使用 macro_rules! 来创建宏 - DRY (不写重复代码)
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2023-12-01
通过提取函数或测试单元的公共部分,宏允许编写 DRY 代码(DRY 是 Don’t Repeat Yourself 的缩写,意思为“不要写重复代码”)。这里给出一个例子,实现并测试了关于 Vec<T>
的 +=
、*=
和 -=
等运算符。
use std::ops::{Add, Mul, Sub};
macro_rules! assert_equal_len {
// `tt` (token tree,令牌树)指示符用于运算符和令牌。
// (原文:The `tt` (token tree) designator is used for
// operators and tokens.)
($a:ident, $b: ident, $func:ident, $op:tt) => (
assert!($a.len() == $b.len(),
"{:?}: dimension mismatch: {:?} {:?} {:?}",
stringify!($func),
($a.len(),),
stringify!($op),
($b.len(),));
)
}
macro_rules! op {
($func:ident, $bound:ident, $op:tt, $method:ident) => (
fn $func<T: $bound<T, Output=T> + Copy>(xs: &mut Vec<T>, ys: &Vec<T>) {
assert_equal_len!(xs, ys, $func, $op);
for (x, y) in xs.iter_mut().zip(ys.iter()) {
*x = $bound::$method(*x, *y);
// *x = x.$method(*y);
}
}
)
}
// 实现 `add_assign`、`mul_assign` 和 `sub_assign` 等函数。
op!(add_assign, Add, +=, add);
op!(mul_assign, Mul, *=, mul);
op!(sub_assign, Sub, -=, sub);
mod test {
use std::iter;
macro_rules! test {
($func: ident, $x:expr, $y:expr, $z:expr) => {
#[test]
fn $func() {
for size in 0usize..10 {
let mut x: Vec<_> = iter::repeat($x).take(size).collect();
let y: Vec<_> = iter::repeat($y).take(size).collect();
let z: Vec<_> = iter::repeat($z).take(size).collect();
super::$func(&mut x, &y);
assert_eq!(x, z);
}
}
}
}
// 测试 `add_assign`、`mul_assign` 和 `sub_assign`
test!(add_assign, 1u32, 2u32, 3u32);
test!(mul_assign, 2u32, 3u32, 6u32);
test!(sub_assign, 3u32, 2u32, 1u32);
}
$ rustc --test dry.rs && ./dry
running 3 tests
test test::mul_assign ... ok
test test::add_assign ... ok
test test::sub_assign ... ok
test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured