01-1-Library

Library

Libraries are similar to contracts, but you can't declare any state variable and you can't send ether.

A library is embedded into the contract if all library functions are internal.

Otherwise the library must be deployed and then linked before the contract is deployed.

库与合约类似，但你不能声明任何状态变量，也不能发送以太币。

如果所有库函数都是内部的，则库将嵌入到合约中。

否则，必须在部署合约之前部署并链接库。

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

library Math {
    function sqrt(uint y) internal pure returns (uint z) {
        if (y > 3) {
            z = y;
            uint x = y / 2 + 1;
            while (x < z) {
                z = x;
                x = (y / x + x) / 2;
            }
        } else if (y != 0) {
            z = 1;
        }
        // else z = 0 (default value)
    }
}

contract TestMath {
    function testSquareRoot(uint x) public pure returns (uint) {
        return Math.sqrt(x);
    }
}

// Array function to delete element at index and re-organize the array
// so that there are no gaps between the elements.
library Array {
    function remove(uint[] storage arr, uint index) public {
        // Move the last element into the place to delete
        require(arr.length > 0, "Can't remove from empty array");
        arr[index] = arr[arr.length - 1];
        arr.pop();
    }
}

contract TestArray {
    using Array for uint[];

    uint[] public arr;

    function testArrayRemove() public {
        for (uint i = 0; i < 3; i++) {
            arr.push(i);
        }

        arr.remove(1);

        assert(arr.length == 2);
        assert(arr[0] == 0);
        assert(arr[1] == 2);
    }
}

总结

用ERC721的引用的库合约String为例介绍solidity中的库合约（library），并总结了常用的库函数。

库函数是一种特殊的合约，为了提升solidity代码的复用性和减少gas而存在。库合约一般都是一些好用的函数合集（库函数），由大神或者项目方创作

和普通合约主要有以下几点不同：

1. 不能存在状态变量
2. 不能够继承或被继承
3. 不能接收以太币
4. 不可以被销毁

01.String库合约

String库合约是将uint256类型转换为相应的string类型的代码库，主要包含两个函数，toString()将uint256转为string，toHexString()将uint256转换为16进制，再转换为string。

样例代码如下：

library Strings {
    bytes16 private constant _HEX_SYMBOLS = "0123456789abcdef";

    /**
     * @dev Converts a `uint256` to its ASCII `string` decimal representation.
     */
    function toString(uint256 value) public pure returns (string memory) {
        // Inspired by OraclizeAPI's implementation - MIT licence
        // https://github.com/oraclize/ethereum-api/blob/b42146b063c7d6ee1358846c198246239e9360e8/oraclizeAPI_0.4.25.sol

        if (value == 0) {
            return "0";
        }
        uint256 temp = value;
        uint256 digits;
        while (temp != 0) {
            digits++;
            temp /= 10;
        }
        bytes memory buffer = new bytes(digits);
        while (value != 0) {
            digits -= 1;
            buffer[digits] = bytes1(uint8(48 + uint256(value % 10)));
            value /= 10;
        }
        return string(buffer);
    }

    /**
     * @dev Converts a `uint256` to its ASCII `string` hexadecimal representation.
     */
    function toHexString(uint256 value) public pure returns (string memory) {
        if (value == 0) {
            return "0x00";
        }
        uint256 temp = value;
        uint256 length = 0;
        while (temp != 0) {
            length++;
            temp >>= 8;
        }
        return toHexString(value, length);
    }

    /**
     * @dev Converts a `uint256` to its ASCII `string` hexadecimal representation with fixed length.
     */
    function toHexString(uint256 value, uint256 length) public pure returns (string memory) {
        bytes memory buffer = new bytes(2 * length + 2);
        buffer[0] = "0";
        buffer[1] = "x";
        for (uint256 i = 2 * length + 1; i > 1; --i) {
            buffer[i] = _HEX_SYMBOLS[value & 0xf];
            value >>= 4;
        }
        require(value == 0, "Strings: hex length insufficient");
        return string(buffer);
    }
}

02.如何使用库合约

用String库函数的toHexString()来演示两种使用库合约中函数的办法。

2-1.using for指令

指令using A for B;可用于附加库函数（从库 A）到任何类型（B）。添加完指令后，库A中的函数会自动添加为B类型变量的成员，可以直接调用。注意：在调用的时候，这个变量会被当作第一个参数传递给函数：

// 利用using for指令
using Strings for uint256;
function getString1(uint256 _number) public pure returns(string memory){
    // 库函数会自动添加为uint256型变量的成员
    return _number.toHexString();
}

2-2.通过库合约名称调用库函数

// 直接通过库合约名调用
function getString2(uint256 _number) public pure returns(string memory){
    return Strings.toHexString(_number);
}