fmt
A modern formatting library
{fmt} is a modern C++ library for formatting and printing text with a clean Python-like syntax, faster than printf and safer than iostreams, with compile-time error checking in C++20.
{fmt} is a modern C++ library for formatting and printing text. The problem it solves is that C++'s built-in ways to format output — printf from the C standard library, and iostreams (the cout style) — are either unsafe, slow, or awkward to use. Printf uses format strings like "%d" and "%s" that the compiler can't check, so a mismatch between the format and the actual value causes crashes or wrong output. Iostreams are type-safe but verbose and slow.
{fmt} gives you a cleaner syntax similar to Python's string formatting, where you write placeholders in curly braces: fmt::format("The answer is {}.", 42) produces "The answer is 42." The library catches format errors at compile time in C++20, so mistakes are caught before the program even runs. It also handles dates and times, colored terminal output, printing collections like lists and vectors, and writing to files. Benchmarks show it is faster than both printf and standard C++ streams.
You would use {fmt} in any C++ project where you need to produce formatted strings or output — logging, building strings for display or serialization, or replacing slow printf calls. It has no external dependencies, works across platforms (Linux, macOS, Windows), and is licensed under the MIT license. It also implements the C++20 standard std::format interface, making code written with it forward-compatible with the standard library.
Where it fits
- Replace printf or cout in a C++ logging system with faster, type-safe formatted output using curly-brace placeholders.
- Format dates, terminal colors, and collections like vectors using a single consistent API across platforms.
- Port legacy printf-based code to type-safe output that catches format errors at compile time in C++20.