2.3 Rust函数

2.3 函数

[root@itoracle src]# cargo new functions
     Created binary (application) `functions` package
[root@itoracle src]# cd functions/
[root@itoracle functions]# vim src/main.rs 

Rust code uses snake case as the conventional style for function and variable names. In snake case, all letters are lowercase and underscores separate words. Here’s a program that contains an example function definition:

fn main() {
    println!("Hello, world!");

    another_function();
}

fn another_function() {
    println!("Another function.");
}

Rust doesn’t care where you define your functions, only that they’re defined somewhere.

[root@itoracle functions]# cargo run
   Compiling functions v0.1.0 (/usr/local/automng/src/goapp/src/functions)
    Finished dev [unoptimized + debuginfo] target(s) in 6.41s
     Running `target/debug/functions`
Hello, world!
Another function.

函数参数

In function signatures, you must declare the type of each parameter. This is a deliberate decision in Rust’s design: requiring type annotations in function definitions means the compiler almost never needs you to use them elsewhere in the code to figure out what you mean.

fn main() {
    another_function(5, 6);
}

fn another_function(x: i32, y: i32) {
    println!("The value of x is: {}", x);
    println!("The value of y is: {}", y);
}

[root@itoracle functions]# cargo run
   Compiling functions v0.1.0 (/usr/local/automng/src/goapp/src/functions)
    Finished dev [unoptimized + debuginfo] target(s) in 3.03s
     Running `target/debug/functions`
The value of x is: 5
The value of y is: 6

函数体

Function bodies are made up of a series of statements optionally ending in an expression.

We’ve actually already used statements and expressions. Statements are instructions that perform some action and do not return a value. Expressions evaluate to a resulting value. Let’s look at some examples.

Creating a variable and assigning a value to it with the let keyword is a statement. let y = 6; is a statement.

fn main() {
    let y = 6;
}

Function definitions are also statements; the entire preceding example is a statement in itself.

Statements do not return values. Therefore, you can’t assign a let statement to another variable, as the following code tries to do; you’ll get an error:

fn main() {
    let x = (let y = 6);
}

[root@itoracle functions]# cargo run
   Compiling functions v0.1.0 (/usr/local/automng/src/goapp/src/functions)
error: expected expression, found statement (`let`)
 --> src/main.rs:2:14
  |
2 |     let x = (let y = 6);
  |              ^^^ expected expression
  |
  = note: variable declaration using `let` is a statement                                                                

error: aborting due to previous error                                                                                    

error: Could not compile `functions`.                                                                                    

To learn more, run the command again with --verbose.

The let y = 6 statement does not return a value, so there isn’t anything for x to bind to.

Expressions evaluate to something and make up most of the rest of the code that you’ll write in Rust. Consider a simple math operation, such as 5 + 6, which is an expression that evaluates to the value 11.Expressions can be part of statements: The 6 in the statement let y = 6; is an expression that evaluates to the value 6. Calling a function is an expression. Calling a macro is an expression. The block that we use to create new scopes, {}, is an expression, for example

fn main() {
    let _x = 5;

    let _y = {
        let _x = 3;
        _x + 1
    };

    println!("The value of y is: {}", _y);
}

let y = {
        let x = 3;
        x + 1
    };

is a block that, in this case, evaluates to 4. That value gets bound to y as part of the letstatement. Note the x + 1 line without a semicolon at the end, which is unlike most of the lines you’ve seen so far. Expressions do not include ending semicolons. If you add a semicolon to the end of an expression, you turn it into a statement, which will then not return a value. Keep this in mind as you explore function return values and expressions next.

[root@itoracle functions]# cargo run src/main.rs
   Compiling functions v0.1.0 (/usr/local/automng/src/goapp/src/functions)
    Finished dev [unoptimized + debuginfo] target(s) in 0.63s
     Running `target/debug/functions src/main.rs`
The value of y is: 4

函数返回值

We don’t name return values, but we do declare their type after an arrow (->). In Rust, the return value of the function is synonymous with the value of the final expression in the block of the body of a function. You can return early from a function by using the return keyword and specifying a value, but most functions return the last expression implicitly. Here’s an example of a function that returns a value:

fn five() -> i32 {
    5
}

fn main() {
    let x = five();

    println!("The value of x is: {}", x);
}

[root@itoracle functions]# cargo run
   Compiling functions v0.1.0 (/usr/local/automng/src/rust/test/functions)
    Finished dev [unoptimized + debuginfo] target(s) in 4.96s
     Running `target/debug/functio

There are two important bits: first, the line let x = five(); shows that we’re using the return value of a function to initialize a variable. Because the function five returns a 5, that line is the same as the following:

let x = 5;

Second, the five function has no parameters and defines the type of the return value, but the body of the function is a lonely 5 with no semicolon because it’s an expression whose value we want to return.

fn main() {
    let x = plus_one(5);

    println!("The value of x is: {}", x);
}

fn plus_one(x: i32) -> i32 {
    x + 1
}

注意，函数体结束的时候没有分号“;”

[root@itoracle functions]# cargo run
   Compiling functions v0.1.0 (/usr/local/automng/src/rust/test/functions)
    Finished dev [unoptimized + debuginfo] target(s) in 1.76s
     Running `target/debug/functions`
The value of x is: 6

如果不是在函数体最后一行返回，则可以使用return语句

fn main() {
    let mut x = plus_one(5);
    println!("The value of x is: {}", x);
    x = plus_one(15);
    println!("The value of x is: {}", x);
}

fn plus_one(mut x: i32) -> i32 {
    if x < 10 {
       return  10;
    }
    x = x + 10;
    x
}

[root@itoracle functions]# cargo run
   Compiling functions v0.1.0 (/usr/local/automng/src/rust/test/functions)
    Finished dev [unoptimized + debuginfo] target(s) in 1.22s
     Running `target/debug/functions`
The value of x is: 10
The value of x is: 25

另外，最后一句使用return语句也是可以的，但rust建议使用不加分号的表达式

fn main() {
    let mut x = plus_one(5);
    println!("The value of x is: {}", x);
    x = plus_one(15);
    println!("The value of x is: {}", x);
}

fn plus_one(mut x: i32) -> i32 {
    if x < 10 {
       return  10;
    }
    x = x + 10;
    return x;
}

[root@itoracle functions]# cargo run
   Compiling functions v0.1.0 (/usr/local/automng/src/rust/test/functions)
    Finished dev [unoptimized + debuginfo] target(s) in 2.43s
     Running `target/debug/functions`
The value of x is: 10
The value of x is: 25

 没有返回值的函数

fn justgo(){
    println!("永远是多远？");
}

[root@itoracle functions]# cat src/main.rs
fn main() {
    let mut x = plus_one();
    println!("The value of x is: {}", x);
    x = plus_one();
    println!("The value of x is: {}", x);
    justgo();
}

fn plus_one(mut x: i32) -> i32 {
    if x <  {
       return  ;
    }
    x = x + ;
    x
}

fn justgo(){
    println!("永远是多远？");
}

运行结果

[root@itoracle functions]# cargo run
   Compiling functions v0.1.0 (/usr/local/automng/src/rust/test/functions)
    Finished dev [unoptimized + debuginfo] target(s) in .95s
     Running `target/debug/functions`
The value of x is:
The value of x is:
永远是多远？

函数定义后，必须被调用，不然运行时会给出警告

warning: function is never used: `justgo`
  --> src/main.rs::
   |
 | fn justgo(){
   | ^^^^^^^^^^^                                                                                                       

    Finished dev [unoptimized + debuginfo] target(s) in .48s
     Running `target/debug/functions`

使用元组让函数返回多个值

fn testFnReturn() -> (u8,String){
    (,"good".to_string())
}

let a = testFnReturn();
println!("{}",a.);