Eridian
eridian.xyzx.comGitHub
  • 📖Eridian Docs
  • Ethereum Dev
    • ✏️Ethereum Notes
      • 🎛️Technical Basics
      • 🪧Ethereum Addresses
      • 📚Ethereum State Explained
      • ⛽Gas Fees Explained
    • 🔧Useful Tools
      • ☁️Ethers
      • *️⃣Ethernal
    • 📝Solidity Notes
      • ❔Interview Questions
        • 🟢1. Easy - Interview Questions
        • 🟠2. Medium - Interview Questions
        • 🟡3. Hard - Interview Questions
        • 🔴4. Advanced - Interview Questions
      • 💡Note Ideas
      • ABI
      • abi.encodePacked
      • Abstract Contracts
      • Arrays
      • Casting
      • CEI - Checks, Effects, Interactions
      • Comments (NATSPEC)
      • Constructor
      • Contract Structure & Versions
      • Data - Storage vs Memory
      • Data - Storage Layout
      • Enum
      • Errors (require & revert)
      • Events
      • EVM Opcodes
      • External Contract Interaction
      • 🏗️External Dependencies
      • Functions
      • Function Modifiers
      • If / Else / For / While Loops
      • Inheritance
      • Interfaces
      • Keccak256
      • Library
      • Mappings
      • msg.sender
      • Objects & Types
      • OpenZeppelin
      • Payable
      • Public State Variable vs Function
      • Receive & Fallback
      • Security
      • Self Destruct
      • Send ETH (transfer, send, call)
      • Stack Too Deep
      • Structs
      • Style Guide
      • Time Units
      • Try / Catch
      • Typecasting
      • Using Directive
      • Variables, Consts & Immutable
      • Withdraws
    • ⚒️Foundry Notes
      • 📖Docs & GitHub Pages
      • 🤝Useful Commands
        • 🔨Anvil
        • 🪄Cast
        • 🔥Forge
      • 🧪Tests
        • Cheatcodes
      • 📝Useful Scripts
        • Deploy Contract Using Hex
    • 👾DeFi Challenges
      • 👨‍🚀Ethernaut
        • Ethernaut - Template
        • Level 1 - Fallback ⏺
        • Level 2 - Fallout ⏺
        • Level 3 - Coin Flip ⏺⏺
        • Level 4 - Telephone ⏺
        • Level 5 - Token ⏺⏺
        • Level 6 - Delegation ⏺⏺
        • Level 7 - Force ⏺⏺⏺
        • Level 8 - Vault ⏺⏺
        • Level 9 - King ⏺⏺⏺
        • Level 10 - Re-entrancy ⏺⏺⏺
        • Level 11 - Elevator ⏺⏺
        • Level 12 - Privacy ⏺⏺⏺
        • Level 13 - Gatekeeper 1 ⏺⏺⏺⏺
        • Level 14 - Gatekeeper 2 ⏺⏺⏺
        • Level 15 - Naught Coin ⏺⏺⏺
        • Level 16 - Preservation ⏺⏺⏺⏺
        • Level 17 - Recovery ⏺⏺⏺
        • Level 18 - Magic Number ⏺⏺⏺
        • Level 19 - Alien Codex ⏺⏺⏺⏺
        • Level 20 - Denial ⏺⏺⏺
        • Level 21 - Shop ⏺⏺
        • Level 22 - Dex ⏺⏺
        • Level 23 - Dex Two ⏺⏺
        • Level 24 - Puzzle Wallet ⏺⏺⏺⏺
        • Level 25 - Motorbike ⏺⏺⏺
        • Level 26 - DoubleEntryPoint ⏺⏺
        • Level 27 - Good Samaritan ⏺⏺⏺
        • Level 28 - Gatekeeper 3 ⏺⏺⏺
        • Level 29 - Switch ⏺⏺⏺⏺
        • Level 30 - Higher Order ⏺⏺⏺⏺
        • Level 31 - Stake ⏺⏺⏺
      • 💸Damn Vulnerable DeFi
    • 🔍Auditing
      • 🗞️Exploit Resources
      • 🔧Audit Tools
    • 🤖MEV
  • Infrastructure Docs
    • 💻Hardware
    • 🐧Linux
      • 📖Linux Glossary
      • ⌨️Linux Commands
      • 💾Installation
      • 🏗️Maintenance
      • 🖥️Ubuntu Desktop
      • 🛜ZeroTier
      • 🎞️TMUX
      • 🔵Bluetooth
    • ⛓️Ethereum Clients
      • ⚙️Execution Clients
        • ⛏️Geth
          • 💾Installation
          • ⌨️Useful Commands
          • 🏗️Maintenance
        • 🐻Erigon
          • 💾Installation
          • ⌨️Useful Commands
          • 🏗️Maintenance
        • 🧶Besu
          • 💾Installation
          • ⌨️Useful Commands
          • 🏗️Maintenance
      • 🤝Beacon Clients
        • 💡Lighthouse
          • 💾Installation
          • ⌨️Useful Commands
          • 🏗️Maintenance
        • 🪅Teku
          • 💾Installation
          • ⌨️Useful Commands
          • 🏗️Maintenance
      • 💎Validator Clients
        • 💡Lighthouse
          • 💾Installation
          • ⌨️Useful Commands
          • 🏗️Maintenance
      • ➕L2 Clients
        • 🔵Base
          • 💾Installation
          • ⌨️Useful Commands
          • 🏗️Maintenance
      • 💰MEV Boost
        • 💾Installation
        • 🏗️Maintenance
    • 🚨Alerting and Monitoring
      • 🔥Prometheus
      • 🌡️HealthChecks.io
      • 📟PagerDuty
  • General Dev
    • 💾Git Notes
      • Repos
      • Committing changes
      • Branches
      • Merging & Rebasing
      • PRs
Powered by GitBook
On this page
  • Declaring and Importing Struct
  • Passing structs as arguments
  • Struct packing to save gas
  • Example
Edit on GitHub
  1. Ethereum Dev
  2. Solidity Notes

Structs

  • Structs can be declared outside of a contract and imported in another contract.

  • Sometimes you need a more complex data type.

  • For this, Solidity provides structs, allowing you to create more complicated data types that have multiple properties.

struct People {
  uint256 favoriteNumber;
  string name;
}

// create a New Person:
People public person = People({favoriteNumber: 7, name: "Eridian"});

Declaring and Importing Struct

File that the struct is declared in
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;
// This is saved as 'StructDeclaration.sol'

struct Todo {
    string text;
    bool completed;
}
File that imports the struct above
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;

import "./StructDeclaration.sol";

contract Todos {
    // An array of 'Todo' structs
    Todo[] public todos;
}

Passing structs as arguments

  • You can pass a storage pointer to a struct as an argument to a private or internal function.

  • This is useful, for example, for passing around our Zombie structs between functions.

function _doStuff(People storage _person) internal {
  // do stuff with _person
}
  • This way we can pass a reference to our person into a function instead of passing in a person ID and looking it up.

Struct packing to save gas

  • There are other types of uints: uint8, uint16, uint32, etc.

  • Normally there's no benefit to using these sub-types because Solidity reserves 256 bits of storage regardless of the uint size.

    • For example, using uint8 instead of uint (uint256) won't save you any gas.

  • But there's an exception to this: inside structs.

  • If you have multiple uints inside a struct, using a smaller-sized uintwhen possible will allow Solidity to pack these variables together to take up less storage.

struct NormalStruct {
  uint a;
  uint b;
  uint c;
}

struct MiniMe {
  uint32 a;
  uint32 b;
  uint c;
}

// `mini` will cost less gas than `normal` because of struct packing
NormalStruct normal = NormalStruct(10, 20, 30);
MiniMe mini = MiniMe(10, 20, 30);
  • For this reason, inside a struct you'll want to use the smallest integer sub-types you can get away with.

  • You'll also want to cluster identical data types together (i.e. put them next to each other in the struct) so that Solidity can minimize the required storage space.

  • For example, a struct with fields uint c; uint32 a; uint32 b; will cost less gas than a struct with fields uint32 a; uint c; uint32 b; because the uint32 fields are clustered together.

Example

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;

contract Todos {
    struct Todo {
        string text;
        bool completed;
    }

    // An array of 'Todo' structs
    Todo[] public todos;

    function create(string calldata _text) public {
        // 3 ways to initialize a struct
        // - calling it like a function
        todos.push(Todo(_text, false));

        // key value mapping
        todos.push(Todo({text: _text, completed: false}));

        // initialize an empty struct and then update it
        Todo memory todo;
        todo.text = _text;
        // todo.completed initialized to false

        todos.push(todo);
    }

    // Solidity automatically created a getter for 'todos' so you don't actually need this function.
    function get(uint _index) public view returns (string memory text, bool completed) {
        Todo storage todo = todos[_index];
        return (todo.text, todo.completed);
    }

    // update text
    function updateText(uint _index, string calldata _text) public {
        Todo storage todo = todos[_index];
        todo.text = _text;
    }

    // update completed
    function toggleCompleted(uint _index) public {
        Todo storage todo = todos[_index];
        todo.completed = !todo.completed;
    }
}

Last updated 1 year ago

📝
https://solidity-by-example.org/structs/