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通过例子学习Rust

49.9 格式化文本

We've seen that formatting is specified via a format string:

  • format!("{}", foo) -> "3735928559"
  • format!("0x{:X}", foo) -> "0xDEADBEEF"
  • format!("0o{:o}", foo) -> "0o33653337357"

The same variable (foo) can be formatted differently depending on which argument type is used: X vs o vs unspecified.

This formatting functionality is implemented via traits, and there is one trait for each argument type. The most common formatting trait is Show, which handles cases where the argument type is left unspecified: {} for instance.

use std::fmt::{mod,Formatter,Show}; // For .abs() use std::num::Float; struct City { name: &'static str, // Latitude lat: f32, // Longitude lon: f32, } impl Show for City { // `f` is a buffer, this method must write the formatted string into it fn fmt(&self, f: &mut Formatter) -> fmt::Result { let lat_c = if self.lat >= 0.0 { 'N' } else { 'S' }; let lon_c = if self.lon >= 0.0 { 'E' } else { 'W' }; // `write!` is like `format!`, but it will write the formatted string // into a buffer (the first argument) write!(f, "{}: {:.3}°{} {:.3}°{}", self.name, self.lat.abs(), lat_c, self.lon.abs(), lon_c) } } fn main() { for city in [ City { name: "Dublin", lat: 53.347778, lon: -6.259722 }, City { name: "Oslo", lat: 59.95, lon: 10.75 }, City { name: "Vancouver", lat: 49.25, lon: -123.1 }, ].iter() { println!("{}", city); } }

Here's the full list of formatting traits and their respective argument types:

  • unspecified -> Show
  • d and i -> Signed
  • u -> Unsigned
  • b -> Bool
  • c -> Char
  • o -> Octal
  • x -> LowerHex
  • X -> UpperHex
  • s -> String
  • p -> Pointer
  • t -> Binary
  • f -> Float
  • e -> LowerExp
  • E -> UpperExp
  • ? -> Poly