7.17.1. Define generics in a function #

This is a method of selecting and sorting integer numbers:

fn max(array: &[i32]) -> i32 {
    let mut max_index = 0;
    let mut i = 1;
    while i < array.len() {
        if array[i] > array[max_index] {
            max_index = i;
        }
        i += 1;
    }
    array[max_index]
}
fn main() {
    let a = [2, 4, 6, 3, 1];
    println!("max = {}", max(&a));
}

max = 6

fn max<T>(array: &[T]) -> T {
    let mut max_index = 0;
    let mut i = 1;
    while i < array.len() {
        if array[i] > array[max_index] {
            max_index = i;
        }
        i += 1;
    }
    array[max_index]
}

7.17.2. Structures and generics in enumerated classes #

The Option and Result enumeration classes we learned earlier are generic.

Both structures and enumerated classes in Rust can implement generic mechanisms.

struct Point<T> {
    x: T,
    y: T
}

This is a point coordinate structure, and T represents the numerical type that describes the point coordinates. We can use it like this:

let p1 = Point {x: 1, y: 2};
let p2 = Point {x: 1.0, y: 2.0};

The type is not declared when it is used. The automatic type mechanism is used here, but type mismatches are not allowed as follows:

let p = Point {x: 1, y: 2.0};

When x is bound to 1, T is set to i32, so the type of f64 is no longer allowed. If we want x and y to be represented by different data types, we can use two generic identifiers:

struct Point<T1, T2> {
    x: T1,
    y: T2
}

Methods that represent generics in enumerated classes such as Option and Result:

enum Option<T> {
    Some(T),
    None,
}

enum Result<T, E> {
    Ok(T),
    Err(E),
}

Both structures and enumerated classes can define methods, so methods should also implement generic mechanisms, otherwise generic classes will not be effectively manipulated by methods.

struct Point<T> {
    x: T,
    y: T,
}
impl<T> Point<T> {
    fn x(&self) -> &T {
        &self.x
    }
}
fn main() {
    let p = Point { x: 1, y: 2 };
    println!("p.x = {}", p.x());
}

p.x = 1

impl Point<f64> {
    fn x(&self) -> f64 {
        self.x
    }
}

impl<T, U> Point<T, U> {
    fn mixup<V, W>(self, other: Point<V, W>) -> Point<T, W> {
        Point {
            x: self.x,
            y: other.y,
        }
    }
}

7.17.3. Characteristics #

The concept of trait is close to the Interface in Java, but the two are not exactly the same. Features are similar to interfaces in that they are behavioral specifications that can be used to identify which classes have which methods.

Features are used in Rust trait indicates:

trait Descriptive {
    fn describe(&self) -> String;
}

Descriptive is stipulated that the implementer must have describe(&self) -> String method.

We use it to implement a structure:

struct Person {
    name: String,
    age: u8
}
impl Descriptive for Person {
    fn describe(&self) -> String {
        format!("{} {}", self.name, self.age)
    }
}

Impl<attribute name>for<implemented type name>

7.17.4. Default Properties #

This is the difference between a property and an interface: an interface canonly standardize a method, not define a method, but a property can define amethod as the default method, and because it is “default”, the object canredefine the method or not redefine the method using the default method:

trait Descriptive {
    fn describe(&self) -> String {
        String::from("[Object]")
    }
}
struct Person {
    name: String,
    age: u8
}
impl Descriptive for Person {
    fn describe(&self) -> String {
        format!("{} {}", self.name, self.age)
    }
}
fn main() {
    let cali = Person {
        name: String::from("Cali"),
        age: 24
    };
    println!("{}", cali.describe());
}

Cali 24

[Object]

7.17.5. Characteristics as parameters #

In many cases, we need to pass a function as an argument, such as a callbackfunction, setting button events, and so on. In Java, the function must be passed as an instance of the class implemented by the interface, and it can be achieved by passing property parameters in Rust:

fn output(object: impl Descriptive) {
    println!("{}", object.describe());
}

Anything that comes true Descriptive , the object of the property can be used as an argument to this function. It is not necessary for this function to know whether there are any other properties or methods in the incoming object, just know that it must have Descriptive . The method of feature specification is fine. Of course, other properties and methods cannot be used within this function.

Property parameters can also be implemented using this equivalent syntax:

fn output<T: Descriptive>(object: T) {
    println!("{}", object.describe());
}

This is a syntactic sugar with a style similar to generics, which is useful when multiple parameter types are properties:

fn output_two<T: Descriptive>(arg1: T, arg2: T) {
    println!("{}", arg1.describe());
    println!("{}", arg2.describe());
}

If multiple properties are involved in the type representation of a property, it can be represented by a + symbol, for example:

fn notify(item: impl Summary + Display)
fn notify<T: Summary + Display>(item: T)

Note: when used only to represent types, it does not mean that it can be used in the impl block.

Complex implementation relationships can be used where keyword simplification, for example:

fn some_function<T: Display + Clone, U: Clone + Debug>(t: T, u: U)

Can be simplified to:

fn some_function<T, U>(t: T, u: U) -> i32
    where T: Display + Clone,
          U: Clone + Debug

After understanding this syntax, the “take the maximum” case in the generic chapter can be really implemented:

trait Comparable {
    fn compare(&self, object: &Self) -> i8;
}
fn max<T: Comparable>(array: &[T]) -> &T {
    let mut max_index = 0;
    let mut i = 1;
    while i < array.len() {
        if array[i].compare(&array[max_index]) > 0 {
            max_index = i;
        }
        i += 1;
    }
    &array[max_index]
}
impl Comparable for f64 {
    fn compare(&self, object: &f64) -> i8 {
        if &self > &object { 1 }
        else if &self == &object { 0 }
        else { -1 }
    }
}
fn main() {
    let arr = [1.0, 3.0, 5.0, 4.0, 2.0];
    println!("maximum of arr is {}", max(&arr));
}

maximum of arr is 5

7.17.6. Property as the return value #

The format of the return value of the property is as follows:

fn person() -> impl Descriptive {
    Person {
        name: String::from("Cali"),
        age: 24
    }
}

fn some_function(bool bl) -> impl Descriptive {
    if bl {
        return A {};
    } else {
        return B {};
    }
}

7.17.7. Conditional realization method #

impl is so powerful that we can use it to implement the methods of the class. But for a generic class, sometimes we need to distinguish betweenthe methods that the generic class to which it belongs has been implementedto determine which method it should implement next:

struct A<T> {}

impl<T: B + C> A<T> {
    fn d(&self) {}
}

This code declares A<T> type can effectively implement this only ifT has already implemented the B and C features with impl block.