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Counter Example¶

A simple counter contract demonstrating basic state management.

Contract¶

contract Counter {
    uint256 public count;
    address public owner;

    event Incremented(address indexed by, uint256 newValue);
    event Decremented(address indexed by, uint256 newValue);
    event Reset(address indexed by);

    error Underflow();

    modifier onlyOwner() {
        require(msg.sender == owner, "Not owner");
        _;
    }

    constructor() {
        owner = msg.sender;
        count = 0;
    }

    function increment() public {
        count += 1;
        emit Incremented(msg.sender, count);
    }

    function decrement() public {
        if (count == 0) revert Underflow();
        count -= 1;
        emit Decremented(msg.sender, count);
    }

    function reset() public onlyOwner {
        count = 0;
        emit Reset(msg.sender);
    }

    function incrementBy(uint256 amount) public {
        count += amount;
        emit Incremented(msg.sender, count);
    }

    function getCount() public view returns (uint256) {
        return count;
    }
}

Build¶

solscript build counter.sol -o ./build

Test¶

import * as anchor from "@coral-xyz/anchor";
import { expect } from "chai";

describe("Counter", () => {
  const provider = anchor.AnchorProvider.env();
  anchor.setProvider(provider);

  it("initializes to zero", async () => {
    // Test initialization
  });

  it("increments correctly", async () => {
    // Test increment
  });

  it("decrements correctly", async () => {
    // Test decrement
  });

  it("prevents underflow", async () => {
    // Test underflow protection
  });

  it("only owner can reset", async () => {
    // Test access control
  });
});

Key Concepts¶

State variables: count and owner
Events: Track state changes
Modifiers: Access control with onlyOwner
Custom errors: Gas-efficient error handling

See Also¶