[OpenSource 사용하기] fmt: C++ formatting library (feat. python styled print)

본 문서는 'OpenSource 뜯어보기'라는 프로젝트의 일환으로.
fmt라는 C++ formatting library를 설치, 실행 등 사용하는 방법에 대해 정리해보았습니다. 해당 라이브러리는 python처럼 print를 할 수 있게 도와주는 라이브러리로 C++20 부터는 std::format으로 사용할 수 있으니 참고하시기 바랍니다.

💡 본 문서는 'OpenSource 사용하기'라는 프로젝트의 일환으로.
gRPC라는 c++ RPC library를 설치, 실행 등 사용하는 방법과 더불어, library 내 구조, 소스 분석 및 패턴 분석까지 다룰 예정이니 해당 오픈소스에 관심이 있다면 봐두길 권장합니다.

1. 오픈소스의 목적

1.1 '{fmt} C++ formatting library'란?

• Simple format API with positional arguments for localization
• Implementation of C++20 std::format
• Format string syntax similar to Python's format
• Fast IEEE 754 floating-point formatter with correct rounding, shortness and round-trip guarantees
• Safe printf implementation including the POSIX extension for positional arguments
• Extensibility: support for user-defined types
• High performance: faster than common standard library implementations of (s)printf, iostreams, to_string and to_chars, see Speed tests and Converting a hundred million integers to strings per second
• Small code size both in terms of source code with the minimum configuration consisting of just three files, core.h, format.h and format-inl.h, and compiled code; see Compile time and code bloat
• Reliability: the library has an extensive set of tests and is continuously fuzzed
• Safety: the library is fully type safe, errors in format strings can be reported at compile time, automatic memory management prevents buffer overflow errors
• Ease of use: small self-contained code base, no external dependencies, permissive MIT license
• Portability with consistent output across platforms and support for older compilers
• Clean warning-free codebase even on high warning levels such as -Wall -Wextra -pedantic
• Locale-independence by default
• Optional header-only configuration enabled with the FMT_HEADER_ONLY macro

See the documentation for more details.
See the benchmark for compare with iostream, printf

1.2 'fmt: C++ formatting library' 추가 정보

Git: https://github.com/fmtlib/fmt

• Note that master is generally a work in progress, and you probably want to use a tagged release version.

Lisence: MIT License: Copyright (c) 2012 - present, Victor Zverovich

GitHub - fmtlib/fmt: A modern formatting library

 

2. 환경 구축

using CMake (FetchContent)

FetchContent_Declare(
  fmt
  GIT_REPOSITORY https://github.com/fmtlib/fmt.git
  GIT_TAG master
)
FetchContent_MakeAvailable(fmt)

add_library(fmtlib main.cpp)
target_link_libraries(fmtlib PUBLIC fmt::fmt)
add_executable(fmtexe main.cpp)

fmt::format은 C++20을 사용한다면 std::format으로 standard library에 포함되었으니 참고바랍니다.

3. 간단한 사용

간단한 사용 (in fmt library)

Example

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();
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);

Header

<fmt/core.h>	lightweight subset of formatting API
<fmt/format.h>	full API = core.h + compile-time format checks, iterators, user-def. types, …
<fmt/args.h>	core.h + dynamic format arguments
<fmt/chrono.h>	format.h + date and time formatting
<fmt/compile.h>	format.h + format string compilation
<fmt/color.h>	format.h + terminal color and text style
<fmt/os.h>	format.h + file output, system APIs
<fmt/ostream.h>	format.h + std::ostream support
<fmt/printf.h>	format.h + printf formatting
<fmt/ranges.h>	format.h + formatting support for ranges and tuples
<fmt/xchar.h>	format.h + wchar_t support

간단한 사용 (in C++20 standard library)

#include <format>
#include <cassert>
int main() {
    std::string message = std::format("The answer is {}.", 42);
    assert( message == "The answer is 42." );
}

fmt library cheat sheets

참고

• [Github] fmtlib/fmt: https://github.com/fmtlib/fmt
• {fmt} A modern formatting library: https://fmt.dev/latest/index.html
• [stackoverflow] cmake add fmt library: https://stackoverflow.com/questions/66531225/cmake-add-fmt-library 
• [cpp reference] cpp reference format library: https://en.cppreference.com/w/cpp/utility/format