Overview — fmt 10.2.1 documentation
Author: Victor Zverovich
Description: Small, safe and fast formatting library
Keywords: C++, formatting, printf, string, library

# {fmt}

A modern formatting library

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# Overview

{fmt} is an open-source formatting library providing a fast and safe alternative to C stdio and C++ iostreams.

What users say:
Thanks for creating this library. It’s been a hole in C++ for a long time. I’ve used both `boost::format` and `loki::SPrintf`, and neither felt like the right answer. This does.

## Format API

The format API is similar in spirit to the C `printf` family of function but is safer, simpler and several times faster than common standard library implementations. The format string syntax is similar to the one used by str.format in Python:

    std::string s = fmt::format("The answer is {}.", 42);

The `fmt::format` function returns a string “The answer is 42.”. You can use `fmt::memory_buffer` to avoid constructing `std::string`:

    auto out = fmt::memory_buffer();
    fmt::format_to(std::back_inserter(out),
              "For a moment, {} happened.", "nothing");
    auto data = out.data(); // pointer to the formatted data
    auto size = out.size(); // size of the formatted data

The `fmt::print` function performs formatting and writes the result to a stream:

    fmt::print(stderr, "System error code = {}\n", errno);

If you omit the file argument the function will print to `stdout`:

    fmt::print("Don't {}\n", "panic");

The format API also supports positional arguments useful for localization:

    fmt::print("I'd rather be {1} than {0}.", "right", "happy");

You can pass named arguments with `fmt::arg`:

    fmt::print("Hello, {name}! The answer is {number}. Goodbye, {name}.",
               fmt::arg("name", "World"), fmt::arg("number", 42));

If your compiler supports C++11 user-defined literals, the suffix `_a` offers an alternative, slightly terser syntax for named arguments:

    using namespace fmt::literals;
    fmt::print("Hello, {name}! The answer is {number}. Goodbye, {name}.",
               "name"_a="World", "number"_a=42);

## Safety

The library is fully type safe, automatic memory management prevents buffer overflow, errors in format strings are reported using exceptions or at compile time. For example, the code

    fmt::format("The answer is {:d}", "forty-two");

throws the `format_error` exception because the argument `"forty-two"` is a string while the format code `d` only applies to integers.

The code

    format(FMT_STRING("The answer is {:d}"), "forty-two");

reports a compile-time error on compilers that support relaxed `constexpr`. See here for details.

The following code

    fmt::format("Cyrillic letter {}", L'\x42e');

produces a compile-time error because wide character `L'\x42e'` cannot be formatted into a narrow string. For comparison, writing a wide character to `std::ostream` results in its numeric value being written to the stream (i.e. 1070 instead of letter ‘ю’ which is represented by `L'\x42e'` if we use Unicode) which is rarely desirable.

## Compact Binary Code

The library produces compact per-call compiled code. For example (godbolt),

    #include <fmt/core.h>

    int main() {
      fmt::print("The answer is {}.", 42);
    }

compiles to just

    main: # @main
      sub rsp, 24
      mov qword ptr [rsp], 42
      mov rcx, rsp
      mov edi, offset .L.str
      mov esi, 17
      mov edx, 1
      call fmt::v7::vprint(fmt::v7::basic_string_view<char>, fmt::v7::format_args)
      xor eax, eax
      add rsp, 24
      ret
    .L.str:
      .asciz "The answer is {}."

## Portability

The library is highly portable and relies only on a small set of C++11 features:

- variadic templates

- type traits

- rvalue references

- decltype

- trailing return types

- deleted functions

- alias templates

These are available in GCC 4.8, Clang 3.4, MSVC 19.0 (2015) and more recent compiler version. For older compilers use {fmt} version 4.x which is maintained and only requires C++98.

The output of all formatting functions is consistent across platforms. For example,

    fmt::print("{}", std::numeric_limits<double>::infinity());

always prints `inf` while the output of `printf` is platform-dependent.

## Ease of Use

{fmt} has a small self-contained code base with the core library consisting of just three header files and no external dependencies. A permissive MIT license allows using the library both in open-source and commercial projects.

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