使用文件
现在我们在电脑上使用Rust，我们可以开始处理文件了。你会注意到，现在我们会开始在代码中越来越多的看到Result。这是因为一旦你开始处理文件和类似的事情，很多事情都会出错。一个文件可能不在那里，或者计算机无法读取它。
你可能还记得，如果你想使用?运算符，调用它的函数必须返回一个Result。如果你记不住错误类型，你可以什么都不给它，让编译器告诉你。让我们用一个试图用.parse()创建一个数字的函数来试试。
// ⚠️ fn give_number(input: &str) -> Result<i32, ()> { input.parse::<i32>() } fn main() { println!("{:?}", give_number("88")); println!("{:?}", give_number("5")); }
编译器告诉我们到底该怎么做。
error[E0308]: mismatched types
--> src\main.rs:4:5
|
3 | fn give_number(input: &str) -> Result<i32, ()> {
| --------------- expected `std::result::Result<i32, ()>` because of return type
4 | input.parse::<i32>()
| ^^^^^^^^^^^^^^^^^^^^ expected `()`, found struct `std::num::ParseIntError`
|
= note: expected enum `std::result::Result<_, ()>`
found enum `std::result::Result<_, std::num::ParseIntError>`
很好! 所以我们只要把返回值改成编译器说的就可以了:
use std::num::ParseIntError; fn give_number(input: &str) -> Result<i32, ParseIntError> { input.parse::<i32>() } fn main() { println!("{:?}", give_number("88")); println!("{:?}", give_number("5")); }
现在程序可以运行了!
Ok(88)
Ok(5)
所以现在我们想用?，如果能用就直接给我们数值，如果不能用就给错误。但是如何在fn main()中做到这一点呢？如果我们尝试在main中使用?，那就不行了。
// ⚠️ use std::num::ParseIntError; fn give_number(input: &str) -> Result<i32, ParseIntError> { input.parse::<i32>() } fn main() { println!("{:?}", give_number("88")?); println!("{:?}", give_number("5")?); }
它说:
error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
--> src\main.rs:8:22
|
7 | / fn main() {
8 | | println!("{:?}", give_number("88")?);
| | ^^^^^^^^^^^^^^^^^^ cannot use the `?` operator in a function that returns `()`
9 | | println!("{:?}", give_number("5")?);
10 | | }
| |_- this function should return `Result` or `Option` to accept `?`
但实际上main()可以返回一个Result，就像其他函数一样。如果我们的函数能工作，我们不想返回任何东西(main()并没有给其他任何东西)。而如果它不工作，我们将错误返回。所以我们可以这样写:
use std::num::ParseIntError; fn give_number(input: &str) -> Result<i32, ParseIntError> { input.parse::<i32>() } fn main() -> Result<(), ParseIntError> { println!("{:?}", give_number("88")?); println!("{:?}", give_number("5")?); Ok(()) }
不要忘了最后的Ok(()):这在Rust中是很常见的，它的意思是Ok，里面是()，也就是我们的返回值。现在它打印出来了:
88
5
只用.parse()的时候不是很有用，但是用文件就很有用。这是因为?也为我们改变了错误类型。下面是用简单英语写的?运算符页面:
If you get an `Err`, it will get the inner error. Then `?` does a conversion using `From`. With that it can change specialized errors to more general ones. The error it gets is then returned.
另外，Rust在使用Files和类似的东西时，有一个方便的Result类型。它叫做std::io::Result，当你在使用?对文件进行打开和操作时，通常在main()中看到的就是这个。这其实是一个类型别名。它的样子是这样的:
type Result<T> = Result<T, Error>;
所以这是一个Result<T, Error>，但我们只需要写出Result<T>部分。
现在让我们第一次尝试使用文件。std::fs是处理文件的方法所在，有了std::io::Write，你就可以写。有了它，我们就可以用.write_all()来写进文件。
use std::fs; use std::io::Write; fn main() -> std::io::Result<()> { let mut file = fs::File::create("myfilename.txt")?; // Create a file with this name. // CAREFUL! If you have a file with this name already, // it will delete everything in it. file.write_all(b"Let's put this in the file")?; // Don't forget the b in front of ". That's because files take bytes. Ok(()) }
然后如果你打开新文件myfilename.txt，会看到内容Let's put this in the file。
不过我们不需要写两行，因为我们有?操作符。如果有效，它就会传递我们想要的结果，有点像在迭代器上很多方法一样。这时候?就变得非常方便了。
use std::fs; use std::io::Write; fn main() -> std::io::Result<()> { fs::File::create("myfilename.txt")?.write_all(b"Let's put this in the file")?; Ok(()) }
所以这是说 "请尝试创建一个文件，然后检查是否成功。如果成功了，那就使用.write_all()，然后检查是否成功。"
而事实上，也有一个函数可以同时做这两件事。它叫做std::fs::write。在它里面，你给它你想要的文件名，以及你想放在里面的内容。再次强调，要小心! 如果该文件已经存在，它将删除其中的所有内容。另外，它允许你写一个&str，前面不写b，因为这个:
#![allow(unused)] fn main() { pub fn write<P: AsRef<Path>, C: AsRef<[u8]>>(path: P, contents: C) -> Result<()> }
AsRef<[u8]>就是为什么你可以给它任何一个。
很简单的:
use std::fs; fn main() -> std::io::Result<()> { fs::write("calvin_with_dad.txt", "Calvin: Dad, how come old photographs are always black and white? Didn't they have color film back then? Dad: Sure they did. In fact, those photographs *are* in color. It's just the *world* was black and white then. Calvin: Really? Dad: Yep. The world didn't turn color until sometimes in the 1930s...")?; Ok(()) }
所以这就是我们要用的文件。这是一个名叫Calvin的漫画人物和他爸爸的对话，他爸爸对他的问题并不认真。有了这个，每次我们都可以创建一个文件来使用。
打开一个文件和创建一个文件一样简单。你只要用open()代替create()就可以了。之后(如果它找到了你的文件)，你就可以做read_to_string()这样的事情。要做到这一点，你可以创建一个可变的 String，然后把文件读到那里。它看起来像这样:
use std::fs; use std::fs::File; use std::io::Read; // this is to use the function .read_to_string() fn main() -> std::io::Result<()> { fs::write("calvin_with_dad.txt", "Calvin: Dad, how come old photographs are always black and white? Didn't they have color film back then? Dad: Sure they did. In fact, those photographs *are* in color. It's just the *world* was black and white then. Calvin: Really? Dad: Yep. The world didn't turn color until sometimes in the 1930s...")?; let mut calvin_file = File::open("calvin_with_dad.txt")?; // Open the file we just made let mut calvin_string = String::new(); // This String will hold it calvin_file.read_to_string(&mut calvin_string)?; // Read the file into it calvin_string.split_whitespace().for_each(|word| print!("{} ", word.to_uppercase())); // Do things with the String now Ok(()) }
会打印:
#![allow(unused)] fn main() { CALVIN: DAD, HOW COME OLD PHOTOGRAPHS ARE ALWAYS BLACK AND WHITE? DIDN'T THEY HAVE COLOR FILM BACK THEN? DAD: SURE THEY DID. IN FACT, THOSE PHOTOGRAPHS *ARE* IN COLOR. IT'S JUST THE *WORLD* WAS BLACK AND WHITE THEN. CALVIN: REALLY? DAD: YEP. THE WORLD DIDN'T TURN COLOR UNTIL SOMETIMES IN THE 1930S... }
好吧，如果我们想创建一个文件，但如果已经有另一个同名的文件就不做了怎么办？也许你不想为了创建一个新的文件而删除已经存在的其他文件。要做到这一点，有一个结构叫OpenOptions。其实，我们一直在用OpenOptions，却不知道。看看File::open的源码吧。
#![allow(unused)] fn main() { pub fn open<P: AsRef<Path>>(path: P) -> io::Result<File> { OpenOptions::new().read(true).open(path.as_ref()) } }
有意思，这好像是我们学过的建造者模式。File::create也是如此。
#![allow(unused)] fn main() { pub fn create<P: AsRef<Path>>(path: P) -> io::Result<File> { OpenOptions::new().write(true).create(true).truncate(true).open(path.as_ref()) } }
如果你去OpenOptions的页面，你可以看到所有可以选择的方法。大多数采取bool。
append(): 意思是 "添加到已经存在的内容中，而不是删除"。create(): 这让OpenOptions创建一个文件。create_new(): 意思是只有在文件不存在的情况下才会创建文件。read(): 如果你想让它读取文件，就把这个设置为true。truncate(): 如果你想在打开文件时把文件内容剪为0(删除内容)，就把这个设置为true。write(): 这可以让它写入一个文件。
然后在最后你用.open()加上文件名，就会得到一个Result。我们来看一个例子。
// ⚠️ use std::fs; use std::fs::OpenOptions; fn main() -> std::io::Result<()> { fs::write("calvin_with_dad.txt", "Calvin: Dad, how come old photographs are always black and white? Didn't they have color film back then? Dad: Sure they did. In fact, those photographs *are* in color. It's just the *world* was black and white then. Calvin: Really? Dad: Yep. The world didn't turn color until sometimes in the 1930s...")?; let calvin_file = OpenOptions::new().write(true).create_new(true).open("calvin_with_dad.txt")?; Ok(()) }
首先我们用new做了一个OpenOptions(总是以new开头)。然后我们给它的能力是write。之后我们把create_new()设置为true，然后试着打开我们做的文件。打不开，这是我们想要的。
Error: Os { code: 80, kind: AlreadyExists, message: "The file exists." }
让我们尝试使用.append()，这样我们就可以向一个文件写入。为了写入文件，我们可以使用 .write_all()，这是一个尝试写入你给它的一切内容的方法。
另外，我们将使用 write! 宏来做同样的事情。你会记得这个宏，我们在为结构体做impl Display的时候用到过。这次我们是在文件上使用它，而不是在缓冲区上。
use std::fs; use std::fs::OpenOptions; use std::io::Write; fn main() -> std::io::Result<()> { fs::write("calvin_with_dad.txt", "Calvin: Dad, how come old photographs are always black and white? Didn't they have color film back then? Dad: Sure they did. In fact, those photographs *are* in color. It's just the *world* was black and white then. Calvin: Really? Dad: Yep. The world didn't turn color until sometimes in the 1930s...")?; let mut calvin_file = OpenOptions::new() .append(true) // Now we can write without deleting it .read(true) .open("calvin_with_dad.txt")?; calvin_file.write_all(b"And it was a pretty grainy color for a while too.\n")?; write!(&mut calvin_file, "That's really weird.\n")?; write!(&mut calvin_file, "Well, truth is stranger than fiction.")?; println!("{}", fs::read_to_string("calvin_with_dad.txt")?); Ok(()) }
这个打印:
Calvin: Dad, how come old photographs are always black and white? Didn't they have color film back then?
Dad: Sure they did. In fact, those photographs *are* in color. It's just the *world* was black and white then.
Calvin: Really?
Dad: Yep. The world didn't turn color until sometimes in the 1930s...And it was a pretty grainy color for a while too.
That's really weird.
Well, truth is stranger than fiction.