Copy and destroy hooks in Mojo

A struct can define what copying and destruction mean for it.

struct Handle(Copyable, Movable):
    var name: String

    def __init__(out self, name: String):
        self.name = name

    def copy(self) -> Self:
        return Handle(self.name + "-copy")

    def __del__(deinit self):
        print("closing", self.name)


def main():
    var primary = Handle("db")
    var backup = primary.copy()
    print(primary.name, backup.name)

How it works

1. copy(self) -> Self satisfies Copyable and renames the duplicate.
2. __del__(deinit self) runs when a value dies, printing as it closes.
3. Both handles are destroyed right after their last use, newest first.

Keywords and builtins used here

• Copyable
• Movable
• String
• copy
• def
• main
• print
• return
• self
• struct
• var

The run, in numbers

Lines

17

Characters to type

323

Tokens

91

Three-star pace

75 tpm

At the three-star pace of 75 tokens a minute, this run takes about 73 seconds.

Type this snippet

Step 5 of 5 in Value semantics, step 10 of 22 in Structs & traits.

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