Solidity

Enum (Enumeration)

• An enum is a user-defined type whose value must be one of a fixed list of named constants.
• It gives meaningful names to a small set of related values, for example the states of an order: Placed, Shipped, Delivered, Cancelled.
• Syntax: enum <Name> { Member1, Member2, ... } – no semicolon after the closing brace.
• An enum can be declared inside a contract or at file level (outside any contract) so several contracts can share it.

Rules You Must Know

Benefits of Using Enumerations

• Readability – Status.Shipped is clearer than the number 1.
• Fewer errors – a variable can only hold valid members, so mistyped or out-of-range numbers are rejected.
• Easier maintenance – names are used throughout the code, so adding a state is simpler.
• Ideal for state machines – contracts often move through stages (open → closed → paid) and enums make each stage explicit.
• Gas-efficient – stored as a small integer.

Example : Order Status State Machine

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

contract OrderTracker {
    enum Status { Placed, Shipped, Delivered, Cancelled }   // 0, 1, 2, 3

    Status public status;          // default value: Status.Placed (0)

    function ship() external {
        require(status == Status.Placed, "Order not in Placed state");
        status = Status.Shipped;
    }

    function deliver() external {
        require(status == Status.Shipped, "Order not shipped yet");
        status = Status.Delivered;
    }

    function cancel() external {
        require(status == Status.Placed, "Too late to cancel");
        status = Status.Cancelled;
    }

    function statusNumber() external view returns (uint) {
        return uint(status);       // explicit conversion to an integer
    }

    function setByNumber(uint n) external {
        status = Status(n);        // reverts if n > 3
    }

    function lastStatus() external pure returns (Status) {
        return type(Status).max;   // Status.Cancelled
    }
}

Key Takeaways

The PDF E-Notes are the earlier printable version of this chapter. The notes on this page are the latest, revised version.

Video : Enums in Solidity

Resource File

• Chapter 11 : Solidity File

Question / Answer (15 questions)

MCQ Test : Check Your Understanding (5 questions)

Select an option and press Check Answer, or answer all questions and press Submit Test to see your score. Each answer comes with an explanation.

Bibliography/References

The notes, questions and MCQs on this page are original explanations written for this course. The works listed below were consulted and are cited for attribution and further reading. Product names, logos and trademarks belong to their respective owners. Short quotations, where used, are marked and attributed.

1. Solidity Team. (n.d.). Types (value types, reference types, mappings, operators, conversions). Solidity Documentation. https://docs.soliditylang.org/en/latest/types.html
2. Solidity Team. (n.d.). Cheatsheet. Solidity Documentation. https://docs.soliditylang.org/en/latest/cheatsheet.html
3. Microsoft Learn. (n.d.). When to Use an Enumeration (Visual Basic programming guide). https://learn.microsoft.com/en-us/dotnet/visual-basic/programming-guide/language-features/constants-enums/when-to-use-an-enumeration
4. Modi, R. (2018). Solidity Programming Essentials: A beginner’s guide to build smart contracts for Ethereum and blockchain. Packt Publishing. ISBN 978-1-78883-138-3.