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//! Utilities for formatting and printing `String`s. //! //! This module contains the runtime support for the [`format!`] syntax extension. //! This macro is implemented in the compiler to emit calls to this module in //! order to format arguments at runtime into strings. //! //! # Usage //! //! The [`format!`] macro is intended to be familiar to those coming from C's //! `printf`/`fprintf` functions or Python's `str.format` function. //! //! Some examples of the [`format!`] extension are: //! //! ``` //! format!("Hello"); // => "Hello" //! format!("Hello, {}!", "world"); // => "Hello, world!" //! format!("The number is {}", 1); // => "The number is 1" //! format!("{:?}", (3, 4)); // => "(3, 4)" //! format!("{value}", value=4); // => "4" //! format!("{} {}", 1, 2); // => "1 2" //! format!("{:04}", 42); // => "0042" with leading zeros //! ``` //! //! From these, you can see that the first argument is a format string. It is //! required by the compiler for this to be a string literal; it cannot be a //! variable passed in (in order to perform validity checking). The compiler //! will then parse the format string and determine if the list of arguments //! provided is suitable to pass to this format string. //! //! To convert a single value to a string, use the [`to_string`] method. This //! will use the [`Display`] formatting trait. //! //! ## Positional parameters //! //! Each formatting argument is allowed to specify which value argument it's //! referencing, and if omitted it is assumed to be "the next argument". For //! example, the format string `{} {} {}` would take three parameters, and they //! would be formatted in the same order as they're given. The format string //! `{2} {1} {0}`, however, would format arguments in reverse order. //! //! Things can get a little tricky once you start intermingling the two types of //! positional specifiers. The "next argument" specifier can be thought of as an //! iterator over the argument. Each time a "next argument" specifier is seen, //! the iterator advances. This leads to behavior like this: //! //! ``` //! format!("{1} {} {0} {}", 1, 2); // => "2 1 1 2" //! ``` //! //! The internal iterator over the argument has not been advanced by the time //! the first `{}` is seen, so it prints the first argument. Then upon reaching //! the second `{}`, the iterator has advanced forward to the second argument. //! Essentially, parameters which explicitly name their argument do not affect //! parameters which do not name an argument in terms of positional specifiers. //! //! A format string is required to use all of its arguments, otherwise it is a //! compile-time error. You may refer to the same argument more than once in the //! format string. //! //! ## Named parameters //! //! Rust itself does not have a Python-like equivalent of named parameters to a //! function, but the [`format!`] macro is a syntax extension which allows it to //! leverage named parameters. Named parameters are listed at the end of the //! argument list and have the syntax: //! //! ```text //! identifier '=' expression //! ``` //! //! For example, the following [`format!`] expressions all use named argument: //! //! ``` //! format!("{argument}", argument = "test"); // => "test" //! format!("{name} {}", 1, name = 2); // => "2 1" //! format!("{a} {c} {b}", a="a", b='b', c=3); // => "a 3 b" //! ``` //! //! It is not valid to put positional parameters (those without names) after //! arguments which have names. Like with positional parameters, it is not //! valid to provide named parameters that are unused by the format string. //! //! ## Argument types //! //! Each argument's type is dictated by the format string. //! There are various parameters which require a particular type, however. //! An example is the `{:.*}` syntax, which sets the number of decimal places //! in floating-point types: //! //! ``` //! let formatted_number = format!("{:.*}", 2, 1.234567); //! //! assert_eq!("1.23", formatted_number) //! ``` //! //! If this syntax is used, then the number of characters to print precedes the //! actual object being formatted, and the number of characters must have the //! type [`usize`]. //! //! ## Formatting traits //! //! When requesting that an argument be formatted with a particular type, you //! are actually requesting that an argument ascribes to a particular trait. //! This allows multiple actual types to be formatted via `{:x}` (like [`i8`] as //! well as [`isize`]). The current mapping of types to traits is: //! //! * *nothing* ⇒ [`Display`] //! * `?` ⇒ [`Debug`] //! * `x?` ⇒ [`Debug`] with lower-case hexadecimal integers //! * `X?` ⇒ [`Debug`] with upper-case hexadecimal integers //! * `o` ⇒ [`Octal`](trait.Octal.html) //! * `x` ⇒ [`LowerHex`](trait.LowerHex.html) //! * `X` ⇒ [`UpperHex`](trait.UpperHex.html) //! * `p` ⇒ [`Pointer`](trait.Pointer.html) //! * `b` ⇒ [`Binary`] //! * `e` ⇒ [`LowerExp`](trait.LowerExp.html) //! * `E` ⇒ [`UpperExp`](trait.UpperExp.html) //! //! What this means is that any type of argument which implements the //! [`fmt::Binary`][`Binary`] trait can then be formatted with `{:b}`. Implementations //! are provided for these traits for a number of primitive types by the //! standard library as well. If no format is specified (as in `{}` or `{:6}`), //! then the format trait used is the [`Display`] trait. //! //! When implementing a format trait for your own type, you will have to //! implement a method of the signature: //! //! ``` //! # #![allow(dead_code)] //! # use std::fmt; //! # struct Foo; // our custom type //! # impl fmt::Display for Foo { //! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { //! # write!(f, "testing, testing") //! # } } //! ``` //! //! Your type will be passed as `self` by-reference, and then the function //! should emit output into the `f.buf` stream. It is up to each format trait //! implementation to correctly adhere to the requested formatting parameters. //! The values of these parameters will be listed in the fields of the //! [`Formatter`] struct. In order to help with this, the [`Formatter`] struct also //! provides some helper methods. //! //! Additionally, the return value of this function is [`fmt::Result`] which is a //! type alias of [`Result`]`<(), `[`std::fmt::Error`]`>`. Formatting implementations //! should ensure that they propagate errors from the [`Formatter`][`Formatter`] (e.g., when //! calling [`write!`]). However, they should never return errors spuriously. That //! is, a formatting implementation must and may only return an error if the //! passed-in [`Formatter`] returns an error. This is because, contrary to what //! the function signature might suggest, string formatting is an infallible //! operation. This function only returns a result because writing to the //! underlying stream might fail and it must provide a way to propagate the fact //! that an error has occurred back up the stack. //! //! An example of implementing the formatting traits would look //! like: //! //! ``` //! use std::fmt; //! //! #[derive(Debug)] //! struct Vector2D { //! x: isize, //! y: isize, //! } //! //! impl fmt::Display for Vector2D { //! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { //! // The `f` value implements the `Write` trait, which is what the //! // write! macro is expecting. Note that this formatting ignores the //! // various flags provided to format strings. //! write!(f, "({}, {})", self.x, self.y) //! } //! } //! //! // Different traits allow different forms of output of a type. The meaning //! // of this format is to print the magnitude of a vector. //! impl fmt::Binary for Vector2D { //! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { //! let magnitude = (self.x * self.x + self.y * self.y) as f64; //! let magnitude = magnitude.sqrt(); //! //! // Respect the formatting flags by using the helper method //! // `pad_integral` on the Formatter object. See the method //! // documentation for details, and the function `pad` can be used //! // to pad strings. //! let decimals = f.precision().unwrap_or(3); //! let string = format!("{:.*}", decimals, magnitude); //! f.pad_integral(true, "", &string) //! } //! } //! //! fn main() { //! let myvector = Vector2D { x: 3, y: 4 }; //! //! println!("{}", myvector); // => "(3, 4)" //! println!("{:?}", myvector); // => "Vector2D {x: 3, y:4}" //! println!("{:10.3b}", myvector); // => " 5.000" //! } //! ``` //! //! ### `fmt::Display` vs `fmt::Debug` //! //! These two formatting traits have distinct purposes: //! //! - [`fmt::Display`][`Display`] implementations assert that the type can be faithfully //! represented as a UTF-8 string at all times. It is **not** expected that //! all types implement the [`Display`] trait. //! - [`fmt::Debug`][`Debug`] implementations should be implemented for **all** public types. //! Output will typically represent the internal state as faithfully as possible. //! The purpose of the [`Debug`] trait is to facilitate debugging Rust code. In //! most cases, using `#[derive(Debug)]` is sufficient and recommended. //! //! Some examples of the output from both traits: //! //! ``` //! assert_eq!(format!("{} {:?}", 3, 4), "3 4"); //! assert_eq!(format!("{} {:?}", 'a', 'b'), "a 'b'"); //! assert_eq!(format!("{} {:?}", "foo\n", "bar\n"), "foo\n \"bar\\n\""); //! ``` //! //! ## Related macros //! //! There are a number of related macros in the [`format!`] family. The ones that //! are currently implemented are: //! //! ```ignore (only-for-syntax-highlight) //! format! // described above //! write! // first argument is a &mut io::Write, the destination //! writeln! // same as write but appends a newline //! print! // the format string is printed to the standard output //! println! // same as print but appends a newline //! eprint! // the format string is printed to the standard error //! eprintln! // same as eprint but appends a newline //! format_args! // described below. //! ``` //! //! ### `write!` //! //! This and [`writeln!`] are two macros which are used to emit the format string //! to a specified stream. This is used to prevent intermediate allocations of //! format strings and instead directly write the output. Under the hood, this //! function is actually invoking the [`write_fmt`] function defined on the //! [`std::io::Write`] trait. Example usage is: //! //! ``` //! # #![allow(unused_must_use)] //! use std::io::Write; //! let mut w = Vec::new(); //! write!(&mut w, "Hello {}!", "world"); //! ``` //! //! ### `print!` //! //! This and [`println!`] emit their output to stdout. Similarly to the [`write!`] //! macro, the goal of these macros is to avoid intermediate allocations when //! printing output. Example usage is: //! //! ``` //! print!("Hello {}!", "world"); //! println!("I have a newline {}", "character at the end"); //! ``` //! ### `eprint!` //! //! The [`eprint!`] and [`eprintln!`] macros are identical to //! [`print!`] and [`println!`], respectively, except they emit their //! output to stderr. //! //! ### `format_args!` //! //! This is a curious macro which is used to safely pass around //! an opaque object describing the format string. This object //! does not require any heap allocations to create, and it only //! references information on the stack. Under the hood, all of //! the related macros are implemented in terms of this. First //! off, some example usage is: //! //! ``` //! # #![allow(unused_must_use)] //! use std::fmt; //! use std::io::{self, Write}; //! //! let mut some_writer = io::stdout(); //! write!(&mut some_writer, "{}", format_args!("print with a {}", "macro")); //! //! fn my_fmt_fn(args: fmt::Arguments) { //! write!(&mut io::stdout(), "{}", args); //! } //! my_fmt_fn(format_args!(", or a {} too", "function")); //! ``` //! //! The result of the [`format_args!`] macro is a value of type [`fmt::Arguments`]. //! This structure can then be passed to the [`write`] and [`format`] functions //! inside this module in order to process the format string. //! The goal of this macro is to even further prevent intermediate allocations //! when dealing formatting strings. //! //! For example, a logging library could use the standard formatting syntax, but //! it would internally pass around this structure until it has been determined //! where output should go to. //! //! # Syntax //! //! The syntax for the formatting language used is drawn from other languages, //! so it should not be too alien. Arguments are formatted with Python-like //! syntax, meaning that arguments are surrounded by `{}` instead of the C-like //! `%`. The actual grammar for the formatting syntax is: //! //! ```text //! format_string := <text> [ maybe-format <text> ] * //! maybe-format := '{' '{' | '}' '}' | <format> //! format := '{' [ argument ] [ ':' format_spec ] '}' //! argument := integer | identifier //! //! format_spec := [[fill]align][sign]['#']['0'][width]['.' precision][type] //! fill := character //! align := '<' | '^' | '>' //! sign := '+' | '-' //! width := count //! precision := count | '*' //! type := identifier | '?' | '' //! count := parameter | integer //! parameter := argument '$' //! ``` //! //! # Formatting Parameters //! //! Each argument being formatted can be transformed by a number of formatting //! parameters (corresponding to `format_spec` in the syntax above). These //! parameters affect the string representation of what's being formatted. //! //! ## Fill/Alignment //! //! The fill character is provided normally in conjunction with the //! [`width`](#width) //! parameter. This indicates that if the value being formatted is smaller than //! `width` some extra characters will be printed around it. The extra //! characters are specified by `fill`, and the alignment can be one of the //! following options: //! //! * `<` - the argument is left-aligned in `width` columns //! * `^` - the argument is center-aligned in `width` columns //! * `>` - the argument is right-aligned in `width` columns //! //! Note that alignment may not be implemented by some types. In particular, it //! is not generally implemented for the `Debug` trait. A good way to ensure //! padding is applied is to format your input, then use this resulting string //! to pad your output. //! //! ## Sign/`#`/`0` //! //! These can all be interpreted as flags for a particular formatter. //! //! * `+` - This is intended for numeric types and indicates that the sign //! should always be printed. Positive signs are never printed by //! default, and the negative sign is only printed by default for the //! `Signed` trait. This flag indicates that the correct sign (`+` or `-`) //! should always be printed. //! * `-` - Currently not used //! * `#` - This flag is indicates that the "alternate" form of printing should //! be used. The alternate forms are: //! * `#?` - pretty-print the [`Debug`] formatting //! * `#x` - precedes the argument with a `0x` //! * `#X` - precedes the argument with a `0x` //! * `#b` - precedes the argument with a `0b` //! * `#o` - precedes the argument with a `0o` //! * `0` - This is used to indicate for integer formats that the padding should //! both be done with a `0` character as well as be sign-aware. A format //! like `{:08}` would yield `00000001` for the integer `1`, while the //! same format would yield `-0000001` for the integer `-1`. Notice that //! the negative version has one fewer zero than the positive version. //! Note that padding zeroes are always placed after the sign (if any) //! and before the digits. When used together with the `#` flag, a similar //! rule applies: padding zeroes are inserted after the prefix but before //! the digits. //! //! ## Width //! //! This is a parameter for the "minimum width" that the format should take up. //! If the value's string does not fill up this many characters, then the //! padding specified by fill/alignment will be used to take up the required //! space. //! //! The default [fill/alignment](#fillalignment) for non-numerics is a space and //! left-aligned. The //! defaults for numeric formatters is also a space but with right-alignment. If //! the `0` flag is specified for numerics, then the implicit fill character is //! `0`. //! //! The value for the width can also be provided as a [`usize`] in the list of //! parameters by using the dollar syntax indicating that the second argument is //! a [`usize`] specifying the width, for example: //! //! ``` //! // All of these print "Hello x !" //! println!("Hello {:5}!", "x"); //! println!("Hello {:1$}!", "x", 5); //! println!("Hello {1:0$}!", 5, "x"); //! println!("Hello {:width$}!", "x", width = 5); //! ``` //! //! Referring to an argument with the dollar syntax does not affect the "next //! argument" counter, so it's usually a good idea to refer to arguments by //! position, or use named arguments. //! //! ## Precision //! //! For non-numeric types, this can be considered a "maximum width". If the resulting string is //! longer than this width, then it is truncated down to this many characters and that truncated //! value is emitted with proper `fill`, `alignment` and `width` if those parameters are set. //! //! For integral types, this is ignored. //! //! For floating-point types, this indicates how many digits after the decimal point should be //! printed. //! //! There are three possible ways to specify the desired `precision`: //! //! 1. An integer `.N`: //! //! the integer `N` itself is the precision. //! //! 2. An integer or name followed by dollar sign `.N$`: //! //! use format *argument* `N` (which must be a `usize`) as the precision. //! //! 3. An asterisk `.*`: //! //! `.*` means that this `{...}` is associated with *two* format inputs rather than one: the //! first input holds the `usize` precision, and the second holds the value to print. Note that //! in this case, if one uses the format string `{<arg>:<spec>.*}`, then the `<arg>` part refers //! to the *value* to print, and the `precision` must come in the input preceding `<arg>`. //! //! For example, the following calls all print the same thing `Hello x is 0.01000`: //! //! ``` //! // Hello {arg 0 ("x")} is {arg 1 (0.01) with precision specified inline (5)} //! println!("Hello {0} is {1:.5}", "x", 0.01); //! //! // Hello {arg 1 ("x")} is {arg 2 (0.01) with precision specified in arg 0 (5)} //! println!("Hello {1} is {2:.0$}", 5, "x", 0.01); //! //! // Hello {arg 0 ("x")} is {arg 2 (0.01) with precision specified in arg 1 (5)} //! println!("Hello {0} is {2:.1$}", "x", 5, 0.01); //! //! // Hello {next arg ("x")} is {second of next two args (0.01) with precision //! // specified in first of next two args (5)} //! println!("Hello {} is {:.*}", "x", 5, 0.01); //! //! // Hello {next arg ("x")} is {arg 2 (0.01) with precision //! // specified in its predecessor (5)} //! println!("Hello {} is {2:.*}", "x", 5, 0.01); //! //! // Hello {next arg ("x")} is {arg "number" (0.01) with precision specified //! // in arg "prec" (5)} //! println!("Hello {} is {number:.prec$}", "x", prec = 5, number = 0.01); //! ``` //! //! While these: //! //! ``` //! println!("{}, `{name:.*}` has 3 fractional digits", "Hello", 3, name=1234.56); //! println!("{}, `{name:.*}` has 3 characters", "Hello", 3, name="1234.56"); //! println!("{}, `{name:>8.*}` has 3 right-aligned characters", "Hello", 3, name="1234.56"); //! ``` //! //! print two significantly different things: //! //! ```text //! Hello, `1234.560` has 3 fractional digits //! Hello, `123` has 3 characters //! Hello, ` 123` has 3 right-aligned characters //! ``` //! //! # Escaping //! //! The literal characters `{` and `}` may be included in a string by preceding //! them with the same character. For example, the `{` character is escaped with //! `{{` and the `}` character is escaped with `}}`. //! //! [`usize`]: ../../std/primitive.usize.html //! [`isize`]: ../../std/primitive.isize.html //! [`i8`]: ../../std/primitive.i8.html //! [`Display`]: trait.Display.html //! [`Binary`]: trait.Binary.html //! [`fmt::Result`]: type.Result.html //! [`Result`]: ../../std/result/enum.Result.html //! [`std::fmt::Error`]: struct.Error.html //! [`Formatter`]: struct.Formatter.html //! [`write!`]: ../../std/macro.write.html //! [`Debug`]: trait.Debug.html //! [`format!`]: ../../std/macro.format.html //! [`to_string`]: ../../std/string/trait.ToString.html //! [`writeln!`]: ../../std/macro.writeln.html //! [`write_fmt`]: ../../std/io/trait.Write.html#method.write_fmt //! [`std::io::Write`]: ../../std/io/trait.Write.html //! [`print!`]: ../../std/macro.print.html //! [`println!`]: ../../std/macro.println.html //! [`eprint!`]: ../../std/macro.eprint.html //! [`eprintln!`]: ../../std/macro.eprintln.html //! [`write!`]: ../../std/macro.write.html //! [`format_args!`]: ../../std/macro.format_args.html //! [`fmt::Arguments`]: struct.Arguments.html //! [`write`]: fn.write.html //! [`format`]: fn.format.html #![stable(feature = "rust1", since = "1.0.0")] #[unstable(feature = "fmt_internals", issue = "0")] pub use core::fmt::rt; #[stable(feature = "rust1", since = "1.0.0")] pub use core::fmt::{Formatter, Result, Write}; #[stable(feature = "rust1", since = "1.0.0")] pub use core::fmt::{Binary, Octal}; #[stable(feature = "rust1", since = "1.0.0")] pub use core::fmt::{Debug, Display}; #[stable(feature = "rust1", since = "1.0.0")] pub use core::fmt::{LowerHex, Pointer, UpperHex}; #[stable(feature = "rust1", since = "1.0.0")] pub use core::fmt::{LowerExp, UpperExp}; #[stable(feature = "rust1", since = "1.0.0")] pub use core::fmt::Error; #[stable(feature = "rust1", since = "1.0.0")] pub use core::fmt::{write, ArgumentV1, Arguments}; #[stable(feature = "rust1", since = "1.0.0")] pub use core::fmt::{DebugList, DebugMap, DebugSet, DebugStruct, DebugTuple}; #[stable(feature = "fmt_flags_align", since = "1.28.0")] pub use core::fmt::{Alignment}; use crate::string; /// The `format` function takes an [`Arguments`] struct and returns the resulting /// formatted string. /// /// The [`Arguments`] instance can be created with the [`format_args!`] macro. /// /// # Examples /// /// Basic usage: /// /// ``` /// use std::fmt; /// /// let s = fmt::format(format_args!("Hello, {}!", "world")); /// assert_eq!(s, "Hello, world!"); /// ``` /// /// Please note that using [`format!`] might be preferable. /// Example: /// /// ``` /// let s = format!("Hello, {}!", "world"); /// assert_eq!(s, "Hello, world!"); /// ``` /// /// [`Arguments`]: struct.Arguments.html /// [`format_args!`]: ../../std/macro.format_args.html /// [`format!`]: ../../std/macro.format.html #[stable(feature = "rust1", since = "1.0.0")] pub fn format(args: Arguments<'_>) -> string::String { let capacity = args.estimated_capacity(); let mut output = string::String::with_capacity(capacity); output .write_fmt(args) .expect("a formatting trait implementation returned an error"); output }