10.3 Enums with Data

A hallmark of Rust enums is that their variants can hold data, combining aspects of both enums and unions in C.

10.3.1 Defining Enums with Data

enum Message {
    Quit,
    Move { x: i32, y: i32 },       // Struct-like variant
    Write(String),                 // Tuple variant
    ChangeColor(i32, i32, i32),    // Tuple variant
}
  • Variants:
    • Quit: No data.
    • Move: Struct-like with named fields.
    • Write: A single String in a tuple variant.
    • ChangeColor: Three i32 values in a tuple variant.

10.3.2 Creating Instances

enum Message {
    Quit,
    Move { x: i32, y: i32 },
    Write(String),
    ChangeColor(i32, i32, i32),
}
fn main() {
let msg1 = Message::Quit;
let msg2 = Message::Move { x: 10, y: 20 };
let msg3 = Message::Write(String::from("Hello"));
let msg4 = Message::ChangeColor(255, 255, 0);
}

10.3.3 Comparison with C Unions

In C, you would typically combine a union with a separate tag enum:

#include <stdio.h>
#include <string.h>

enum MessageType {
    Quit,
    Move,
    Write,
    ChangeColor,
};

struct MoveData {
    int x;
    int y;
};

struct WriteData {
    char text[50];
};

struct ChangeColorData {
    int r;
    int g;
    int b;
};

union MessageData {
    struct MoveData move;
    struct WriteData write;
    struct ChangeColorData color;
};

struct Message {
    enum MessageType type;
    union MessageData data;
};

int main() {
    struct Message msg;
    msg.type = Write;
    strcpy(msg.data.write.text, "Hello");

    if (msg.type == Write) {
        printf("Write message: %s\n", msg.data.write.text);
    }
    return 0;
}
  • Complexity: You must track which field is valid at any time.
  • No Safety: There’s no enforced check to prevent reading the wrong union field.

10.3.4 Advantages of Rust’s Enums with Data

  • Type Safety: It’s impossible to read the wrong variant by accident.
  • Pattern Matching: Straightforward branching and data extraction.
  • Single Type: Functions and collections can deal with multiple variants without extra tagging.