fmt Knowledge Base

fmt is a fast, type-safe formatting library with Python-style replacement fields — {} holes in a string, filled by arguments checked at compile time wherever possible. It is not just "a formatting library": it is the reference implementation that P0645 turned into C++20's std::format, and it still ships features the standard hasn't caught up with, from named arguments to color output to support on toolchains that don't have std::format at all.

How this is organised

Roughly outside-in: Overview → Basics get you formatting strings today; the Format spec mini-language is the reference you keep coming back to for every {:...} you write; Custom types and Compile-time checks are what you need to use fmt across a real codebase instead of just in a script; Advanced features and Performance are the allocation- and throughput-sensitive corners you reach for once fmt is on a hot path.

Sections

	Section	What it covers
	Overview	What it is, installing it, its relationship to std::format, why not printf
	Basics	Replacement fields, the format/print/format_to family, named arguments
	Format Spec Mini-Language	The full spec grammar: fill, align, sign, width, precision, type, locale
	Formatting Custom Types	fmt::formatter<T>, the ostream bridge, ranges, std::chrono
	Compile-Time Checks	FMT_STRING, consteval checking, reading the errors
	Advanced Features	format_to, memory_buffer, color and styles, Unicode
	Performance & Best Practices	Where the speed comes from, build modes, migration, pitfalls

Suggested reading paths

• Coming from printf: Comparison with printf → Format strings → Format spec syntax.
• Formatting your own types: formatter specialization → Ranges → Compile-time checks.
• Chasing allocations: format_to → memory_buffer → Performance characteristics.

Quick reference

the 90% of the API

#include <fmt/format.h>
#include <fmt/ranges.h>

std::string s = fmt::format("{} scored {:.1f}", name, score);
fmt::print("{:>10} | {:<10}\n", left, right);          // aligned columns
fmt::print(stderr, "error: {}\n", msg);

fmt::memory_buffer buf;                                // no std::string allocation
fmt::format_to(std::back_inserter(buf), "{:08.3f}", x);

fmt::print("{}\n", std::vector{1, 2, 3});              // [1, 2, 3] via fmt/ranges.h
fmt::print("{}\n", fmt::join(v, ", "));                // 1, 2, 3

Want	Spec
Right-align in 10	{:>10}
Zero-pad to 8	{:08}
3 decimal places	{:.3f}
Hex, with 0x	{:#x}
Binary	{:b}
Thousands separators	{:L}
Escape a brace	{{
Named argument	fmt::format("{n}", fmt::arg("n", x))