状态变量储存结构¶

静态大小的变量（除映射（mapping）和动态数组之外的所有类型）都从位置 0 开始连续放置在存储（storage）中。如果可能的话，存储大小少于 32 字节的多个变量会被打包到一个存储插槽（storage slot）中，规则如下：

存储插槽（storage slot）的第一项会以低位对齐（即右对齐）的方式储存。
基本类型仅使用存储它们所需的字节。
如果存储插槽（storage slot）中的剩余空间不足以储存一个基本类型，那么它会被移入下一个存储插槽（storage slot）。
结构体（struct）和数组数据总是会占用一整个新插槽（但结构体或数组中的各项，都会以这些规则进行打包）。

对于使用继承的合约，状态变量的排序由C3线性化合约顺序（顺序从最基类合约开始）确定。如果上述规则成立，那么来自不同的合约的状态变量会共享一个存储插槽（storage slot）。

结构体和数组中的成员变量会存储在一起，就像它们在显式声明中的一样。

警告

在使用小于 32 字节的元素（变量）时，合约的 gas 使用量可能会高于使用 32 字节的元素。这是因为以太坊虚拟机（Ethereum Virtual Machine(EVM)）每次操作 32 个字节，所以如果元素比 32 字节小，以太坊虚拟机（Ethereum Virtual Machine(EVM)）必须执行额外的操作以便将其大小缩减到到所需的大小。

当我们在处理状态变量时，只有当编译器会将多个元素打包到一个存储插槽（storage slot）中，使用缩减的大小（小于32字节）的变量才更有益处。因为它会将多个读或写合并为单次操作。而在处理函数参数或内存（memory）中的值时，因为编译器不会打包这些值，所以没有什么益处。

最后，为了允许以太坊虚拟机（Ethereum Virtual Machine(EVM)）对此进行优化，请确保存储（storage）中的变量和 struct 成员的书写顺序允许它们被紧密地打包。例如，应该按照 uint128，uint128，uint256 的顺序来声明状态变量，而不是使用 uint128，uint256，uint128 ，因为前者只占用两个存储插槽（storage slot），而后者将占用三个。

备注

由于存储（storage）中的指针可以传递给库（library），所以存储（storage）中状态变量的布局被认为是 solidity 外部接口的一部分。这意味着，本节所述规则的任何变更均被视为语言的重大变更，由于其关键性，请在执行前应仔细考虑。

映射和动态数组¶

由于映射（mapping）和动态数组的大小是不可预知的，他们使用 Keccak-256 哈希计算来找到值的位置或数组的起始位置。这些起始位置本身的数值总是会占满堆栈插槽。

映射（mapping）或动态数组本身会根据上述规则来在某个位置 p 处占用一个（未填充的）存储中的插槽（或递归地将该规则应用到映射（mapping）的映射（mapping）或数组的数组）。对于动态数组，此插槽中会存储数组中元素的数量（字节数组和字符串除外，见下文）。

对于映射（mapping），该插槽未被使用（但它仍是需要的，以使两个相同的映射（mapping）在彼此之后会使用不同的散列分布）。数组的数据会位于 keccak256(p) ；映射（mapping）中的键 k 所对应的值会位于 keccak256(k . p) ，其中 . 是连接符。如果该值又是一个非基本类型，则通过添加 keccak256(k . p) 作为偏移量来找到位置。

所以对于以下合约片段``data[4][9].b`` 的位置将是 keccak256(uint256(9) . keccak256(uint256(4) . uint256(1))) + 1 。

// SPDX-License-Identifier: GPL-3.0 pragma solidity >=0.4.0 <0.7.0;

contract C {
struct S { uint a; uint b; } uint x; mapping(uint => mapping(uint => S)) data;

}

`bytes` 和 `string`¶

bytes 和 string 编码是一样的。对于短字节数组，它们将数据和它们的长度存储在同一个插槽中。具体地说：如果数据长度小于等于 31 字节，则它存储在高位字节（左对齐），最低位字节存储 length * 2。如果数据长度超出 31 字节，则在主插槽存储 length * 2 + 1 ，数据照常存储在 keccak256(slot) 中。

这意味着可以通过检查是否设置了最低位：短（未设置）和长（设置）来区分短数组和长数组。

备注

目前不支持处理无效编码的插槽，但可能在将来添加。

JSON Output¶

The storage layout of a contract can be requested via the standard JSON interface. The output is a JSON object containing two keys, storage and types. The storage object is an array where each element has the following form:

{
    "astId": 2,
    "contract": "fileA:A",
    "label": "x",
    "offset": 0,
    "slot": "0",
    "type": "t_uint256"
}

The example above is the storage layout of contract A { uint x; } from source unit fileA and

astId is the id of the AST node of the state variable’s declaration
contract is the name of the contract including its path as prefix
label is the name of the state variable
offset is the offset in bytes within the storage slot according to the encoding
slot is the storage slot where the state variable resides or starts. This number may be very large and therefore its JSON value is represented as a string.
type is an identifier used as key to the variable’s type information (described in the following)

The given type, in this case t_uint256 represents an element in types, which has the form:

{
    "encoding": "inplace",
    "label": "uint256",
    "numberOfBytes": "32",
}

where

encoding how the data is encoded in storage, where the possible values are:
- inplace: data is laid out contiguously in storage (see above).
- mapping: Keccak-256 hash-based method (see above).
- dynamic_array: Keccak-256 hash-based method (see above).
- bytes: single slot or Keccak-256 hash-based depending on the data size (see above).
label is the canonical type name.
numberOfBytes is the number of used bytes (as a decimal string). Note that if numberOfBytes > 32 this means that more than one slot is used.

Some types have extra information besides the four above. Mappings contain its key and value types (again referencing an entry in this mapping of types), arrays have its base type, and structs list their members in the same format as the top-level storage (see above).

备注

The JSON output format of a contract’s storage layout is still considered experimental and is subject to change in non-breaking releases of Solidity.

The following example shows a contract and its storage layout, containing value and reference types, types that are encoded packed, and nested types.

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.0 <0.7.0;
contract A {
    struct S {
        uint128 a;
        uint128 b;
        uint[2] staticArray;
        uint[] dynArray;
    }

    uint x;
    uint y;
    S s;
    address addr;
    mapping (uint => mapping (address => bool)) map;
    uint[] array;
    string s1;
    bytes b1;
}

"storageLayout": {
  "storage": [
    {
      "astId": 14,
      "contract": "fileA:A",
      "label": "x",
      "offset": 0,
      "slot": "0",
      "type": "t_uint256"
    },
    {
      "astId": 16,
      "contract": "fileA:A",
      "label": "y",
      "offset": 0,
      "slot": "1",
      "type": "t_uint256"
    },
    {
      "astId": 18,
      "contract": "fileA:A",
      "label": "s",
      "offset": 0,
      "slot": "2",
      "type": "t_struct(S)12_storage"
    },
    {
      "astId": 20,
      "contract": "fileA:A",
      "label": "addr",
      "offset": 0,
      "slot": "6",
      "type": "t_address"
    },
    {
      "astId": 26,
      "contract": "fileA:A",
      "label": "map",
      "offset": 0,
      "slot": "7",
      "type": "t_mapping(t_uint256,t_mapping(t_address,t_bool))"
    },
    {
      "astId": 29,
      "contract": "fileA:A",
      "label": "array",
      "offset": 0,
      "slot": "8",
      "type": "t_array(t_uint256)dyn_storage"
    },
    {
      "astId": 31,
      "contract": "fileA:A",
      "label": "s1",
      "offset": 0,
      "slot": "9",
      "type": "t_string_storage"
    },
    {
      "astId": 33,
      "contract": "fileA:A",
      "label": "b1",
      "offset": 0,
      "slot": "10",
      "type": "t_bytes_storage"
    }
  ],
  "types": {
    "t_address": {
      "encoding": "inplace",
      "label": "address",
      "numberOfBytes": "20"
    },
    "t_array(t_uint256)2_storage": {
      "base": "t_uint256",
      "encoding": "inplace",
      "label": "uint256[2]",
      "numberOfBytes": "64"
    },
    "t_array(t_uint256)dyn_storage": {
      "base": "t_uint256",
      "encoding": "dynamic_array",
      "label": "uint256[]",
      "numberOfBytes": "32"
    },
    "t_bool": {
      "encoding": "inplace",
      "label": "bool",
      "numberOfBytes": "1"
    },
    "t_bytes_storage": {
      "encoding": "bytes",
      "label": "bytes",
      "numberOfBytes": "32"
    },
    "t_mapping(t_address,t_bool)": {
      "encoding": "mapping",
      "key": "t_address",
      "label": "mapping(address => bool)",
      "numberOfBytes": "32",
      "value": "t_bool"
    },
    "t_mapping(t_uint256,t_mapping(t_address,t_bool))": {
      "encoding": "mapping",
      "key": "t_uint256",
      "label": "mapping(uint256 => mapping(address => bool))",
      "numberOfBytes": "32",
      "value": "t_mapping(t_address,t_bool)"
    },
    "t_string_storage": {
      "encoding": "bytes",
      "label": "string",
      "numberOfBytes": "32"
    },
    "t_struct(S)12_storage": {
      "encoding": "inplace",
      "label": "struct A.S",
      "members": [
        {
          "astId": 2,
          "contract": "fileA:A",
          "label": "a",
          "offset": 0,
          "slot": "0",
          "type": "t_uint128"
        },
        {
          "astId": 4,
          "contract": "fileA:A",
          "label": "b",
          "offset": 16,
          "slot": "0",
          "type": "t_uint128"
        },
        {
          "astId": 8,
          "contract": "fileA:A",
          "label": "staticArray",
          "offset": 0,
          "slot": "1",
          "type": "t_array(t_uint256)2_storage"
        },
        {
          "astId": 11,
          "contract": "fileA:A",
          "label": "dynArray",
          "offset": 0,
          "slot": "3",
          "type": "t_array(t_uint256)dyn_storage"
        }
      ],
      "numberOfBytes": "128"
    },
    "t_uint128": {
      "encoding": "inplace",
      "label": "uint128",
      "numberOfBytes": "16"
    },
    "t_uint256": {
      "encoding": "inplace",
      "label": "uint256",
      "numberOfBytes": "32"
    }
  }
}

状态变量储存结构¶

映射和动态数组¶

bytes 和 string¶

JSON Output¶

`bytes` 和 `string`¶