std::format (C++20)

In a nutshell

A type-safe printf alternative—format strings with {} placeholders, compile-time checking of the argument count, and support for formatting custom types.

Header file

#include <format>

Core API Cheat Sheet

Operation	Signature	Description
Format a string	string format(fmt, args...)	Returns the formatted string
Format to output	void vformat_to(out_it, fmt, args)	Outputs to an iterator
Format to buffer	format_to_n(out, n, fmt, args...)	writes into a fixed buffer, returns the output end and the total needed length
Format to stdout	(C++23) void print(fmt, args...)	Writes directly to standard output
Positional arguments	"{0} {1} {0}"	References arguments by index
Width/precision	"{:>10.2f}"	Right-aligned, width 10, precision 2
Custom formatting	template<> struct formatter<T>	Specialize std::formatter to support custom types

Minimal Example

// Standard: C++20
#include <format>
#include <iostream>
#include <string>

int main() {
    std::string s = std::format("Hello, {}!", "world");
    std::cout << s << "\n"; // Hello, world!

    int version = 2;
    double pi = 3.14159265;
    std::cout << std::format("v{}. pi={:.2f}", version, pi) << "\n";
    // v2. pi=3.14

    // Positional arguments
    std::cout << std::format("{0} + {0} = {1}", 3, 6) << "\n";
    // 3 + 3 = 6
}

Embedded Suitability: Medium

• Replaces printf, eliminating the risk of runtime crashes from a format string that does not match the argument types
• Replaces std::stringstream, avoiding heap allocation overhead
• Checks the argument count at compile time, but full compile-time validation of format specifiers requires the help of mandatory compile-time checking of format strings (C++23, P2216)
• Flash overhead can be significant (the code size of the formatting engine); evaluate before use on extremely resource-constrained devices
• The {fmt} library can serve as a fallback option from C++11 onward

Compiler Support

GCC	Clang	MSVC
13	17	19.29

See Also

• cppreference: std::format

---

Part of the content is referenced from cppreference.com and is licensed under CC-BY-SA 4.0