Solidity之mapping类型

　　映射是一种引用类型，存储键值对。它的定义是：mapping(key => value)，概念上与java中的map，python中的字典类型相似，但在使用上有比较多的限制。java

一.mapping定义
　　在mapping中， key能够是整型、字符串等基本数据类型，但不能使用动态数组、contract、枚举、struct，以及mapping这些类型。 value 的类型没有限制，甚至使用一个mapping做为value也是容许的。python

pragma solidity ^0.4.24; contract Mappings { struct Employee { string name; uint8 no; } mapping (bytes => Employee) bytesMapping;  //字节数组做为key
    mapping (address => Employee) addressMapping; // address做为key 
    mapping (string => mapping(uint => Employee)) complexMapping;  // mapping做为value，是能够的。 
    // mapping (Employee ==> uint8) structMapping; //不能使用struct来做为key
  
    function len() public returns(uint8) { // 编译错误： TypeError: Member "length" not found or not visible after argument-dependent lookup in mapping(bytes memory => struct Mappings.Employee // storage ref)
        // return bytesMapping.length;
        // ^-----------------^
 } }

　　在solidity中，mapping能够看作是一个哈希表。在这个表中，已经列举了全部可能的key值，并把这些key映射到已经将全部字段初始化为0的value上。但在存储上，key的值并非直接保存在这个哈希表中，而是它的keccak256值。使用的时候，须要经过这个值来查到对应的真实的key值。因此，在solidity中，mapping没有长度length的概念，也没法使用set来设置或者添加映射值的必要。git

二.存储模型
　　到当前版本，mapping 还仅能被声明为storage变量中，不能使用在memory上。因为在mapping中键的数量是任意的，致使映射的大小也是变长的。映射只能声明为storage的状态变量，或者在局部变量中引用到某个状态变量。github

pragma solidity ^0.4.24; contract Mappings { struct Employee { string name; uint8 no; } mapping (bytes => Employee) bytesMapping; function declaring() public returns(string) { bytesMapping["alex"] = Employee("Alex John", 1); mapping (bytes => Employee) ref = bytesMapping; ref["alex"].name = "Alexanda Jackson"; return bytesMapping["alex"].name; } }

　　如上咱们声明了bytesMapping的状态变量，而后在函数中申明ref局部变量来引用它。数组

三.mapping状态变量
　　若是在contract中声明了mapping状态变量， solidity编译器会自动生成get方法，输入参数为key，输出参数为value。若是使用mapping来做为value，则会生成两个输入参数的get方法，以此类推。app

pragma solidity ^0.4.24; contract Mappings { struct Employee { string name; uint8 no; } mapping (string => mapping(uint8 => Employee)) complexMapping; function setter() public returns(string){ complexMapping["alex"][1] = Employee("Alex John", 1); complexMapping["alex"][2] = Employee("Alex Mike", 2); complexMapping["tom"][1] = Employee("Tom Cruise", 6); return complexMapping["tom"][1].name; } }

　　在上面的示例中，咱们声明了storage的状态变量complexMapping，可使用key来访问或者设置对应的值。函数

四.mapping扩展
　　因为原生的mapping限制比较多，以太坊在官方的github上提供了一个扩展IterableMapping库。增长了相似其余语言的映射类型的操做。post

library IterableMapping { struct itmap { mapping(uint => IndexValue) data; KeyFlag[] keys; uint size; } struct IndexValue { uint keyIndex; uint value; } struct KeyFlag { uint key; bool deleted; } function insert(itmap storage self, uint key, uint value) returns (bool replaced) { uint keyIndex = self.data[key].keyIndex; self.data[key].value = value; if (keyIndex > 0) return true; else { keyIndex = self.keys.length++; self.data[key].keyIndex = keyIndex + 1; self.keys[keyIndex].key = key; self.size++; return false; } } function remove(itmap storage self, uint key) returns (bool success) { uint keyIndex = self.data[key].keyIndex; if (keyIndex == 0) return false; delete self.data[key]; self.keys[keyIndex - 1].deleted = true; self.size --; } function contains(itmap storage self, uint key) returns (bool) { return self.data[key].keyIndex > 0; } function iterate_start(itmap storage self) returns (uint keyIndex) { return iterate_next(self, uint(-1)); } function iterate_valid(itmap storage self, uint keyIndex) returns (bool) { return keyIndex < self.keys.length; } function iterate_next(itmap storage self, uint keyIndex) returns (uint r_keyIndex) { keyIndex++; while (keyIndex < self.keys.length && self.keys[keyIndex].deleted) keyIndex++; return keyIndex; } function iterate_get(itmap storage self, uint keyIndex) returns (uint key, uint value) { key = self.keys[keyIndex].key; value = self.data[key].value; } }

　　使用示例以下：ui

contract User { // Just a struct holding our data.
 IterableMapping.itmap data; // Insert something
  function insert(uint k, uint v) returns (uint size) { // Actually calls itmap_impl.insert, auto-supplying the first parameter for us.
 IterableMapping.insert(data, k, v); // We can still access members of the struct - but we should take care not to mess with them.
    return data.size; } // Computes the sum of all stored data.
  function sum() returns (uint s) { for (var i = IterableMapping.iterate_start(data); IterableMapping.iterate_valid(data, i); i = IterableMapping.iterate_next(data, i)) { var (key, value) = IterableMapping.iterate_get(data, i); s += value; } } }

文章来源：https://toutiao.io/posts/vg6mip/previewspa