這裡整理之前上課的筆記，都是入門級別一共 4 章，分別如下：

• 專有名詞（Terms）
• solidity basics（Data types & Control flow）
• functions
• 實踐 ERC-20

在 EVM 中有三個地方儲存變數

1. storage
   1. holds state variables
   2. persistent and expensive(cost gas)
   3. like HDD
2. memory
   1. local variables with memory keyword
   2. like RAM
   3. free to be used
   4. holds function args
3. stack
   1. hold local variables inside functions if they are not reference types(e.g: int)
   2. free to be used
   3. reference types: string, array, struct, mapping

variables

solidity 的變數大致上分以下兩種

1. state variable: declare at contract level
   1. store in contract storage
   2. can be constant
   3. cost gas to declare
   4. set by init, construct or setter
2. local variable: declare at function level
   1. can allocate at runtime(use memory keyword and in array or struct data types)
   2. string default saves in storage, use memory if you want string be local variable

variable types

可以分為以下兩種

• value type
  • boolean
  • int / uint
  • fixed / ufixed
  • address
  • dynamically-sized byte array(bytes, byte[]), fixed-size byte array(byte[2])
• reference type
  • array
  • struct
  • mapping
  • string

每個 reference type 在定義的時候必須指定他的儲存位置，分為storage, memory, calldata

boolean

bool myBoolean = true;

int and uint

有 uint8,uint16,uint32…uint256，如果都不指定預設是 uint256

int noAssign; // int256 noAssign = 0
uint256 myUnsignedInt = 42;
int256 mySignedInt = -42;

address

20-byte 的 eth address

address myAddress = 0x1234567890123456789012345678901234567890;

bytes

array of byte，alias of byte[]

bytes myBytes = "1234567890abcdef";
byte[] sameMyBytes = "1234567890abcdef";

string

string myString = "Hello, World!";

enum

enum State {
    Open,
    Closed,
    Unknown
}

State xxx = State.Open;

struct

自定義的 key-value 結構，通常用來定義單數的物件

struct Person {
    string name;
    uint age;
}

Person myStruct;
myStruct.name = "Jon Doe";
myStruct.age = 18;
// or
Person myStruct = Person("Jon Doe", 18);

mapping

自定義的 key-value 結構，通常用來定義複數的物件

• not iterable(key is hash and not saved into mapping)
• key 不可以是 mapping, array, enum or struct
• 如果對不存在的 key 進行存取的話，會取得 value 變數的預設值

mapping (uint256 => string) myMapping;
myMapping[0] = "Hello";
myMapping[1] = "World";

array

• fixed-size(compile time)

uint[3] numbers = [2, 3, 4];

• dynamic-size(runtime)

uint[] numbers;
byte[] byteNums;

能見度（visibility）

變數與 function 都有能見度，宣告在 contract level，共有 4 個層級

• public
  • 可在同個 contract 中被呼叫或是被其他 account 呼叫
  • variable 預設會建立對應的 getter
• private
  • internal 的子集
  • state variable 的預設能見度
  • 只活在該 contract 中
• internal
  • 有點類似 protected, 活在 contract 本身與繼承的 contract
• external
  • 可視為 contract 的 interface，只能被其他 contract 或是 EOA 呼叫
  • not available for state variables

實際撰寫時的語法如下

// SPDX-License-Identifier: GPL-3.0

pragma solidity ^0.8.7;

contract MyContract {
    // <variable type> <visibility> <variable name>;
    uint public myNum;
    // function <function name>(<variable type> <variable name>) <visibility> {}
    function addNum(uint _myNum) public {
        myNum += _myNum;
    }
}

global variable

預設的全域變數，比較常用的有

• msg: 如專有名詞那裡提到，CA 會產生 message，只的就是這個 msg
  • msg.sender: account address of tx maker
  • msg.value: eth value (in wei) sent to this contract
  • msg.gas: remaining gas(will be replaced by gasleft())
  • msg.data: data field from tx or called this function’s data
• gasleft(): remain gas of this transaction, can calculate a function’s gas
• this: the current contract, e.g: address(this).balance => this contract’s balance
• block: the block itself
  • block.timestmap: alias of now, unix epoch
• tx.gasprice: gas price of the transaction

references

• solidity global variables