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Value types: structs and enums

Structs copied on every bind, mutating methods, and enums with a closed set of members.

Structs

When you want data with no identity (a coordinate, a row, a small record), use a struct. A struct is copied on every bind, pass, return, and store (deeply), has no identity, and is final. A method that writes this must be marked mutating.

A struct is copied, not shared

struct Point {
    int x;  int y;
    int dot() => x * x + y * y;
    mutating void translate(int dx, int dy) { x = x + dx; y = y + dy; }
}

Point a = Point(3, 4);
Point b = a;              // a full copy: b is independent
b.translate(1, 1);        // mutates b only; a is still (3, 4)
console.writeln("${a.x}, ${a.y}");
console.writeln("${b.x}, ${b.y}");
console.writeln(a.dot());

Run it ▶

3, 4
4, 5
25

Enums

An enum is a value type with a closed set of members carried by int. Members live on the static side, reached with ::.

Enum members, codes, and lookup by code

enum Status : int { OK = 200, NotFound = 404, Teapot = 418 }

Status s = Status::NotFound;
int   code = s.code();              // 404
string txt = s.toString();          // "NotFound"
console.writeln(code);
console.writeln(txt);

Status? found = Status::fromCode(200);   // Status?: None if no member matches
if (found != None) {
    console.writeln(found.toString());
}
Status? missing = Status::fromCode(1);
if (missing == None) {
    console.writeln("no member has code 1");
}

Run it ▶

404
NotFound
OK
no member has code 1

Because value structs with all-scalar fields have no identity, an Array<Point> can be stored column-major automatically for big speedups, with zero change to your code. The reference entry on columnar arrays explains when that applies.

Value types: structs and enums

Structs

Enums

See also