rust-vec-len.md

title	date	draft
[Rust学习]创建Vec指定长度	2018-06-27	false

golang中可创建指定长度的切片，Rust中的Vec只能在创建时指定容量。

let v: Vec<i32> = Vec::with_capacity(3);
println!("{}", v[2]);// index out of bounds

Unsafe 的 set_len()

let mut v: Vec<i32> = Vec::new();
unsafe { v.set_len(3); }
println!("v:{}", v[2]);// SIGSEGV, 段错误

点进去发现，

    /// Sets the length of a vector.
    ///
    /// This will explicitly set the size of the vector, without actually
    /// modifying its buffers, so it is up to the caller to ensure that the
    /// vector is actually the specified size.

因此实用 set_len 需要确保容量

 let mut v: Vec<i32> = Vec::with_capacity(3);
 unsafe { v.set_len(3); }
 println!("v:{}", v[2]);  // v:0

通过 vec! 宏创建

let v = vec![0;10];

通过 std::vec::from_elem 创建

let v = std::vec::from_elem(0, 10);