std::format (C++20)

In a Nutshell

A type-safe replacement for printf -- format strings with {} placeholders, compile-time argument count checking, and support for custom type formatting.

Header

#include <format>

Core API Quick Reference

Operation	Signature	Description
Format string	string format(fmt, args...)	Returns the formatted string
Format to output	void vformat_to(out_it, fmt, args)	Output to an iterator
Format to buffer	size_t formatted_size(fmt, args...)	Pre-compute output length
Format to stdout	(C++23) void print(fmt, args...)	Direct output to standard output
Positional arguments	"{0} {1} {0}"	Reference arguments by index
Width/precision	"{:>10.2f}"	Right-align, 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 Applicability: Medium

• Replaces printf, eliminating the risk of runtime crashes from format-string/argument type mismatches
• Replaces std::stringstream, avoiding heap allocation overhead
• Compile-time argument count checking; full compile-time format specifier validation requires C++23's std::is_constant_evaluated
• Flash overhead may be significant (formatting engine code size); evaluate for extremely resource-constrained devices
• The {fmt} library can serve as a C++11-compatible fallback

Compiler Support

GCC	Clang	MSVC
13	17	19.29

See Also

• cppreference: std::format

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