作者:张富春(ahfuzhang)，转载时请注明作者和引用链接，谢谢！

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1.在编译阶段就可以识别出来的溢出

fn main(){
    let a : usize = 1;
    println!("{}",a-2);
}

执行rustc报以下错误：

rustc usize_test.rs 
error: this arithmetic operation will overflow
 --> usize_test.rs:4:19
  |
4 |     println!("{}",a-2);
  |                   ^^^ attempt to compute `1_usize - 2_usize`, which would overflow
  |
  = note: `#[deny(arithmetic_overflow)]` on by default

error: aborting due to previous error

2.运行期溢出

use std::time::{SystemTime, UNIX_EPOCH};

fn main() {
    let start = SystemTime::now();
    let since_the_epoch = start
        .duration_since(UNIX_EPOCH)
        .expect("Time went backwards");
    let ts = since_the_epoch.as_secs() as usize;
    //
    let a:usize = 1;
    let b :usize= ts % 10 + 2;
    println!("a={:?}, b={:?}", a,b);
    //
    println!("{:?}", a-b);
}

当溢出发生的时候，程序panic了：

thread 'main' panicked at 'attempt to subtract with overflow', usize_test_3.rs:16:22
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

由此也说明，rust不是真正的零成本抽象。为了防止无符号整形的溢出，必然需要在减法之前编译进去一个范围判断。

3.如何才能做溢出不panic的减法

use std::time::{SystemTime, UNIX_EPOCH};

fn main() {
    let start = SystemTime::now();
    let since_the_epoch = start
        .duration_since(UNIX_EPOCH)
        .expect("Time went backwards");
    let ts = since_the_epoch.as_secs() as usize;
    //
    let a:usize = 1;
    let b :usize= ts % 10 + 2;
    println!("a={:?}, b={:?}", a,b);
    //
    println!("{:?}", a.wrapping_sub(b));
}

使用wrapping_sub()这个方法。

rust是如此的严谨，情愿使用的时候很啰嗦，也不要在运行时挖坑。

标签：since,ts,整数,let,rust,usize,println,溢出
来源： https://www.cnblogs.com/ahfuzhang/p/16149527.html