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solidity

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smart contract solidity

What is Smart Contract

In the 1990s, cryptographer Nick Szabo coined the term and defined it as “a set of promises, specified in digital form, including protocols within which the parties perform on the other promises.”

In ethereum,use immutable computer programs that run deterministically in the context of an Ethereum Virtual Machine as part of the Ethereum network protocol

Life Cycle of a Smart Contract

  • called by a transaction
  • call another contract
  • no parallel and single-threaded
  • atomic
  • cannot be changed. However, a contract can be “deleted”

Solidity

  • imperative language: Imperative programming, by contrast, is where a programmer writes a set of procedures that combine the logic and flow of a program

  • created by Dr. Gavin Wood

https://docs.soliditylang.org/en/v0.7.4/

Install Solidity Compiler

install solidity

Check current solc version

$ solc --version
solc, the solidity compiler commandline interface
Version: 0.7.5+commit.eb77ed08.Darwin.appleclang

Simple Solidity Program

https://github.com/ethereumbook/ethereumbook/blob/develop/code/Solidity/Faucet.sol

compile

✗ solc --optimize --bin Faucet.sol
Error: Source file requires different compiler version (current compiler is 0.7.5+commit.eb77ed08.Darwin.appleclang) - note that nightly builds are considered to be strictly less than the released version
 --> Faucet.sol:2:1:
  |
2 | pragma solidity ^0.6.0;
  | ^^^^^^^^^^^^^^^^^^^^^^^

resolve version issue

pragma solidity ^0.6.0 means this program can be compiled by a Solidity compiler with a minimum version of 0.6.0,that means you can use 0.6.x but not 0.7.x.

Our current version is 0.7.5, So we need to modify the first line as:

pragma solidity ^0.7.0;

compile again

✗ solc --optimize --bin Faucet.sol
Warning: SPDX license identifier not provided in source file. Before publishing, consider adding a comment containing "SPDX-License-Identifier: <SPDX-License>" to each source file. Use "SPDX-License-Identifier: UNLICENSED" for non-open-source code. Please see https://spdx.org for more information.
--> Faucet.sol


======= Faucet.sol:Faucet =======
Binary:
608060405234801561001057600080fd5b5060cc8061001f6000396000f3fe608060405260043610601f5760003560e01c80632e1a7d4d14602a576025565b36602557005b600080fd5b348015603557600080fd5b50605060048036036020811015604a57600080fd5b50356052565b005b67016345785d8a0000811115606657600080fd5b604051339082156108fc029083906000818181858888f193505050501580156092573d6000803e3d6000fd5b505056fea2646970667358221220aeb8ae5bf7323de1c04cf037f9612e67a487fbc2d44df50efcc65d22a49f52ba64736f6c63430007050033

Add SPDX-License-Identifier: UNLICENSED to the head of file to resolve the compiler’s warning

get abi

✗ solc --abi Faucet.sol

======= Faucet.sol:Faucet =======
Contract JSON ABI
[{"inputs":[{"internalType":"uint256","name":"withdraw_amount","type":"uint256"}],"name":"withdraw","outputs":[],"stateMutability":"nonpayable","type":"function"},{"stateMutability":"payable","type":"receive"}]

Get transaction info by infura

$ curl https://ropsten.infura.io/v3/YOUR-PROJECT-ID \
>     -X POST \
>     -H "Content-Type: application/json" \
>     -d '{"jsonrpc":"2.0","method":"eth_getTransactionByHash","params": ["0xf874eb0fb2fdcd10842ebe787d2a18f9a2bf3d51c575d6be434c90692de148c4"],"id":1}'     | jq
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  1749  100  1608  100   141   1574    138  0:00:01  0:00:01 --:--:--  1714
{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "blockHash": "0x3f5c606ff44ccba84bcf2b1317de67bd93540079812c0ff9ca438098bb4fc4c4",
    "blockNumber": "0x90e933",
    "from": "0xa45e280704b00547cd5aaab4b41c722e83f2d4b8",
    "gas": "0x293d9",
    "gasPrice": "0x796b6cb0",
    "hash": "0xf874eb0fb2fdcd10842ebe787d2a18f9a2bf3d51c575d6be434c90692de148c4",
    "input": "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",
    "nonce": "0x1e",
    "r": "0x99c226063cf95c7329f64401252168bd45ba6b1b67c3d47011aab3c6b9ac94f8",
    "s": "0x6ac17565e15d0615d4e722b3b4d50022fa39f0a704fe09451adc392a6811d670",
    "to": null,
    "transactionIndex": "0x18",
    "v": "0x29",
    "value": "0x0"
  }
}

Install truffle

npm install -g truffle
✗ truffle version
Truffle v5.1.56 (core: 5.1.56)
Solidity v0.5.16 (solc-js)
Node v15.3.0
Web3.js v1.2.9

Call other contracts

  • Creating a new instance
  • Addressing an existing instance
  • Raw call, delegatecall

whereas a call changes the value of msg.sender to be the calling contract, a delegatecall keeps the same msg.sender as in the calling contract.

pragma solidity ^0.5.16;

contract calledContract {
    event callEvent(address sender, address origin, address from);

    function calledFunction() public {
        emit callEvent(msg.sender, tx.origin, address(this));
    }
}

library calledLibrary {
    event callEvent(address sender, address origin, address from);

    function calledFunction() public {
        emit callEvent(msg.sender, tx.origin, address(this));
    }
}

contract caller {
    function make_calls(calledContract _calledContract) public {
        // Calling calledContract and calledLibrary directly
        _calledContract.calledFunction();
        calledLibrary.calledFunction();

        // Low-level calls using the address object for calledContract
        address(_calledContract).call(
            abi.encodeWithSignature("calledFunction()")
        );
        address(_calledContract).delegatecall(
            abi.encodeWithSignature("calledFunction()")
        );
    }
}

image-20210120001040159

Gas Considerations

  • Avoid Dynamically Sized Arrays

  • Avoid Calls to Other Contracts (don’t agree this)

Estimating Gas Cost

  • call estimateGas function
  • hard code maybe a easier way
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