试图编写一个将向量填充范围内的泛型函数
fn fill_vec<T: From<usize> + Copy>(target: &mut Vec<T>, to: usize, step: usize) {
let mut start = 0_usize;
for i in 0..to {
if start >= to {
break;
} else {
if start > 0 {
target.push(T::from(start));
}
start += step;
}
};
}
但是我得到了错误
error[E0277]: the trait bound `i32: std::convert::From<usize>` is not satisfied
--> src/main.rs:28:14
fill_vec(&mut target, 30, 4);
| ^^^^^^^^^^^ the trait `std::convert::From<usize>` is not implemented for `i32`
答案 0 :(得分:0)
实际上,您不能从usize
隐式转换为i32
。
对于这种类型的转换(可能会导致截断),您
需要as
关键字。
fn main() {
let mut v: Vec<i32> = Vec::new();
let a: usize = 123;
// v.push(a);
// expected `i32`, found `usize`
// v.push(i32::from(a));
// the trait `std::convert::From<usize>` is not implemented for `i32`
v.push(a as i32); // cast
}
关于您的特定示例,使用num_traits
条板箱可能会有所帮助。
fn fill_vec<T: num_traits::cast::FromPrimitive>(
target: &mut Vec<T>,
to: usize,
step: usize,
) {
let mut start = 0_usize;
for i in 0..to {
if start >= to {
break;
} else {
if start > 0 {
target.push(T::from_usize(start).unwrap());
}
start += step;
}
}
}