Introduction to Smart Contracts

What is a Smart Contract?

The concept of smart contracts was first proposed by Nick Szabo in 1995. A Smart contract is a computer protocol designed to disseminate, verify or execute contracts in an informational manner. Smart contracts allow transactions to be executed without a trusted third party, and the transactions are traceable and irreversible.

Its purpose is to provide a secure method that outperforms traditional contracts and reduces other transaction costs associated with contracts.

For Conflux network, a smart contract is a simple program that runs on the Conflux Network. Each instance of a smart contract is a collection of code and data that resides at a specific address on the Conflux Network. Since the state on the blockchain is fully deterministic, operations on smart contracts are guaranteed to produce the same results on all blockchain nodes. Furthermore, since smart contracts run on the blockchain, the decentralization and non-tamperable characteristics of the blockchain ensure certainty and security in the operation of the contract. Therefore, a wide range of decentralized applications (dApps) are created based on smart contracts, including games, digital collectibles (NFT), online voting systems, financial products (DeFi), etc.

There are two account types on the Conflux Network: user accounts and smart contract accounts. Once the smart contract is deployed, a corresponding smart contract account is created. Smart contract accounts have a CFX balance and they can also interact with other accounts. However, they are not controlled by users but by the program deployed on the network. User accounts are able to interact with smart contracts by submitting transactions that execute the functions defined on the smart contract. The smart contract enables us to define rules just like traditional contracts and automates the execution through codes.

Nick Szabo used the example of a vending machine to describe how real-world contractual obligations can be programmed into software and hardware systems. Each person simply puts the correct number of coins into the machine and can expect to receive a product in exchange. Similarly, on Conflux, a smart contract is able to perform a certain task and get a certain result under specified conditions.

Example Vending Machine Smart Contract

Let's look at one of the simplest smart contract implementations of a vending machine.

pragma solidity ^0.8.0;

contract VendingMachine {

    // Declare state variables of the contract
    address public owner;
    mapping (address => uint) public cupcakeBalances;

    // Declare event for record purchase event
    event Purchase(address customer, uint amount);

    // When 'VendingMachine' contract is deployed:
    // 1. set the deploying address as the owner of the contract
    // 2. set the deployed smart contract's cupcake balance to 100
    constructor() {
        owner = msg.sender;
        cupcakeBalances[address(this)] = 100;
    }

    // Get user's cupcake balance
    function balanceOf(address user) public view returns(uint) {
        return cupcakeBalances[user];
    }

    // Allow the owner to increase the smart contract's cupcake balance
    function refill(uint amount) public {
        require(msg.sender == owner, "Only the owner can refill.");
        cupcakeBalances[address(this)] += amount;
    }

    // Allow anyone to purchase cupcakes
    function purchase(uint amount) public payable {
        require(msg.value >= amount * 1 ether, "You must pay at least 1 CFX per cupcake");
        require(cupcakeBalances[address(this)] >= amount, "Not enough cupcakes in stock to complete this purchase");

        unchecked {
            cupcakeBalances[address(this)] -= amount;
        }

        cupcakeBalances[msg.sender] += amount;
        emit Purchase(msg.sender,amount);
    }
}

This contract has two functions: method refill enables administrators to refill the products. Method purchase enables the customers to purchase products using CFX tokens. Just like how vending machines eliminate the need for salesmen, smart contracts can eliminate intermediaries in many industries.

The smart contract is a set of code (contract functions) and data (contract state variables). For the line mapping (address => uint) public cupcakeBalances; , it declares a state variable named cupcakeBalances with type mapping (address => uint). You can see it as a table in the database. Meanwhile, the contract methods refill and purchase are used to read or modify the state of the data in that database table. Also this state variable is public and this means it's public-accessible.

This contract is written in solidity, whose syntax is similar to javascript.

• pragma solidity ^0.8.0; indicates that this contract needs to be compiled with a >=0.8.0<0.9.0 compiler.

• contract VendingMachine assigns the contract name as VendingMachine ;

• address public owner; defines a public state variable with the name owner and type address;

• event Purchase(address customer, uint amount); defines the event with the name Purchase. event is similar to the logging function in other languages. Its role is mainly to record some important information when the contract is executed. For example, Purchase is an event that generates a purchase record when a customer purchases an item.

• The constructor function is executed exactly once the contract is deployed.

• The balanceOf function marked with the view is a read-only function that can't be used to modify the contract state. All read-only functions must be declared as view or pure functions.

• refill function is an ordinary function, can be called by sending a transaction. This function will change the contract state.

• require(msg.sender == owner, "Only the owner can refill"); indicates that only when the condition msg.sender == owner is fullfilled (that is, the caller is the contract admin), the state of contract variable cupcakeBalances can be modified to refill the products. If the condition is not fullfilled then a messaged will be returner (in this case "Only the owner can refill") and the transaction will be reverted.

• purchase function includes a payable, indicating you can send CFX at the same time the function is called. Since customers need to pay CFX to purchase it's marked as a payable function.

• require(msg.value >= amount 1 ether, "You must pay at least 1 CFX per cupcake") indicates the full amount of CFX must be paid in order to complete the purchase. Otherwhise the function will be reverted and will return the "You must pay at least 1 CFX per cupcake" message.

  Solidity has built-in literal Ether Units

  A literal number can take a suffix of wei, gwei or ether to specify a subdenomination of Ether, where Ether numbers without a postfix are assumed to be Wei. In Conflux 1 ether = 1 CFX.

  • assert(1 wei == 1);
  • assert(1 gwei == 1e9);
  • assert(1 ether == 1e18);

• The number of products in the vending machine decreases after a successful purchase and the number of goods owned by the customers (indicated by cupcakeBalances[msg.sender]) increases.

After writing the smart contract, you need to compile it and then deploy it to the Conflux Network by sending a transaction. Then you can interact with the smart contract functions.

Compilation

Smart contract compilation is the process of generating the elements required when deploying the contract from the contract code through the compiler. There are two main parts in the compilation result, The Contract Application Binary Interface (abi) and bytecode.

• Bytecode: Smart contracts are executed on the Ethereum Virtual Machine (EVM). The bytecode is the hexadecimal value corresponding to the smart contract that the EVM can recognize.
• ABI: ABI refers to the Application Binary Interface, which describes each function name, modifier, visibility, parameters name, and its type, returns value name and its type and description of events in the public interface of contract (in JSON format). When we call the contract function externally and encode it in a certain way based on the description of the function in the ABI, we can get a value that the EVM can recognize and display in hexadecimal format. You can use this value to interact with the smart contract.

You can use solc, conflux-truffle for Conflux Core or truffle for Conflux eSpace to compile the smart contract.

Here we take solc as example.

Install solc

npm install -g solc

Attention: The compiler version needs to match the version specified in the contract. To download the v0.8.1 version, use npm install -g solc@v0.8.1

Compilation

solcjs ./VendingMachine.sol --bin --abi

Generate the bytecode file __...VendingMachine.bin and the ABI file __...VendingMachine.abi

__...VendingMachine.bin

60806040523480156100...bfea2646970667358221220761301cb41bc1e4c37cc823f17fd695c6c09521a3e09fe1e8a7c51f6e77181a464736f6c63430008000033

__...VendingMachine.abi

[
    {
      "inputs": [],
      "stateMutability": "nonpayable",
      "type": "constructor"
    },
    {
      "anonymous": false,
      "inputs": [
        {
          "indexed": false,
          "internalType": "address",
          "name": "customer",
          "type": "address"
        },
        {
          "indexed": false,
          "internalType": "uint256",
          "name": "amount",
          "type": "uint256"
        }
      ],
      "name": "Purchase",
      "type": "event"
    },
    ......
]

Deploy

The smart contract deployment is creating a contract on the Conflux Network by sending a transaction with the data as bytecode and to left empty.

If the constructor contains parameters, data should be a combination of bytecode and the ABI encoding of the constructor.

We will use js-conflux-sdk to demostrate:

const { Conflux } = require("js-conflux-sdk");

(async function () {
    const cfx = await Conflux.create({ url: "https://test.confluxrpc.com" })
    const account = cfx.wallet.addPrivateKey("0x2139FB4C55CB9AF7F0086CD800962C2E9013E2292BAE77978A9209E3BEE71D49")

    // your bytecode
    let bytecode = "0x608060405234801561001057600080fd5b50336000806101000a81548173ffffff......0008000033"

    let deployReceipt = await cfx.sendTransaction({
        from: account.address,
        data: bytecode
    }).executed()

    // or use contract instance
    // let vendingMachine = cfx.Contract({ bytecode })
    // let deployReceipt = await vendingMachine.constructor().sendTransaction({from:account.address}).executed()

    console.log("deploy tx receipt:", deployReceipt)
})()

As shown in the example above, the contract is deployed after sending a transaction with data as bytecode. The contractCreated field of the transaction receipt is the contract address after deployment.

deploy tx receipt: {
  blockHash: '0xe1b7f118447d3f945db4c2cf5752e592e542d4b9d24d0312b4ca5fce925c1ae5',
  contractCreated: 'CFXTEST:TYPE.CONTRACT:ACCYSPEUUR469BCA0EXRFXKXMKX651W45JFW2RN5M0',
  epochNumber: 27675623,
  from: 'CFXTEST:TYPE.USER:AAP9KTHVCTUNVF030RBKK9K7ZBZYZ12DAJP1U3SP4G',
  gasCoveredBySponsor: false,
  gasFee: 646047n,
  gasUsed: 646047n,
  index: 1,
  logs: [],
  logsBloom: '0x...',
  outcomeStatus: 0,
  stateRoot: '0xd2ada6e3c04d6e8260446deaf1b8289d57ba84e2d82387155bbb397be93e2a30',
  storageCollateralized: 1664n,
  storageCoveredBySponsor: false,
  storageReleased: [],
  to: null,
  transactionHash: '0xe19d8a7527a7f655f0325374a5d483ed4459f465a2f7f9d3ac9a23a548eac5c4',
  txExecErrorMsg: null
}

The example directly sending transaction for demostration purpose. If the contract constructor contains parameters, a better way is to use smart contract development tools as conflux-truffle (for Conflux Core) and hardhat or truffle (for Conflux eSpace) to develop, compilate and deploy process.

Calling smart contract functions

After the contract is deployed you can interact with the functions. There are two ways:

• Calling through the RPC cfx_call (for Conflux Core) or eth_call (for Conflux eSpace): this type of contract calling is only executed in the EVM but does not actually change the state. This is usually used to call read-only contract functions or to simulate the execution of a transaction to see if it can be executed successfully.
• Sending a transaction: this type of contract calling will change the contract state when executed.

The data used when calling the contract is generated by ABI encoding based on the function information described by the ABI. The first 4 bytes are the function selector (the first 4 bytes of the Keccak (SHA-3) hash of the function signature), and the fifth byte starts with the ABI-encoded parameter.

We will use js-conflux-sdk to demonstrate.

const { Conflux } = require("js-conflux-sdk");

(async function () {
    const cfx = await Conflux.create({ url: "https://test.confluxrpc.com", logger: console })
    const me = cfx.wallet.addPrivateKey("0x2139FB4C55CB9AF7F0086CD800962C2E9013E2292BAE77978A9209E3BEE71D49")

    const abi = [...]
    const contract = cfx.Contract({ address: "CFXTEST:TYPE.CONTRACT:ACCYSPEUUR469BCA0EXRFXKXMKX651W45JFW2RN5M0", abi })
    let myBalance = await contract.balanceOf(me.address)
    console.log("my cupcake balance :", myBalance)

    const receipt = await contract.purchase(2).sendTransaction({ from: me.address, value: 2e18 }).executed()
    console.log("purchase receipt", receipt)

    const event = contract.abi.decodeLog(receipt.logs[0])
    console.log("purchase event:", event)

    myBalance = await contract.balanceOf(me.address)
    console.log("after purchase, my cupcake balance :", myBalance)
})()

From the log, we can see that the corresponding RPC information of contract.cupcakeBalances(me.address) is as follows.

{
  data: {
    jsonrpc: '2.0',
    id: '1794b9e755639b5164925a8e',
    method: 'cfx_call',
    params: [
      {
        to: 'CFXTEST:TYPE.CONTRACT:ACCYSPEUUR469BCA0EXRFXKXMKX651W45JFW2RN5M0',
        data: '0xe18a7b9200000000000000000000000019f4bcf113e0b896d9b34294fd3da86b4adf0302'
      },
      undefined
    ]
  },
  result: '0x0000000000000000000000000000000000000000000000000000000000000000',
  duration: 33
}

rpc method is cfx_call, data is the result of function selector + ABI-encoded result of parameter list. The first 4 bytes 0xe18a7b92 is the function selector of function balanceOf. The calculation takes keccak operation keccak256("balanceOf(address)") on the signature balanceOf(address) of balanceOf and then takes the first 4 bytes. 00000000000000000000000019f4bcf113e0b896d9b34294fd3da86b4adf0302 is the ABI-encoded value of paramter 0x19f4bcf113e0b896d9b34294fd3da86b4adf0302.

The returned value is 0x0000000000000000000000000000000000000000000000000000000000000000 wich is the result of ABI-encoded value 0 with the uint256 type.

The RPC method for purchase is cfx_sendRawTransaction, which is sending transaction. This will change the state of the contract. The encoding method for the transaction's data is also function selector + ABI-encoded result of parameter list. You can check this through getTransactionByHash.

{
  "jsonrpc": "2.0",
  "result": {
    "data": "0xefef39a10000000000000000000000000000000000000000000000000000000000000002",
    "hash": "0x2c188c67247d7e2bf12fb96f17ced61da8ea4143447887a10a2cc597c1fa66e1",
    "to": "CFXTEST:TYPE.CONTRACT:ACCYSPEUUR469BCA0EXRFXKXMKX651W45JFW2RN5M0",
    "value": "0x1bc16d674ec80000"
    ...
  },
  "id": 1
}

We can see a record from the logs field of transaction receipt. logs indicate the event that happened in the transaction.

purchase receipt {
  blockHash: '0x3d4111b299e65c279184aa83021e59f9d134baa9c78969dd6d94a92bfbd340df',
  contractCreated: null,
  epochNumber: 27677382,
  from: 'CFXTEST:TYPE.USER:AAP9KTHVCTUNVF030RBKK9K7ZBZYZ12DAJP1U3SP4G',
  ......
  logs: [
    {
      address: 'CFXTEST:TYPE.CONTRACT:ACCYSPEUUR469BCA0EXRFXKXMKX651W45JFW2RN5M0',
      data: '0x00000000000000000000000019f4bcf113e0b896d9b34294fd3da86b4adf03020000000000000000000000000000000000000000000000000000000000000002',
      topics: [Array]
    }
  ],
}

When analysing the result:

It indicates that one Purchase event happened, customer is cfxtest:aap9kthvctunvf030rbkk9k7zbzyz12dajp1u3sp4g, quantity is 2.

After purchase is complete, cupcakeBalances[0x19f4bcf113e0b896d9b34294fd3da86b4adf0302] changed from 0 to 2. State has changed.

{
  data: {
    jsonrpc: '2.0',
    id: '179a19eb98ed23dda1d1d516',
    method: 'cfx_call',
    params: [ [Object], undefined ]
  },
  result: '0x0000000000000000000000000000000000000000000000000000000000000002',
  duration: 30
}

Attention: When deploying or calling contracts on the Conflux Core, if new storage space is occupied in the contract, some CFX will be collateralized for the occupied space; this will be refunded after the storage is released. For more information, please visit storage collateral mechanism of Conflux Core.

Dev spaces

Remember that when developing your smart contract and deploying it on the Conflux network you must select one of the two development Conflux spaces:

• Conflux Core
• Conflux eSpace.

Conflux eSpace is 100% compatible with the EVM ecosystem and you can use tools like Remix, Hardat, MetaMask and services like Unifra.

Additional resources

In addition to Solidity you can use also Vyper for developing smart contracts.

• Solidity
• Vyper

Other

• Conflux Basics
• Ethereum's introduction of smart contract