> For the complete Mojo documentation index, see [llms.txt](/llms.txt). > Markdown versions of all pages are available by appending .md to any URL (e.g. /docs/manual/basics.md). # Mojo structs A struct is Mojo's primary way to define your own type. When you want to model both data and behavior—whether that's a small value type, a numeric abstraction, or the foundation of a larger system—you'll use a struct. At a high level, a Mojo struct lets you bundle data together with the operations that act on that data. This makes structs a natural way to represent concepts in your program, rather than passing loosely related values through functions. Each Mojo `struct` is a data structure that lets you encapsulate _fields_ and _methods_ to store and operate on data. Structs can define the following members: - **Fields** are variables that store data relevant to the struct. - **Methods** are functions defined in a struct that normally act upon the field data. - **Static methods** are functions provided by the type to perform behaviors, provide constants, or create specialized instances. - **Dunder methods** are named for their _d_ouble _under_-scored form, with `__` on both sides. Also called "special methods", [they help define behaviors](/docs/manual/structs/#special-methods) such as initialization and allow structs to conform to [traits](/docs/manual/traits/). - **`comptime` members** enable compile-time references that can be used for optimization. For example, if you're building a graphics program, you can use a struct to define an `Image` that has fields to store information about each image (such as its component pixels) and methods that perform actions on it (such as rotating the image). Mojo's struct format is designed to provide a static, memory-safe data structure that's both powerful and performant. Unlike dynamic objects (such as Python classes) that can be modified freely at runtime, structs are defined at compile time, which allows Mojo to generate highly optimized code. All struct fields must be declared using `var` and include a type annotation. This requirement is part of Mojo's compile-time guarantees, helping ensure both performance and memory safety. ## Struct definition You can define a simple struct called `MyPair` with two fields like this: ```mojo struct MyPair: var first: Int var second: Int ``` However, you can't instantiate this struct because it has no constructor method. So here it is with a constructor to initialize the two fields: ```mojo struct MyPair: var first: Int var second: Int def __init__(out self, first: Int, second: Int): self.first = first self.second = second ``` Notice that the first argument in the `__init__()` method is `out self`. You'll have a `self` argument as the first argument on all struct methods. It references the current struct instance (it allows code in the method to refer to "itself"). _When you call the constructor, you never pass a value for `self`—Mojo passes it in automatically._ The `out` portion of `out self` is an [argument convention](/docs/manual/values/ownership#argument-conventions) that declares `self` as a mutable reference that starts out as uninitialized and must be initialized before the function returns. Many types use a field-wise constructor like the one shown for `MyPair` above: it takes an argument for each field, and initializes the fields directly from the arguments. To save typing, Mojo provides a [`@fieldwise_init`](/docs/reference/decorators/fieldwise-init/) decorator, which generates a field-wise constructor for the struct. So you can rewrite the `MyPair` example above like this: ```mojo @fieldwise_init struct MyPair: var first: Int var second: Int ``` The `__init__()` method is one of many [special methods](#special-methods) (also known as "dunder methods" because they have *d*ouble *under*scores) with pre-determined names. :::note You can't assign values when you declare fields. You must initialize all of the struct's fields in the constructor. (If you try to leave a field uninitialized, the code won't compile.) ::: ## Constructing a struct type Once you have a constructor, with `__init__` or using `@fieldwise_init`, you can create an instance of `MyPair` and set the fields: ```mojo title="Construct an instance" var mine = MyPair(2, 4) print(mine.first) ``` ```output 2 ``` :::note Initializer lists Mojo initializer lists let you construct instances without spelling out the full type name and parameters. If the full type can be inferred from context, pass the constructor arguments directly between braces, with or without keywords. For example `{0.5, fish="salmon"}` calls `__init__(0.5, fish="salmon")` on the appropriate struct type. This is equivalent to `MyStruct(0.5, fish="salmon")` if the type is inferred to be `MyStruct`. ::: ## Mutating a struct By default, a struct's methods receive an immutable `self`, so they can't modify the struct's fields. For example: ```mojo struct MyStruct: var value: Int def increment(self): self.value += 1 # ERROR: expression must be mutable in assignment # ... ``` To allow a method to mutate the instance, declare its receiver as `mut self`. This makes `self` mutable inside the method and allows changes to its fields that persist after the method returns: ```mojo struct MyStruct: var value: Int def increment(mut self): self.value += 1 # Works: Mutable `self` allows assignment #... ``` :::info Read more about mutable arguments and the `mut` keyword: [Mutable arguments (`mut`)](/docs/manual/values/ownership/#mutable-arguments-mut) ::: ## Making a struct Copyable {#making-a-struct-copyable-and-movable} Mojo structs are not copyable or movable by default. For example, the following code produces errors: ```mojo var a = MyPair(1, 2) # Implicit copy var b = a # value of type 'MyPair' cannot be implicitly copied, # it does not conform to 'ImplicitlyCopyable' # Explicit copy var c = a.copy() # value of type 'MyPair' cannot be implicitly copied, # it does not conform to 'ImplicitlyCopyable' # Move var d = a^ # value of type 'MyPair' cannot be copied or moved; consider # conforming it to 'Copyable', which also adds 'Movable' # conformance. ``` In most cases, you can make a struct copyable (and movable) just by adding the `Copyable` [trait](/docs/manual/traits/). ### Movability To make a struct move-only, add the `Movable` trait: ```mojo struct MyPair(Movable): ... ``` Mojo will generate a move constructor for you. You rarely need to write your own custom move constructor. For more information, see the section on [move constructors](/docs/manual/lifecycle/life/#move-constructor). ### Copyability To make a struct copyable, add the `Copyable` trait: ```mojo struct MyPair(Copyable): ... ``` In most cases, that's all you need to do. Mojo will generate a copy constructor (`__init__(out self, *, copy: Self)` method) for you. You don't need to write your own unless you need custom logic in the copy constructor; for example, if your struct dynamically allocates memory. For more information, see the section on [copy constructors](/docs/manual/lifecycle/life/#copy-constructor). The `Copyable` trait provides two ways to copy a value: the `copy()` instance method and the copy initializer. In addition, `Copyable` automatically adds movability, so you won't need to add both `Copyable` _and_ `Movable` to your declarations. ### Implicit copyability To make a struct implicitly copyable, add the `ImplicitlyCopyable` trait: ```mojo struct MyPair(ImplicitlyCopyable): ... ``` `ImplicitlyCopyable` automatically implies `Copyable` and `Movable`, so all the notes related to copyability apply here. A type should only be implicitly copyable if copying the type is inexpensive and has no side effects. Unnecessary copies can be a big drain on memory and performance, so use this trait with caution. ## Fields Fields store a struct's data. When you declare a field, it becomes part of the struct's memory layout. Because the compiler knows every field's type at compile time, it can: - Calculate the struct's exact memory footprint - Ensure all fields are initialized before use - Generate fast, direct access to field data - Prevent changes to the struct's layout at runtime Fields share the lifetime of their struct instance. They are created when the struct is created and destroyed when the struct is destroyed. This model avoids dangling references and partially constructed objects. Outside of your struct implementation, you access fields with dot notation (`my_struct.field_name`). Within the struct, your methods access fields using `self` (`self.field_name`). Mojo knows each field's location at compile time, making field access direct and efficient. ### Field requirements **You must** declare field members with `var` in structs: ```mojo struct MyStruct: value: Int # Error. Missing `var` keyword var count: Int # Yes ``` Unlike local variables in functions, this requirement lets Mojo reason about a struct's layout and guarantees that its memory is safe and predictable. **You must** use unique symbols for fields, methods, or `comptime` members. These all exist in the same namespace: ```mojo struct MyStruct: var count: Int var count: String # Error. Invalid redeclaration of `count` ``` **You can** re-use a struct member's name for an argument or method variable. ```mojo struct MyStruct: var foo: Int def use_argument(self, foo: Int): # Argument shadows field print(foo) # Prints argument value def use_local(self, value: Int): var foo = value # Local variable shadows field print(foo, self.foo) # Prints local, then field ``` **You must** mark `self` as mutable if updating a field value. ```mojo struct MyStruct: var foo: Int def update_foo(mut self, new_value: Int): self.foo = new_value ``` **You must** initialize fields within constructors, and not at the point of declaration. ```mojo struct MyStruct: var foo: Int = 10 # Error: Unknown tokens comptime bar = 10 # Yes ``` [`comptime` members](/docs/manual/parameters/#comptime-members) are compile-time constants (not fields) and don't occupy instance storage, so they can be initialized at the point of declaration. ### Field conventions Like other Mojo elements, fields normally adhere to [certain conventions](https://github.com/modular/modular/blob/main/mojo/stdlib/docs/style-guide.md#code-conventions): You should use conventional naming for field members: - Prefer lowercase snake_case for field names (for example, `user_count`, `max_capacity`). - Use descriptive names that indicate purpose, and not their type (for example, `error_msg` not `msg_string`). - For members meant for internal use or to maintain invariants, add an underscore prefix (for example, `_private_field`). - For boolean fields, use `is_` or `has_` prefixes (for example, `is_valid`, `has_data`). - Avoid single-letter names except for common mathematical conventions (such as `x`, `y`, `z` for coordinates). ## Methods In addition to special methods like `__init__()`, you can add any other method you want to your struct. For example: ```mojo @fieldwise_init struct MyPair: var first: Int var second: Int def get_sum(self) -> Int: return self.first + self.second ``` ```mojo var mine = MyPair(6, 8) print(mine.get_sum()) ``` ```output 14 ``` Notice that `get_sum()` also uses the `self` argument, because this is the only way you can access the struct's fields in a method. The name `self` is just a convention, and you can use any name you want to refer to the struct instance that is always passed as the first argument. Methods that take the implicit `self` argument are called _instance methods_ because they act on an instance of the struct. :::note The `self` argument in a struct method is the only argument in a `def` function that does not require a type. You can include the type if you want, but you can elide it because Mojo already knows its type (`MyPair` in this case). ::: ### Static methods A struct can have _static methods_. A static method can be called without creating an instance of the struct. Unlike instance methods, a static method doesn't receive the implicit `self` argument, so it can't access any fields on the struct. To declare a static method, use the `@staticmethod` decorator and don't include a `self` argument: ```mojo struct Logger: def __init__(out self): pass @staticmethod def log_info(message: String): print("Info: ", message) ``` You can invoke a static method by calling it on the type (in this case, `Logger`). You can also call it on an instance of the type. Both forms are shown below: ```mojo Logger.log_info("Static method called.") var l = Logger() l.log_info("Static method called from instance.") ``` ```output Info: Static method called. Info: Static method called from instance. ``` ## Structs compared to classes If you're familiar with other object-oriented languages, then structs might sound a lot like classes, and there are some similarities, but also some important differences. Eventually, Mojo will also support classes to match the behavior of Python classes. So, let's compare Mojo structs to Python classes. They both support methods, fields, operator overloading, decorators for metaprogramming, and more, but their key differences are as follows: - Python classes are dynamic: they allow for dynamic dispatch, monkey-patching (or "swizzling"), and dynamically binding instance fields at runtime. - Mojo structs are static: they are bound at compile-time (you cannot add methods at runtime). Structs allow you to trade flexibility for performance while being safe and easy to use. - Mojo structs do not support inheritance ("sub-classing"), but a struct can implement [traits](/docs/manual/traits/). - Python classes support class attributes—values that are shared by all instances of the class, equivalent to class variables or static data members in other languages. - Mojo structs don't support static data members. Syntactically, the biggest difference compared to a Python class is that all fields in a struct must be explicitly declared with `var`. In Mojo, the structure and contents of a struct are set at compile time and can't be changed while the program is running. Unlike in Python, where you can add, remove, or change attributes of an object on the fly, Mojo doesn't allow that for structs. However, the static nature of structs helps Mojo run your code faster. The program knows exactly where to find the struct's information and how to use it without any extra steps or delays at runtime. Mojo's structs also work really well with features you might already know from Python, like operator overloading (which lets you change how math symbols like `+` and `-` work with your own data, using [special methods](#special-methods)). As mentioned above, all Mojo's standard types (`Int`, `String`, etc.) are made using structs, rather than being hardwired into the language itself. This gives you more flexibility and control when writing your code, and it means you can define your own types with all the same capabilities (there's no special treatment for the standard library types). ## Special methods Special methods (or "dunder methods") such as `__init__()` are pre-determined method names that you can define in a struct to perform a special task. Although it's possible to call special methods with their method names, the point is that you never should, because Mojo automatically invokes them in circumstances where they're needed (which is why they're also called "magic methods"). For example, Mojo calls the `__init__()` method when you create an instance of the struct; and when Mojo destroys the instance, it calls the `__del__()` method (if it exists). Even operator behaviors that appear built-in (`+`, `<`, `==`, `|`, and so on) are implemented as special methods that Mojo implicitly calls upon to perform operations or comparisons on the type that the operator is applied to. Mojo supports a long list of special methods; far too many to discuss here, but they generally match all of [Python's special methods](https://docs.python.org/3/reference/datamodel#special-method-names) and they usually accomplish one of two types of tasks: - Operator overloading: A lot of special methods are designed to overload operators such as `<` (less-than), `+` (add), and `|` (or) so they work appropriately with each type. For more information, see [Implement operators for custom types](/docs/manual/operators/#implement-operators-for-custom-types). - Lifecycle event handling: These special methods deal with the lifecycle and value ownership of an instance. For example, `__init__()` and `__del__()` demarcate the beginning and end of an instance lifetime, and other special methods define the behavior for other lifecycle events such as how to copy or move a value. You can learn all about the lifecycle special methods in the [Value lifecycle](/docs/manual/lifecycle/) section. However, most structs are simple aggregations of other types, so unless your type requires custom behaviors when an instance is created, copied, moved, or destroyed, you can synthesize the essential lifecycle methods you need (and save yourself some time) using the `@fieldwise_init` decorator (described in [Struct definition](#struct-definition)), and the `Copyable` and `Movable` traits (described in [Making a struct copyable](#making-a-struct-copyable-and-movable)).