Unit
- Definition: A smart contract is a self-executing program stored on a blockchain that automatically enforces and executes the terms of an agreement between parties when predefined conditions are met.
- No Intermediaries: Unlike traditional contracts, smart contracts do not require lawyers or notaries because the code itself governs the agreement, reducing costs and delays.
- Immutability: Once deployed, the contract code cannot be modified. This ensures that the terms remain unchanged, but it also means bugs cannot be patched without deploying a new contract.
- Decentralized Execution: Smart contracts run on a network of nodes (e.g., Ethereum), so no single entity can alter the outcome. The result is transparent and tamper-proof.
- EVM Compatibility: Ethereum pioneered Turing-complete smart contracts via the Ethereum Virtual Machine (EVM). Other blockchains like Polygon and BNB Chain also support EVM-compatible contracts.
- State Persistence: Contract state is stored on-chain, making every action auditable. This provides a permanent record of all transactions and changes.
- Gas Fees: Executing contract operations requires gas, a fee paid in cryptocurrency. Efficient coding reduces gas costs and improves usability.
- Cryptocurrency Handling: Smart contracts can hold and transfer native tokens based on conditions, enabling decentralized finance (DeFi) applications like lending and exchanges.
- Composability: Contracts can call other contracts, allowing complex dApps built from interoperable modules.
- Oracle Dependency: Smart contracts cannot access off-chain data (e.g., stock prices) natively. They rely on oracles to feed external information, introducing trust assumptions.
- Development Process: Developers write code in Solidity, compile it to bytecode, and deploy using frameworks like Hardhat or Truffle. Testing on testnets is essential before mainnet release.
- Events and Logging: Smart contracts emit events that are stored in transaction logs. Off-chain applications subscribe to these events to track state changes efficiently. Events are cheaper than storing data directly and enable seamless communication between on-chain logic and user interfaces.
- Access Control: Many contracts implement access control using modifiers like onlyOwner to restrict sensitive functions. OpenZeppelin provides audited contracts for role-based permissions, preventing unauthorized calls and reducing the risk of malicious actions by untrusted users.
- Upgradeability Patterns: Because deployed contracts are immutable, upgradeability patterns like proxy contracts separate logic and data. A proxy delegatecalls to a new implementation, allowing bug fixes without losing contract state, though this introduces additional complexity and trust considerations.
- Reentrancy Protection: Smart contracts are vulnerable to reentrancy attacks where external calls trigger recursive calls before state updates. Using the checks-effects-interactions pattern or OpenZeppelin’s ReentrancyGuard prevents these exploits and ensures contract safety.
Code Example
Solidity
pragma solidity ^0.8.0;
contract SimpleStorage {
uint256 private storedData;
function set(uint256 x) public {
storedData = x;
}
function get() public view returns (uint256) {
return storedData;
}
}This contract stores a single unsigned integer. The set() function updates the value, and get() retrieves it. Both functions are public, but state variable storedData is private to encapsulate state.
Quick Revision
- Smart contracts are self-executing programs on a blockchain that automate agreements.
- They are immutable, decentralized, and transparent, eliminating intermediaries.
- Gas fees compensate network nodes for execution; efficient code reduces costs.
- Contracts can hold cryptocurrency and call other contracts, enabling composable dApps.
- Oracles provide external data, but introduce centralization risks.