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62 lines
1.7 KiB
62 lines
1.7 KiB
# 算术溢出导致的panic
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在 Rust 中，溢出后的数值被截断是很正常的:
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```rust
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let x: u16 = 65535;
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let v = x as u8;
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println!("{}", v)
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```
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最终程序会输出`255`, 因此大家可能会下意识地就觉得算数操作在 Rust 中只会导致结果的不正确，并不会导致异常。但是实际上，如果是因为算术操作符导致的溢出，就会让整个程序 panic:
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```rust
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fn main() {
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let x: u8 = 10;
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let v = x + u8::MAX;
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println!("{}", v)
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}
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```
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输出结果如下:
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```console
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thread 'main' panicked at 'attempt to add with overflow', src/main.rs:5:13
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```
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那么当我们确实有这种需求时，该如何做呢？可以使用 Rust 提供的`checked_xxx`系列方法：
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```rust
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fn main() {
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let x: u8 = 10;
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let v = x.checked_add(u8::MAX).unwrap_or(0);
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println!("{}", v)
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}
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```
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也许你会觉得本章内容其实算不上什么陷阱，但是在实际项目快速迭代中，越是不起眼的地方越是容易出错：
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```rust
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fn main() {
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let v = production_rate_per_hour(5);
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println!("{}", v);
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}
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pub fn production_rate_per_hour(speed: u8) -> f64 {
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let cph: u8 = 221;
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match speed {
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1..=4 => (speed * cph) as f64,
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5..=8 => (speed * cph) as f64 * 0.9,
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9..=10 => (speed * cph) as f64 * 0.77,
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_ => 0 as f64,
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}
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}
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pub fn working_items_per_minute(speed: u8) -> u32 {
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(production_rate_per_hour(speed) / 60 as f64) as u32
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}
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```
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上述代码中，`speed * cph`就会直接 panic:
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```console
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thread 'main' panicked at 'attempt to multiply with overflow', src/main.rs:10:18
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```
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是不是还藏的挺隐蔽的？因此大家在 Rust 中做数学运算时，要多留一个心眼，免得上了生产才发现问题所在。或者，你也可以做好单元测试:) |