1 //! Utilities for formatting and printing `String`s.
3 //! This module contains the runtime support for the [`format!`] syntax extension.
4 //! This macro is implemented in the compiler to emit calls to this module in
5 //! order to format arguments at runtime into strings.
9 //! The [`format!`] macro is intended to be familiar to those coming from C's
10 //! `printf`/`fprintf` functions or Python's `str.format` function.
12 //! Some examples of the [`format!`] extension are:
15 //! format!("Hello"); // => "Hello"
16 //! format!("Hello, {}!", "world"); // => "Hello, world!"
17 //! format!("The number is {}", 1); // => "The number is 1"
18 //! format!("{:?}", (3, 4)); // => "(3, 4)"
19 //! format!("{value}", value=4); // => "4"
20 //! format!("{} {}", 1, 2); // => "1 2"
21 //! format!("{:04}", 42); // => "0042" with leading zeros
24 //! From these, you can see that the first argument is a format string. It is
25 //! required by the compiler for this to be a string literal; it cannot be a
26 //! variable passed in (in order to perform validity checking). The compiler
27 //! will then parse the format string and determine if the list of arguments
28 //! provided is suitable to pass to this format string.
30 //! To convert a single value to a string, use the [`to_string`] method. This
31 //! will use the [`Display`] formatting trait.
33 //! ## Positional parameters
35 //! Each formatting argument is allowed to specify which value argument it's
36 //! referencing, and if omitted it is assumed to be "the next argument". For
37 //! example, the format string `{} {} {}` would take three parameters, and they
38 //! would be formatted in the same order as they're given. The format string
39 //! `{2} {1} {0}`, however, would format arguments in reverse order.
41 //! Things can get a little tricky once you start intermingling the two types of
42 //! positional specifiers. The "next argument" specifier can be thought of as an
43 //! iterator over the argument. Each time a "next argument" specifier is seen,
44 //! the iterator advances. This leads to behavior like this:
47 //! format!("{1} {} {0} {}", 1, 2); // => "2 1 1 2"
50 //! The internal iterator over the argument has not been advanced by the time
51 //! the first `{}` is seen, so it prints the first argument. Then upon reaching
52 //! the second `{}`, the iterator has advanced forward to the second argument.
53 //! Essentially, parameters that explicitly name their argument do not affect
54 //! parameters that do not name an argument in terms of positional specifiers.
56 //! A format string is required to use all of its arguments, otherwise it is a
57 //! compile-time error. You may refer to the same argument more than once in the
60 //! ## Named parameters
62 //! Rust itself does not have a Python-like equivalent of named parameters to a
63 //! function, but the [`format!`] macro is a syntax extension that allows it to
64 //! leverage named parameters. Named parameters are listed at the end of the
65 //! argument list and have the syntax:
68 //! identifier '=' expression
71 //! For example, the following [`format!`] expressions all use named argument:
74 //! format!("{argument}", argument = "test"); // => "test"
75 //! format!("{name} {}", 1, name = 2); // => "2 1"
76 //! format!("{a} {c} {b}", a="a", b='b', c=3); // => "a 3 b"
79 //! It is not valid to put positional parameters (those without names) after
80 //! arguments that have names. Like with positional parameters, it is not
81 //! valid to provide named parameters that are unused by the format string.
83 //! # Formatting Parameters
85 //! Each argument being formatted can be transformed by a number of formatting
86 //! parameters (corresponding to `format_spec` in [the syntax](#syntax)). These
87 //! parameters affect the string representation of what's being formatted.
92 //! // All of these print "Hello x !"
93 //! println!("Hello {:5}!", "x");
94 //! println!("Hello {:1$}!", "x", 5);
95 //! println!("Hello {1:0$}!", 5, "x");
96 //! println!("Hello {:width$}!", "x", width = 5);
99 //! This is a parameter for the "minimum width" that the format should take up.
100 //! If the value's string does not fill up this many characters, then the
101 //! padding specified by fill/alignment will be used to take up the required
102 //! space (see below).
104 //! The value for the width can also be provided as a [`usize`] in the list of
105 //! parameters by adding a postfix `$`, indicating that the second argument is
106 //! a [`usize`] specifying the width.
108 //! Referring to an argument with the dollar syntax does not affect the "next
109 //! argument" counter, so it's usually a good idea to refer to arguments by
110 //! position, or use named arguments.
112 //! ## Fill/Alignment
115 //! assert_eq!(format!("Hello {:<5}!", "x"), "Hello x !");
116 //! assert_eq!(format!("Hello {:-<5}!", "x"), "Hello x----!");
117 //! assert_eq!(format!("Hello {:^5}!", "x"), "Hello x !");
118 //! assert_eq!(format!("Hello {:>5}!", "x"), "Hello x!");
121 //! The optional fill character and alignment is provided normally in conjunction with the
122 //! [`width`](#width) parameter. It must be defined before `width`, right after the `:`.
123 //! This indicates that if the value being formatted is smaller than
124 //! `width` some extra characters will be printed around it.
125 //! Filling comes in the following variants for different alignments:
127 //! * `[fill]<` - the argument is left-aligned in `width` columns
128 //! * `[fill]^` - the argument is center-aligned in `width` columns
129 //! * `[fill]>` - the argument is right-aligned in `width` columns
131 //! The default [fill/alignment](#fillalignment) for non-numerics is a space and
132 //! left-aligned. The
133 //! default for numeric formatters is also a space character but with right-alignment. If
134 //! the `0` flag (see below) is specified for numerics, then the implicit fill character is
137 //! Note that alignment may not be implemented by some types. In particular, it
138 //! is not generally implemented for the `Debug` trait. A good way to ensure
139 //! padding is applied is to format your input, then pad this resulting string
140 //! to obtain your output:
143 //! println!("Hello {:^15}!", format!("{:?}", Some("hi"))); // => "Hello Some("hi") !"
149 //! assert_eq!(format!("Hello {:+}!", 5), "Hello +5!");
150 //! assert_eq!(format!("{:#x}!", 27), "0x1b!");
151 //! assert_eq!(format!("Hello {:05}!", 5), "Hello 00005!");
152 //! assert_eq!(format!("Hello {:05}!", -5), "Hello -0005!");
153 //! assert_eq!(format!("{:#010x}!", 27), "0x0000001b!");
156 //! These are all flags altering the behavior of the formatter.
158 //! * `+` - This is intended for numeric types and indicates that the sign
159 //! should always be printed. Positive signs are never printed by
160 //! default, and the negative sign is only printed by default for the
161 //! `Signed` trait. This flag indicates that the correct sign (`+` or `-`)
162 //! should always be printed.
163 //! * `-` - Currently not used
164 //! * `#` - This flag indicates that the "alternate" form of printing should
165 //! be used. The alternate forms are:
166 //! * `#?` - pretty-print the [`Debug`] formatting
167 //! * `#x` - precedes the argument with a `0x`
168 //! * `#X` - precedes the argument with a `0x`
169 //! * `#b` - precedes the argument with a `0b`
170 //! * `#o` - precedes the argument with a `0o`
171 //! * `0` - This is used to indicate for integer formats that the padding to `width` should
172 //! both be done with a `0` character as well as be sign-aware. A format
173 //! like `{:08}` would yield `00000001` for the integer `1`, while the
174 //! same format would yield `-0000001` for the integer `-1`. Notice that
175 //! the negative version has one fewer zero than the positive version.
176 //! Note that padding zeros are always placed after the sign (if any)
177 //! and before the digits. When used together with the `#` flag, a similar
178 //! rule applies: padding zeros are inserted after the prefix but before
179 //! the digits. The prefix is included in the total width.
183 //! For non-numeric types, this can be considered a "maximum width". If the resulting string is
184 //! longer than this width, then it is truncated down to this many characters and that truncated
185 //! value is emitted with proper `fill`, `alignment` and `width` if those parameters are set.
187 //! For integral types, this is ignored.
189 //! For floating-point types, this indicates how many digits after the decimal point should be
192 //! There are three possible ways to specify the desired `precision`:
194 //! 1. An integer `.N`:
196 //! the integer `N` itself is the precision.
198 //! 2. An integer or name followed by dollar sign `.N$`:
200 //! use format *argument* `N` (which must be a `usize`) as the precision.
202 //! 3. An asterisk `.*`:
204 //! `.*` means that this `{...}` is associated with *two* format inputs rather than one: the
205 //! first input holds the `usize` precision, and the second holds the value to print. Note that
206 //! in this case, if one uses the format string `{<arg>:<spec>.*}`, then the `<arg>` part refers
207 //! to the *value* to print, and the `precision` must come in the input preceding `<arg>`.
209 //! For example, the following calls all print the same thing `Hello x is 0.01000`:
212 //! // Hello {arg 0 ("x")} is {arg 1 (0.01) with precision specified inline (5)}
213 //! println!("Hello {0} is {1:.5}", "x", 0.01);
215 //! // Hello {arg 1 ("x")} is {arg 2 (0.01) with precision specified in arg 0 (5)}
216 //! println!("Hello {1} is {2:.0$}", 5, "x", 0.01);
218 //! // Hello {arg 0 ("x")} is {arg 2 (0.01) with precision specified in arg 1 (5)}
219 //! println!("Hello {0} is {2:.1$}", "x", 5, 0.01);
221 //! // Hello {next arg ("x")} is {second of next two args (0.01) with precision
222 //! // specified in first of next two args (5)}
223 //! println!("Hello {} is {:.*}", "x", 5, 0.01);
225 //! // Hello {next arg ("x")} is {arg 2 (0.01) with precision
226 //! // specified in its predecessor (5)}
227 //! println!("Hello {} is {2:.*}", "x", 5, 0.01);
229 //! // Hello {next arg ("x")} is {arg "number" (0.01) with precision specified
230 //! // in arg "prec" (5)}
231 //! println!("Hello {} is {number:.prec$}", "x", prec = 5, number = 0.01);
237 //! println!("{}, `{name:.*}` has 3 fractional digits", "Hello", 3, name=1234.56);
238 //! println!("{}, `{name:.*}` has 3 characters", "Hello", 3, name="1234.56");
239 //! println!("{}, `{name:>8.*}` has 3 right-aligned characters", "Hello", 3, name="1234.56");
242 //! print three significantly different things:
245 //! Hello, `1234.560` has 3 fractional digits
246 //! Hello, `123` has 3 characters
247 //! Hello, ` 123` has 3 right-aligned characters
252 //! In some programming languages, the behavior of string formatting functions
253 //! depends on the operating system's locale setting. The format functions
254 //! provided by Rust's standard library do not have any concept of locale and
255 //! will produce the same results on all systems regardless of user
258 //! For example, the following code will always print `1.5` even if the system
259 //! locale uses a decimal separator other than a dot.
262 //! println!("The value is {}", 1.5);
267 //! The literal characters `{` and `}` may be included in a string by preceding
268 //! them with the same character. For example, the `{` character is escaped with
269 //! `{{` and the `}` character is escaped with `}}`.
272 //! assert_eq!(format!("Hello {{}}"), "Hello {}");
273 //! assert_eq!(format!("{{ Hello"), "{ Hello");
278 //! To summarize, here you can find the full grammar of format strings.
279 //! The syntax for the formatting language used is drawn from other languages,
280 //! so it should not be too alien. Arguments are formatted with Python-like
281 //! syntax, meaning that arguments are surrounded by `{}` instead of the C-like
282 //! `%`. The actual grammar for the formatting syntax is:
285 //! format_string := text [ maybe_format text ] *
286 //! maybe_format := '{' '{' | '}' '}' | format
287 //! format := '{' [ argument ] [ ':' format_spec ] '}'
288 //! argument := integer | identifier
290 //! format_spec := [[fill]align][sign]['#']['0'][width]['.' precision]type
291 //! fill := character
292 //! align := '<' | '^' | '>'
293 //! sign := '+' | '-'
295 //! precision := count | '*'
296 //! type := '' | '?' | 'x?' | 'X?' | identifier
297 //! count := parameter | integer
298 //! parameter := argument '$'
300 //! In the above grammar, `text` may not contain any `'{'` or `'}'` characters.
302 //! # Formatting traits
304 //! When requesting that an argument be formatted with a particular type, you
305 //! are actually requesting that an argument ascribes to a particular trait.
306 //! This allows multiple actual types to be formatted via `{:x}` (like [`i8`] as
307 //! well as [`isize`]). The current mapping of types to traits is:
309 //! * *nothing* ⇒ [`Display`]
310 //! * `?` ⇒ [`Debug`]
311 //! * `x?` ⇒ [`Debug`] with lower-case hexadecimal integers
312 //! * `X?` ⇒ [`Debug`] with upper-case hexadecimal integers
313 //! * `o` ⇒ [`Octal`]
314 //! * `x` ⇒ [`LowerHex`]
315 //! * `X` ⇒ [`UpperHex`]
316 //! * `p` ⇒ [`Pointer`]
317 //! * `b` ⇒ [`Binary`]
318 //! * `e` ⇒ [`LowerExp`]
319 //! * `E` ⇒ [`UpperExp`]
321 //! What this means is that any type of argument which implements the
322 //! [`fmt::Binary`][`Binary`] trait can then be formatted with `{:b}`. Implementations
323 //! are provided for these traits for a number of primitive types by the
324 //! standard library as well. If no format is specified (as in `{}` or `{:6}`),
325 //! then the format trait used is the [`Display`] trait.
327 //! When implementing a format trait for your own type, you will have to
328 //! implement a method of the signature:
331 //! # #![allow(dead_code)]
333 //! # struct Foo; // our custom type
334 //! # impl fmt::Display for Foo {
335 //! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
336 //! # write!(f, "testing, testing")
340 //! Your type will be passed as `self` by-reference, and then the function
341 //! should emit output into the `f.buf` stream. It is up to each format trait
342 //! implementation to correctly adhere to the requested formatting parameters.
343 //! The values of these parameters will be listed in the fields of the
344 //! [`Formatter`] struct. In order to help with this, the [`Formatter`] struct also
345 //! provides some helper methods.
347 //! Additionally, the return value of this function is [`fmt::Result`] which is a
348 //! type alias of [`Result`]`<(), `[`std::fmt::Error`]`>`. Formatting implementations
349 //! should ensure that they propagate errors from the [`Formatter`] (e.g., when
350 //! calling [`write!`]). However, they should never return errors spuriously. That
351 //! is, a formatting implementation must and may only return an error if the
352 //! passed-in [`Formatter`] returns an error. This is because, contrary to what
353 //! the function signature might suggest, string formatting is an infallible
354 //! operation. This function only returns a result because writing to the
355 //! underlying stream might fail and it must provide a way to propagate the fact
356 //! that an error has occurred back up the stack.
358 //! An example of implementing the formatting traits would look
365 //! struct Vector2D {
370 //! impl fmt::Display for Vector2D {
371 //! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
372 //! // The `f` value implements the `Write` trait, which is what the
373 //! // write! macro is expecting. Note that this formatting ignores the
374 //! // various flags provided to format strings.
375 //! write!(f, "({}, {})", self.x, self.y)
379 //! // Different traits allow different forms of output of a type. The meaning
380 //! // of this format is to print the magnitude of a vector.
381 //! impl fmt::Binary for Vector2D {
382 //! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
383 //! let magnitude = (self.x * self.x + self.y * self.y) as f64;
384 //! let magnitude = magnitude.sqrt();
386 //! // Respect the formatting flags by using the helper method
387 //! // `pad_integral` on the Formatter object. See the method
388 //! // documentation for details, and the function `pad` can be used
389 //! // to pad strings.
390 //! let decimals = f.precision().unwrap_or(3);
391 //! let string = format!("{:.*}", decimals, magnitude);
392 //! f.pad_integral(true, "", &string)
397 //! let myvector = Vector2D { x: 3, y: 4 };
399 //! println!("{}", myvector); // => "(3, 4)"
400 //! println!("{:?}", myvector); // => "Vector2D {x: 3, y:4}"
401 //! println!("{:10.3b}", myvector); // => " 5.000"
405 //! ### `fmt::Display` vs `fmt::Debug`
407 //! These two formatting traits have distinct purposes:
409 //! - [`fmt::Display`][`Display`] implementations assert that the type can be faithfully
410 //! represented as a UTF-8 string at all times. It is **not** expected that
411 //! all types implement the [`Display`] trait.
412 //! - [`fmt::Debug`][`Debug`] implementations should be implemented for **all** public types.
413 //! Output will typically represent the internal state as faithfully as possible.
414 //! The purpose of the [`Debug`] trait is to facilitate debugging Rust code. In
415 //! most cases, using `#[derive(Debug)]` is sufficient and recommended.
417 //! Some examples of the output from both traits:
420 //! assert_eq!(format!("{} {:?}", 3, 4), "3 4");
421 //! assert_eq!(format!("{} {:?}", 'a', 'b'), "a 'b'");
422 //! assert_eq!(format!("{} {:?}", "foo\n", "bar\n"), "foo\n \"bar\\n\"");
427 //! There are a number of related macros in the [`format!`] family. The ones that
428 //! are currently implemented are:
430 //! ```ignore (only-for-syntax-highlight)
431 //! format! // described above
432 //! write! // first argument is a &mut io::Write, the destination
433 //! writeln! // same as write but appends a newline
434 //! print! // the format string is printed to the standard output
435 //! println! // same as print but appends a newline
436 //! eprint! // the format string is printed to the standard error
437 //! eprintln! // same as eprint but appends a newline
438 //! format_args! // described below.
443 //! This and [`writeln!`] are two macros which are used to emit the format string
444 //! to a specified stream. This is used to prevent intermediate allocations of
445 //! format strings and instead directly write the output. Under the hood, this
446 //! function is actually invoking the [`write_fmt`] function defined on the
447 //! [`std::io::Write`] trait. Example usage is:
450 //! # #![allow(unused_must_use)]
451 //! use std::io::Write;
452 //! let mut w = Vec::new();
453 //! write!(&mut w, "Hello {}!", "world");
458 //! This and [`println!`] emit their output to stdout. Similarly to the [`write!`]
459 //! macro, the goal of these macros is to avoid intermediate allocations when
460 //! printing output. Example usage is:
463 //! print!("Hello {}!", "world");
464 //! println!("I have a newline {}", "character at the end");
468 //! The [`eprint!`] and [`eprintln!`] macros are identical to
469 //! [`print!`] and [`println!`], respectively, except they emit their
470 //! output to stderr.
472 //! ### `format_args!`
474 //! This is a curious macro used to safely pass around
475 //! an opaque object describing the format string. This object
476 //! does not require any heap allocations to create, and it only
477 //! references information on the stack. Under the hood, all of
478 //! the related macros are implemented in terms of this. First
479 //! off, some example usage is:
482 //! # #![allow(unused_must_use)]
484 //! use std::io::{self, Write};
486 //! let mut some_writer = io::stdout();
487 //! write!(&mut some_writer, "{}", format_args!("print with a {}", "macro"));
489 //! fn my_fmt_fn(args: fmt::Arguments) {
490 //! write!(&mut io::stdout(), "{}", args);
492 //! my_fmt_fn(format_args!(", or a {} too", "function"));
495 //! The result of the [`format_args!`] macro is a value of type [`fmt::Arguments`].
496 //! This structure can then be passed to the [`write`] and [`format`] functions
497 //! inside this module in order to process the format string.
498 //! The goal of this macro is to even further prevent intermediate allocations
499 //! when dealing with formatting strings.
501 //! For example, a logging library could use the standard formatting syntax, but
502 //! it would internally pass around this structure until it has been determined
503 //! where output should go to.
505 //! [`fmt::Result`]: Result
506 //! [`Result`]: core::result::Result
507 //! [`std::fmt::Error`]: Error
508 //! [`write!`]: core::write
509 //! [`write`]: core::write
510 //! [`format!`]: crate::format
511 //! [`to_string`]: crate::string::ToString
512 //! [`writeln!`]: core::writeln
513 //! [`write_fmt`]: ../../std/io/trait.Write.html#method.write_fmt
514 //! [`std::io::Write`]: ../../std/io/trait.Write.html
515 //! [`print!`]: ../../std/macro.print.html
516 //! [`println!`]: ../../std/macro.println.html
517 //! [`eprint!`]: ../../std/macro.eprint.html
518 //! [`eprintln!`]: ../../std/macro.eprintln.html
519 //! [`format_args!`]: core::format_args
520 //! [`fmt::Arguments`]: Arguments
521 //! [`format`]: crate::format
523 #![stable(feature = "rust1", since = "1.0.0")]
525 #[unstable(feature = "fmt_internals", issue = "none")]
526 pub use core::fmt::rt;
527 #[stable(feature = "fmt_flags_align", since = "1.28.0")]
528 pub use core::fmt::Alignment;
529 #[stable(feature = "rust1", since = "1.0.0")]
530 pub use core::fmt::Error;
531 #[stable(feature = "rust1", since = "1.0.0")]
532 pub use core::fmt::{write, ArgumentV1, Arguments};
533 #[stable(feature = "rust1", since = "1.0.0")]
534 pub use core::fmt::{Binary, Octal};
535 #[stable(feature = "rust1", since = "1.0.0")]
536 pub use core::fmt::{Debug, Display};
537 #[stable(feature = "rust1", since = "1.0.0")]
538 pub use core::fmt::{DebugList, DebugMap, DebugSet, DebugStruct, DebugTuple};
539 #[stable(feature = "rust1", since = "1.0.0")]
540 pub use core::fmt::{Formatter, Result, Write};
541 #[stable(feature = "rust1", since = "1.0.0")]
542 pub use core::fmt::{LowerExp, UpperExp};
543 #[stable(feature = "rust1", since = "1.0.0")]
544 pub use core::fmt::{LowerHex, Pointer, UpperHex};
548 /// The `format` function takes an [`Arguments`] struct and returns the resulting
549 /// formatted string.
551 /// The [`Arguments`] instance can be created with the [`format_args!`] macro.
560 /// let s = fmt::format(format_args!("Hello, {}!", "world"));
561 /// assert_eq!(s, "Hello, world!");
564 /// Please note that using [`format!`] might be preferable.
568 /// let s = format!("Hello, {}!", "world");
569 /// assert_eq!(s, "Hello, world!");
572 /// [`format_args!`]: core::format_args
573 /// [`format!`]: crate::format
574 #[stable(feature = "rust1", since = "1.0.0")]
575 pub fn format(args: Arguments<'_>) -> string::String {
576 let capacity = args.estimated_capacity();
577 let mut output = string::String::with_capacity(capacity);
578 output.write_fmt(args).expect("a formatting trait implementation returned an error");