std::format (C++20)

TL;DR

A type-safe printf alternative—format strings using {} placeholders, checks argument count at compile time, and supports custom type formatting.

Header

<format>

Core API Cheat Sheet

Operation	Signature	Description
Format string	std::format(fmt, args...)	Returns formatted string
Format to output	std::format_to(out, fmt, args...)	Outputs to iterator
Format to buffer	std::formatted_size(fmt, args...)	Pre-calculates output length
Format to stdout	(C++23) std::print(fmt, args...)	Outputs directly to standard output
Positional args	std::format("{0} {1}", a, b)	References arguments by index
Width/precision	std::format("{:>10.2}", v)	Right-aligned, width 10, precision 2
Custom formatting	template<> struct formatter<T>	Specialize formatter to support custom types

Minimal Example

#include <format>
#include <iostream>
#include <string>

int main() {
    // Basic replacement
    std::string s = std::format("The answer is {}.", 42);
    // s == "The answer is 42."

    // Alignment and width
    int x = 42;
    std::cout << std::format("{:>10}", x) << '\n'; // "        42"

    // Type-specific formatting (hex)
    std::cout << std::format("{:#x}", 255) << '\n'; // "0xff"
}

Embedded Applicability: Medium

• Replaces printf, eliminating runtime crash risks from mismatched format strings and argument types.
• Replaces std::stringstream, avoiding heap allocation overhead.
• Checks argument count at compile time, but full compile-time validation of format specifiers requires std::format in C++23.
• Flash overhead may be significant (formatting engine code size); evaluate for resource-constrained devices.
• The {fmt} library can be used as a backport for C++11 and later.

Compiler Support

GCC	Clang	MSVC
13	17	19.29

See Also

• cppreference: std::format

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