solidity/docs/contracts/abstract-contracts.rst

.. index:: ! contract;abstract, ! abstract contract

.. _abstract-contract:

******************
Abstract Contracts
******************

Contracts need to be marked as abstract when at least one of their functions was not implemented.
You can also mark contracts as abstract even though all functions are implemented.
This can be done by using the ``abstract`` keyword as shown in the following example. Note, that this contract need to be
defined as abstract, because the function ``utterance()`` was defined, but no implementation was
provided (no implementation body ``{ }`` was given).::

    pragma solidity >=0.4.0 <0.7.0;

    abstract contract Feline {
        function utterance() public returns (bytes32);
    }

Such abstract contracts can not be instantiated directly. This is also true, if an abstract contract itself does implement
all defined functions. The usage of an abstract contract as a base class is shown in the following example::

    pragma solidity >=0.4.0 <0.7.0;

    abstract contract Feline {
        function utterance() public virtual returns (bytes32);
    }

    contract Cat is Feline {
        function utterance() public override returns (bytes32) { return "miaow"; }
    }

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.

Note that a function without implementation is different from a :ref:`Function Type <function_types>` even though their syntax looks very similar.

Example of function without implementation (a function declaration)::

    function foo(address) external returns (address);

Example of a Function Type (a variable declaration, where the variable is of type ``function``)::

    function(address) external returns (address) foo;

Abstract contracts decouple the definition of a contract from its implementation providing better extensibility and self-documentation and
facilitating patterns like the `Template method <https://en.wikipedia.org/wiki/Template_method_pattern>`_ and removing code duplication.
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".