fmt/doc/index.md

# 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](https://vitaut.net/posts/2020/fast-int-to-string-revisited/)
than common standard library implementations.
The [format string syntax](syntax.md) is similar to the one used by
[str.format](https://docs.python.org/3/library/stdtypes.html#str.format)
in Python:

```c++
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`:

```c++
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:

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

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

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

The format API also supports positional arguments useful for
localization:

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

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

```c++
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:

```c++
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

```c++
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

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

reports a compile-time error on compilers that support relaxed `constexpr`.
See [Compile-Time Format String Checks](api.md#compile-time-format-string-checks)
for details.

The following code

```c++
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](https://godbolt.org/g/TZU4KF)),

```c++
#include <fmt/core.h>

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

compiles to just

```asm
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](https://github.com/fmtlib/fmt/releases/tag/4.1.0) 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](https://github.com/fmtlib/fmt#license) allows
using the library both in open-source and commercial projects.

<a class="btn btn-success" href="https://github.com/fmtlib/fmt">
  GitHub Repository
</a>