Structs & traits

22 steps in 5 sets of Mojo.

A struct is how you name a type in Mojo. It has no hidden header, no vtable, and no allocation you did not ask for — the fields you declare are the memory the value occupies, and the compiler lays them out flat. That is what people mean when they call Mojo structs zero-overhead types.

The heart of it is value semantics. Assigning a struct copies it rather than aliasing it, and every argument spells out its intent: read-only by default, mut when the callee edits the caller's value, var plus a trailing caret when ownership moves. Traits then layer shared behavior on top without giving any of that up — a generic function bound by a trait is compiled fresh for each type it meets, so the abstraction disappears before the program runs.

This tour builds one up from bare fields, follows a value through copies and transfers, teaches a struct the operators it deserves, and finishes with traits and the generic code they unlock.

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A first struct

A struct and its fieldsA struct is a named bundle of fields with a fixed layout the compiler knows exactly.
Methods on a structMethods are functions that take self, so they can read the fields alongside them.
Fieldwise initialization@fieldwise_init writes the constructor for you, one argument per field in order.
Structs holding structsA struct field can be another struct, and the whole thing stays one flat value.
Static methods as constructors@staticmethod gives a struct named constructors that read better than raw arguments.

Value semantics

Assignment copies the valueMojo structs are values, so assigning one makes an independent copy, not an alias.
Transferring ownershipTicket is not copyable, so passing it along means handing over ownership with ^.
Mutating with mutmut marks an argument the callee may change in place, edits landing on the caller's value.
Read-only by defaultArguments are read-only unless you say otherwise, which costs nothing at runtime.
Copy and destroy hooksA struct can define what copying and destruction mean for it.

Operators

Adding with __add__Implement __add__ and the + operator starts working on your own type.
Equality and ordering__eq__ and __lt__ teach a struct how to answer == and <.
Subscripting a struct__getitem__ and __setitem__ put square-bracket syntax on your own container.
Truthiness with __bool____bool__ decides what your struct means inside an if.
Printable structsConform to Writable and your struct prints itself, no formatting helper required.

Traits

Declaring a traitA trait names the methods a type must supply, without saying how.
Two structs, one traitUnrelated types can satisfy the same trait, each in its own way.
Generic functions over traitsA trait bound lets one function serve every conforming type, compiled fresh for each.
Combining trait bounds& joins trait bounds, demanding a type satisfy every one of them.
Sorting your own structConform to Comparable and the standard sort will order your values.

Encore

Bank ledgerAn account struct that guards its own balance and keeps a running history.
Shape galleryOne trait, three shapes, and a single generic function that tabulates them all.