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std::print: Formatted Output (C++23)

Introduced in C++23, std::print and its companion std::println are in the <print> header and provide a modern, type-safe, and high-performance mechanism for formatted output, designed to replace the legacy printf family and the often-verbose std::cout.

Core Concepts

At its heart, std::print is a function that writes formatted text directly to standard output. It leverages the formatting engine from C++20's <format> library.

Pro's

  • Compile-Time Safety: The format string is parsed at compile time. If the type or number of arguments doesn't match the format specifiers, you get a compiler error, not a runtime crash or garbage output.

  • High Performance: It is faster than std::cout as it avoids the overhead of iostreams' complex architecture. It's designed to be a highly efficient, low-level I/O utility.

  • Clean and Readable Syntax: The Python-inspired syntax using {} placeholders is more intuitive and less error-prone than printf's % specifiers or the << chaining of iostreams.

Basic Usage

The syntax is straightforward. You provide a format string with {} placeholders for the arguments that follow.

The library offers two primary functions for convenience:

  • std::print(fmt, args...): Prints the formatted string.
  • std::println(fmt, args...): Prints the formatted string and automatically appends a newline character.
#include <print>
#include <string>

int main() {
    std::string name = "Alice";
    int age = 30;
    double height = 1.68;

    // Using std::print
    std::print("Name: {}, Age: {}, Height: {}\n", name, age, height);

    // Using std::println for automatic newline
    std::println("Name: {}, Age: {}, Height: {}", name, age, height);

    // Positional arguments
    std::println("Hello {0}. {0}, welcome to {1}!", "World", "C++23");
    
    return 0;
}
Name: Alice, Age: 30, Height: 1.68
Name: Alice, Age: 30, Height: 1.68
Hello World. World, welcome to C++23!

Formatting, using on std::format in the backend

  • std::format: Creates and returns a std::string. It performs no I/O.
  • std::print: Performs I/O by writing formatted text directly to stdout.
#include <print>
#include <numbers> // for std::numbers::pi

int main() {
    // 1. Width, Alignment, and Fill
    // <: left-align, >: right-align, ^: center-align
    std::println("|{:<10}|", "left");
    std::println("|{:>10}|", "right");
    std::println("|{:*^10}|", "center"); // Use '*' as a fill character

    // 2. Floating-Point Precision
    double pi = std::numbers::pi;
    std::println("Pi to 3 decimal places: {:.3f}", pi);
    std::println("Pi in scientific notation: {:.4e}", pi);

    // 3. Integer Bases
    int value = 100;
    std::println("Decimal: {}", value);
    std::println("Hex: {:#x}", value);    // # adds the '0x' prefix
    std::println("Binary: {:#b}", value); // # adds the '0b' prefix
    
    // 4. Positional Arguments
    std::println("First: {0}, Second: {1}, First again: {0}", "alpha", "beta");
    
    return 0;
}
|left      |
|     right|
|**center**|
Pi to 3 decimal places: 3.142
Pi in scientific notation: 3.1416e+00
Decimal: 100
Hex: 0x64
Binary: 0b1100100
First: alpha, Second: beta, First again: alpha