@value

You can add the @value decorator on a struct to generate boilerplate lifecycle methods, including the member-wise __init__() constructor, __copyinit__() copy constructor, and __moveinit__() move constructor.

For example, consider a simple struct like this:

@value
struct MyPet:
    var name: String
    var age: Int
@value
struct MyPet:
    var name: String
    var age: Int

Mojo sees the @value decorator and notices that you don't have any constructors and it synthesizes them for you, the result being as if you had actually written this:

struct MyPet:
    var name: String
    var age: Int

    fn __init__(out self, owned name: String, age: Int):
        self.name = name^
        self.age = age

    fn __copyinit__(out self, existing: Self):
        self.name = existing.name
        self.age = existing.age

    fn __moveinit__(out self, owned existing: Self):
        self.name = existing.name^
        self.age = existing.age
struct MyPet:
    var name: String
    var age: Int

    fn __init__(out self, owned name: String, age: Int):
        self.name = name^
        self.age = age

    fn __copyinit__(out self, existing: Self):
        self.name = existing.name
        self.age = existing.age

    fn __moveinit__(out self, owned existing: Self):
        self.name = existing.name^
        self.age = existing.age

Mojo synthesizes each lifecycle method only when it doesn't exist, so you can use @value and still define your own versions to override the default behavior. For example, it is fairly common to use the default member-wise and move constructor, but create a custom copy constructor.

For more information about these lifecycle methods, read Life of a value.