std::format / std::print

What problem does this section solve?

Formatting strings in C++ has always been a bit cumbersome:

• printf: fast but type-unsafe, format string errors can cause crashes.
• std::cout：Type-safe but verbose to write and inconvenient for format control.
• std::stringstream: Full-featured but verbose.

C++20/23 introduced std::format (for formatted strings) and std::print (for direct output), combining Python-like simplicity with C++'s type safety and high performance.

What is this feature?

• std::format (C++20): similar to Python's f"{name}: {score}", returns the formatted std::string.
• std::print (C++23): Similar to Python's print(), it directly outputs formatted results to stdout.

Based on the {fmt} library, it delivers exceptional performance—on par with or even exceeding printf.

C++ standard version

• std::format：C++20
• std::print：C++23

Required header files

#include <format>    // for std::format (C++20)
#include <print>     // for std::print, std::println (C++23)

Basic Syntax

// std::format：返回 string
std::string s = std::format("Hello, {}!", name);
std::string s2 = std::format("{0} + {1} = {2}", a, b, a + b);

// std::print：直接输出（不自动换行）
std::print("x = {}, y = {}", x, y);

// std::println：输出并换行
std::println("Hello, {}!", name);

// 指定输出目标：stdout / stderr 是 FILE*
std::println(stdout, "normal message");
std::println(stderr, "error message");

// 格式控制
std::format("{:.2f}", 3.14159);     // "3.14" —— 保留 2 位小数
std::format("{:>10}", 42);          // "        42" —— 右对齐，宽度 10
std::format("{:<10}", 42);          // "42        " —— 左对齐
std::format("{:^10}", 42);          // "    42    " —— 居中
std::format("{:#x}", 255);          // "0xff" —— 十六进制带前缀
std::format("{:04d}", 7);           // "0007" —— 前导零填充

Output Mode Comparison

std::print outputs to the standard output stream (stdout) by default, similar to std::cout. You can customize the destination by providing a FILE* or std::FILE* as the first argument.

In the C++23 standard library, the identifiers std::print and std::println are commonly used in two typical patterns:

Here, stdout and stderr are FILE* in the C standard library, so you need to include <cstdio>.

Note: In standard C++23, the first parameter of std::println is not std::cout. std::cout is a C++ std::ostream object belonging to the <iostream> namespace. You might have encountered similar fmt::print(std::cout, "...") syntax in certain libraries or extensions, but that interface is not the same as the one provided by the standard library std::println.

Example code

Example 1: basic usage of std::format

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

int main()
{
    std::string name = "Alice";
    int age = 25;
    double score = 92.5;

    // 基本占位符
    std::string s1 = std::format("Name: {}, Age: {}, Score: {}", name, age, score);
    std::cout << s1 << "\n";

    // 指定顺序
    std::string s2 = std::format("{1} is {0} years old", age, name);
    std::cout << s2 << "\n";

    // 保留 2 位小数
    std::string s3 = std::format("Score: {:.2f}", score);
    std::cout << s3 << "\n";

    return 0;
}

Results：

Name: Alice, Age: 25, Score: 92.5
Alice is 25 years old
Score: 92.50

Example 2: Building on Example 1, format control and alignment

#include <iostream>
#include <format>

int main()
{
    // 右对齐，宽度 10
    std::cout << std::format("[{:>10}]\n", 42);

    // 左对齐，宽度 10
    std::cout << std::format("[{:<10}]\n", 42);

    // 居中，宽度 10
    std::cout << std::format("[{:^10}]\n", 42);

    // 前导零
    std::cout << std::format("{:05d}\n", 7);

    // 十六进制
    std::cout << std::format("hex = {:#x}, oct = {:#o}\n", 255, 255);

    // 打印一个简单的表格
    std::cout << std::format("{:<10} {:>5} {:>7}\n", "Name", "Age", "Score");
    std::cout << std::format("{:<10} {:>5} {:>7.1f}\n", "Alice", 25, 92.5);
    std::cout << std::format("{:<10} {:>5} {:>7.1f}\n", "Bob", 22, 88.0);
    std::cout << std::format("{:<10} {:>5} {:>7.1f}\n", "Charlie", 24, 78.5);

    return 0;
}

Results：

[        42]
[42        ]
[    42    ]
00007
hex = 0xff, oct = 0377
Name         Age   Score
Alice         25    92.5
Bob           22    88.0
Charlie       24    78.5

Example 3: std::print Direct Output (C++23)

#include <cstdio>
#include <print>
#include <string>

int main()
{
    std::string name = "World";
    int value = 42;

    // 直接输出，不换行
    std::print("Hello, ");
    std::print("{}", name);
    std::print("!\n");

    // 输出到 stdout 并换行
    std::println("The answer is {}", value);

    // 第一个参数也可以显式写 stdout
    std::println(stdout, "stdout: {}", name);

    // 带格式输出
    std::println("pi = {:.3f}", 3.1415926);

    // 输出到 stderr
    std::println(stderr, "Error: something went wrong!");

    return 0;
}

Results：

Hello, World!
The answer is 42
stdout: World
pi = 3.142
Error: something went wrong!

runtime results

See the "running results" for each example above.

Key syntax explanation in the example

|Here is the translation of the provided Simplified Chinese Markdown fragment into natural American English, following all specified rules.

Common Errors

Error 1: Missing the index before the format specifier :

std::format("{.2f}", 3.14);       // ❌ 缺少 : 前的占位符

Correct approach: std::format("{:.2f}", 3.14); or std::format("{}", 3.14);

Error 2: Parameter count mismatch

std::format("{}, {}", 1);  // ❌ 有 2 个占位符，只有 1 个参数

Compilation errors occur here (which is where format is safer than printf).

Error 3: Strings output with format contain { or }

std::format("Set = {1, 2, 3}");  // ❌ {1, 2, 3} 会被误解为格式说明

Correct approach: use {{ to represent the literal value {, and }} to represent the literal value }:

std::format("Set = {{1, 2, 3}}");  // 输出：Set = {1, 2, 3}

使用建议

• 明确目标：在开始前确定您的具体需求，以便选择最合适的工具或教程。
• 充分利用资源：参考官方文档、教程和博客，这些资料能帮助您快速上手并解决问题。
• 实践应用：通过动手操作项目或编写代码来巩固学习成果，提升实际操作能力。
• 问题解决：遇到困难时，查阅参考资料或寻求社区支持，逐步培养独立解决问题的能力。
• 分享经验：完成项目后，可以撰写文章或博客分享心得，帮助其他学习者。

如果需要针对特定领域（如单片机、机器人或环境搭建）的进一步建议，请提供更多信息，我将为您细化内容。

1. C++20 Project Replaces stringstream and sprintf with std::format: Cleaner code, type-safe, and high performance.
2. C++23 Project Simplifies Formatting Output with std::print/std::println：Output defaults to stdout, and you can also pass stderr.
3. Format specifiers are similar to Python: If you are familiar with Python's formatting syntax, it is easy to pick up.
4. If your compiler doesn't support C++20/23 yet, you can use the {fmt} library (https://github.com/fmtlib/fmt），它是标准化的基础。

Summary

• std::format("{} + {} = {}", a, b, c) returns formatted strings.
• std::print("...") / std::println("...") directly outputs to stdout.
• std::println(stderr, "...") can output to standard error.
• Type-Safe + Powerful Formatting + High Performance: Combines the speed of printf with the safety of cout.
• Formatting controls: {:.2f} decimal places, {:>10} alignment, {:#x} base, {:04d} zero-padding.