fast_io

The README shows extensive benchmarks where fast_io outperforms <iostream>, <cstdio>, and fmtlib by up to 10x, such as outputting 10M integers in 0.049s vs 0.462s for fstream on Windows.

As a header-only library using C++20 concepts, it offers a clean, extensible API without historical baggage like std::endl or traits_type, as emphasized in the philosophy.

Supports UTF-8, UTF-16, UTF-32, and niche codecvt like GB18030, with SSE-optimized conversions that beat GNU iconv in benchmarks (e.g., 0.442s vs 0.844s for UTF-8 to UTF-32LE).

Works in environments without a full standard library and is tested on Windows, Linux, and MinGW, providing consistent RAII wrappers for file descriptors and handles.