by overriding, it needs to be marked as abstract as well.
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Note that a function without implementation is different from a :ref:`Function Type <function_types>` even though their syntax looks very similar.
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Example of function without implementation (a function declaration)::
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function foo(address) external returns (address);
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Example of a Function Type (a variable declaration, where the variable is of type ``function``)::
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function(address) external returns (address) foo;
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Abstract contracts decouple the definition of a contract from its implementation providing better extensibility and self-documentation and
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facilitating patterns like the `Template method <https://en.wikipedia.org/wiki/Template_method_pattern>`_ and removing code duplication.
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Abstract contracts are useful in the same way that defining methods in an interface is useful. It is a way for the designer of the abstract contract to say "any child of mine must implement this method".
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