如何将格式化字符串附加到现有字符串? [英] How can I append a formatted string to an existing String?

问题描述

使用format!，我可以从格式字符串创建一个 String，但是如果我已经有一个想要附加到的 String 怎么办?我想避免分配第二个字符串只是为了复制它并丢弃分配.

Using format!, I can create a String from a format string, but what if I already have a String that I'd like to append to? I would like to avoid allocating the second string just to copy it and throw away the allocation.

let s = "hello ".to_string();
append!(s, "{}", 5); // Doesn't exist

C/C++ 中的近似等效项是 snprintf.

A close equivalent in C/C++ would be snprintf.

推荐答案

我现在看到 String 实现 Write，所以我们可以使用write!:

I see now that String implements Write, so we can use write!:

use std::fmt::Write;

pub fn main() {
    let mut a = "hello ".to_string();
    write!(a, "{}", 5).unwrap();

    println!("{}", a);
    assert_eq!("hello 5", a);
}

(游乐场)

它 这个 write! 调用不可能返回一个 Err，至少从 Rust 1.47 开始，所以 unwrap 不应该导致关注.

It is impossible for this write! call to return an Err, at least as of Rust 1.47, so the unwrap should not cause concern.

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