How do Solidity structs work?

Let’s dive into a Solidity-specific data structure, not typically found in other programming languages like arrays are… 🤨

Structs

Up until now, we’ve looked at all of the defined data types in Solidity. One super cool feature about Solidity is it allows you to define your own custom data type 🤯.

You can define your own custom data type by creating a struct. Struct types are typically used to represent a record, or a grouping of common data.

The syntax to declare a struct in Solidity resembles an object declaration in JavaScript:

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struct structName {
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   type1 typeName1;
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   type2 typeName2;
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   type3 typeName3;
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}

Super easy! 💯 Let’s look at a specific use case for structs to help us further understand the concept..

Struct Use Cases: Library Record Keeping 📚

What if we wanted to have an immutable record of all the library books we cared about? 🤔 Keeping our book records on a centralized server somewhere might not be ideal, since it might be taken down or corrupted by the server admin. In come smart contracts! 💥

We can create a smart contract called Library and equip it with all the functionality we’d need to perform detailed record keeping for books. For now, we can just work off of a single assumption / user story:

- For each book record I add to my Library smart contract, I want to keep track of its title, author and some sort of id for internal record-keeping

The above assumption would mean lines and lines of code… if we didn’t have Solidity structs. 😉

So, let’s define a smart contract called Library and define a struct called Book inside its scope. The Book struct should keep track of all the properties we listed in the assumption above:

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contract Library {
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    struct Book {
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        string title;
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        string author;
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        uint bookId;
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    }
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}

Great! We’ve got the base setup done. Any further functionality will be structured around this same Book struct and its properties.

A Book struct is specific to one book… so we need a way to keep track of many of the same type… anything ring a bell here? 🔔 That’s right, we can use an array. Let’s add it to our Library:

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contract Library {
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    struct Book {
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        string title;
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        string author;
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        uint bookId;
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    }
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    Book[] public books;
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}

Sweet! An array here will make it super easy to keep indexed track of each Book struct, along with each book’s specific properties.

What if we want to add a book? We can do so with a function addBook():

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contract Library {
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    struct Book {
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        string title;
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        string author;
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        uint bookId;
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    }
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    Book[] public books;
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    function addBook(string memory _title, string memory _author) public {
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        books.push(Book(_title, _author, books.length));
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    }
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}

Ok, let’s break down the addBook() function real quick:

• The string parameters use memory. This is a requirement of Solidity whenever you use reference types as a parameter, you must precede the parameter with either memory or calldata. This is just telling the Solidity compiler where the data being passed in lives. Since this is an ephemeral call, we are passing in the value from memory.

• We use books.push() in order to push a brand new Book struct

• We initialize the struct by calling it exactly like you would a function: Book(_param1, param2, ...) - that is the easiest one but you can also initialize structs like so:

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    // key value mapping
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    todos.push(Todo({text: _text, completed: false}));
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    // initialize an empty struct in local memory and then update it
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    Todo memory todo;
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    todo.text = _text;

Ok great! We can now add new books to our library and push em to the books array for record-keeping. What if we want to retrieve a book from this record? We can add a simple getter and retrieve based on the bookId:

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contract Library {
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    struct Book {
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        string title;
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        string author;
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        uint bookId;
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    }
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    Book[] public books;
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    function addBook(string memory _title, string memory _author) public {
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        books.push(Book(_title, _author, books.length));
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    }
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    function get(uint _bookId) public view returns (string memory _title, string memory _author) {
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        return(books[_bookId].title, books[_bookId].author);
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    }
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}

Awesome! Now anyone can pass in a book id and the smart contract will return that book id’s title and author! 💃

One final function to add before we wrap up the awesomeness of structs is update(), that way we can update a book title or author for any reason (mainly for an easy example, since we probably wouldn’t want to update such a record after it’s been sealed!).

But wait!

One more thing… notice how all of our functions are public? This means anyone could mess with our cool library and we don’t want that! Let’s make it so that each Book holds an address property, and ONLY that specific address can perform certain changes in the contract:

Let’s add a registrant of type address on the Book struct. Let’s also add an update() function:

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contract Library {
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    struct Book {
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        string title;
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        string author;
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        uint bookId;
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        address registrant;
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    }
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    Book[] public books;
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    function addBook(string memory _title, string memory _author) public {
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        // whoever adds a book, that is the registrant on this book
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        books.push(Book(_title, _author, books.length, msg.sender));
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    }
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    function get(uint _bookId) public view returns (string memory _title, string memory _author) {
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        return(books[_bookId].title, books[_bookId].author);
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    }
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    function update(uint _bookId, string memory _newTitle, string memory _newAuthor) public {
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        // protect our book record by only making
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        // this function available to the original registrant
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        require(msg.sender == books[_bookId].registrant, 'You must have been the one to add the book to change the record!');
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        books[_bookId].title = _newTitle;
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        books[_bookId].author = _newAuthor;
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    }
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}

Amazing. We’ve just coded a whole contract that has soooo much functionality in so few lines! 🤯 Plus, it protects our records by checking the msg.sender in the update() function! 🔐

Suggested Reading

• Solidity by Example - Structs
• Open Source Project By AU Student

Conclusion

Structs are super useful in Solidity. They are a custom data type… so they are fitting for any custom record-keeping needs you might think of! Another example would be a Player struct that keeps track of that player’s address, level, etc…

Want to see a 15-minute run-through of Structs using the same Library example as above? Here’s Al covering Structs at DevConnect 2022 Amsterdam? 👀 📽

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