Rust: What is the difference between struct and enum?

Struct — custom type with named fields (one fixed shape). Enum — type with multiple possible variants.

Struct:

• Defines a type with named fields.
• All fields exist simultaneously (one "shape").
• Supports methods via impl blocks.
• Three forms: named fields, tuple structs, unit structs.

1struct Person {
2    name: String,
3    age: u32,
4}
5
6impl Person {
7    fn greet(&self) -> String {
8        format!("Hi, I'm {}", self.name)
9    }
10}

Enum:

• Defines a type with multiple variants.
• Only one variant is active at a time.
• Variants can hold data (algebraic data types).
• Powerful with match for exhaustive pattern matching.

1enum Shape {
2    Circle(f64),           // radius
3    Rectangle(f64, f64),   // width, height
4    Square(f64),           // side
5}
6
7fn area(shape: &Shape) -> f64 {
8    match shape {
9        Shape::Circle(r) => std::f64::consts::PI * r * r,
10        Shape::Rectangle(w, h) => w * h,
11        Shape::Square(s) => s * s,
12    }
13}

When to use:

• Structs for data containers with fixed fields (users, configs, records).
• Enums when a value can be one of several kinds (shapes, commands, states).
• Enums with data replace "tagged unions" from other languages.

Common mistakes:

• Forgetting to handle all enum variants in match — compiler errors.
• Using enums when all variants share the same fields — structs are clearer.
• Not leveraging #[derive()] on enums for common traits.