抽象合约

合约函数可以缺少实现，如下例所示（请注意函数声明头由 ; 结尾）:

pragma solidity >=0.4.0 <0.7.0;

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

这些合约无法成功编译（即使它们除了未实现的函数还包含其他已经实现了的函数），但他们可以用作基类合约:

pragma solidity >=0.4.0 <0.7.0;

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

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

如果合约继承自抽象合约，并且没有通过重写来实现所有未实现的函数，那么它本身就是抽象的。

Note that a function without implementation is different from a Function Type 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 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”.