Skip to main content

/

Docs

Nightly

v24.6

Log in

Mojo trait

ExplicitlyCopyable

The ExplicitlyCopyable trait denotes a type whose value can be copied explicitly.

Unlike Copyable, which denotes types that are implicitly copyable, an explicitly copyable type can only be copied when the explicit copy initializer is called intentionally by the programmer.

An explicit copy initializer is just a normal __init__ method that takes a read-only argument of Self.

Example implementing the ExplicitlyCopyable trait on Foo which requires the fn(self) -> Self method:

struct Foo(ExplicitlyCopyable):
    var s: String

    @implicit
    fn __init__(out self, s: String):
        self.s = s

    fn copy(self) -> Self:
        print("explicitly copying value")
        return Foo(self.s)
struct Foo(ExplicitlyCopyable):
    var s: String

    @implicit
    fn __init__(out self, s: String):
        self.s = s

    fn copy(self) -> Self:
        print("explicitly copying value")
        return Foo(self.s)

You can now copy objects inside a generic function:

fn copy_return[T: ExplicitlyCopyable](foo: T) -> T:
    var copy = foo.copy()
    return copy

var foo = Foo("test")
var res = copy_return(foo)
fn copy_return[T: ExplicitlyCopyable](foo: T) -> T:
    var copy = foo.copy()
    return copy

var foo = Foo("test")
var res = copy_return(foo)
explicitly copying value
explicitly copying value

Implemented traits​

AnyType, UnknownDestructibility

Methods​

copy​

copy(self: _Self) -> _Self

Explicitly construct a copy of self.

Returns:

A copy of this value.