Format Library in C++20: Modernizing Text Formatting

The C++20 standard introduced a new text formatting library, called the Format Library. The Format Library provides a modern, safe, and extensible way to format text in C++.

Modern text formatting is important for a number of reasons. First, it allows you to produce more readable and understandable output. Second, it can help you to make your code more efficient and easier to maintain. Third, it can help you to create more visually appealing output.

This article will provide an overview of the Format Library in C++20. We will discuss the shortcomings of traditional C++ formatting methods, and we will show how the Format Library can be used to overcome these shortcomings. We will also discuss the basic syntax and elements of the Format Library, and we will provide code examples demonstrating its basic usage. Finally, we will discuss some of the advanced features of the Format Library, such as format specifications, positional arguments, and dynamic formats.

The Shortcomings of Traditional C++ Formatting

Traditional C++ formatting methods, such as printf and iostream, have a number of shortcomings. These methods are not type-safe, extensible, or efficient.

Not type-safe

Traditional C++ formatting methods are not type-safe. This means that it is possible to pass the wrong type of argument to a formatting function, which can lead to errors. For example, the following code will compile and run, but it will print an incorrect value:

printf("%d", "Hello, world!");Code language: C++ (cpp)

The printf function expects an integer argument, but in this case, we are passing a string argument. This will cause the printf function to print the number of characters in the string, which is not what we want.

Not extensible

Traditional C++ formatting methods are not extensible. This means that it is difficult to add new formatting features or to customize the formatting of existing output. For example, if we want to add a new formatting feature that allows us to print a date in a specific format, we would have to modify the printf function or write our own formatting function. This can be a difficult and time-consuming task.

Not efficient

Traditional C++ formatting methods are not as efficient as they could be. This is because they often involve copying data from one buffer to another. For example, the following code will print the string “Hello, world!” to the console:

std::cout << "Hello, world!";Code language: C++ (cpp)

This code will copy the string “Hello, world!” from the stack to the heap. This can be a performance bottleneck in applications that need to format a lot of text.

The Format Library addresses all of the shortcomings of traditional C++ formatting methods. It is type-safe, extensible, and efficient. This makes it the preferred method for formatting text in C++.

Understanding the Basics of C++20 Format Library

The C++20 Format Library provides a modern, safe, and extensible way to format text in C++. The Format Library is based on the following principles:

Type safety

The Format Library is type-safe. This means that it is not possible to pass the wrong type of argument to a formatting function, which can lead to errors. For example, the following code will not compile:

std::format("%d", "Hello, world!");Code language: C++ (cpp)

The std::format function expects an integer argument, but in this case, we are passing a string argument. This will cause the compiler to generate an error.

Extensible

The Format Library is extensible. This means that it is easy to add new formatting features or to customize the formatting of existing output. For example, if we want to add a new formatting feature that allows us to print a date in a specific format, we can simply write a new formatting function. This is much easier than modifying the existing formatting functions or writing our own formatting functions.

Efficient

The Format Library is efficient. This means that it does not involve copying data from one buffer to another. For example, the following code will print the string “Hello, world!” to the console:

std::format("Hello, world!");Code language: C++ (cpp)

This code will not copy the string “Hello, world!” from the stack to the heap. This makes the Format Library a good choice for applications that need to format a lot of text.

Basic Syntax

The basic syntax of the Format Library is as follows:

std::format("Format string", arguments);Code language: C++ (cpp)

The format string is a string that contains instructions for formatting the arguments. The arguments are the values that will be formatted.

Elements of Format Library

The format string can contain any of the following elements:

Literals

Literals are characters that are not interpreted by the Format Library. For example, the following code will print the string “Hello, world!” to the console:

std::format("Hello, world!");Code language: C++ (cpp)

Placeholders

Placeholders are special characters that are used to indicate where arguments should be placed. For example, the following code will print the value of the variable name to the console:

std::format("Hello, {}!", name);Code language: C++ (cpp)

The placeholder {} will be replaced with the value of the variable name.

Format specifications

Format specifications are used to control the formatting of arguments. For example, the following code will print the value of the variable number as a decimal number with a width of 10 characters:

std::format("The number is {}.", number, 10);Code language: C++ (cpp)

The format specification 10 specifies that the number should be formatted with a width of 10 characters.

The following table lists the available format specifications:

Specification	Description
`d`	Decimal number
`o`	Octal number
`x`	Hexadecimal number
`f`	Floating-point number
`s`	String
`c`	Character
`b`	Boolean

The Format Library also supports a number of advanced features, such as positional arguments, dynamic formats, and localization. These features are beyond the scope of this introduction.

Here are some code examples demonstrating the basic usage of the Format Library:

// Print the string "Hello, world!" to the console.
std::format("Hello, world!");

// Print the value of the variable `name` to the console.
std::format("Hello, {}!", name);

// Print the value of the variable `number` as a decimal number with a width of 10 characters.
std::format("The number is {}.", number, 10);Code language: C++ (cpp)

Digging Deeper: Advanced Features

Format specifications

Python’s format specifications allow you to control the output of a formatted string. For example, the following code will print the following output:

>>> "This is a string with {0} characters".format(len("This is a string"))
'This is a string with 12 characters'Code language: C++ (cpp)

The format specification {0} refers to the first argument passed to the format() method. You can also use numbered format specifications, such as {1}, {2}, and so on.

Positional arguments

Positional arguments are arguments that are passed to a function in a specific order. For example, the following code defines a function that takes two positional arguments:

def add(a, b):
  return a + bCode language: C++ (cpp)

You can call this function by passing the arguments in the correct order:

>>> add(1, 2)
3Code language: C++ (cpp)

If you pass the arguments in the wrong order, you will get an error:

>>> add(2, 1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: add() missing 1 required positional argument: 'a'Code language: C++ (cpp)

Dynamic formats

Dynamic formats allow you to create formatted strings that are based on the values of variables. For example, the following code will print the following output:

>>> name = "John Doe"
>>> age = 30
>>> "Hello, {name}. You are {age} years old.".format(name=name, age=age)
'Hello, John Doe. You are 30 years old.'Code language: C++ (cpp)

You can use dynamic formats to create more complex and informative output.

Here are some more examples of advanced features in Python:

Lambda functions

Lambda functions are small, anonymous functions that can be used to perform simple tasks. For example, the following code defines a lambda function that squares its input:

>>> square = lambda x: x * x
>>> square(2)
4Code language: C++ (cpp)

Lambda functions can be used in place of regular functions in many cases. They are particularly useful for short, one-line functions.

Generators

Generators are a type of iterator that can be used to create sequences of values. Generators are created using the yield keyword. For example, the following code defines a generator that generates the Fibonacci sequence:

def fibonacci():
  a, b = 0, 1
  while True:
    yield a
    a, b = b, a + bCode language: PHP (php)

You can iterate over a generator using a for loop:

for i in fibonacci():
  print(i)Code language: C++ (cpp)

This will print the following output:

Generators are a powerful tool for creating iterators. They can be used to implement many different algorithms and data structures.

Decorators

Decorators are a way of adding functionality to functions without modifying the function’s code. Decorators are defined using the @ symbol. For example, the following code defines a decorator that prints the name of the function being decorated:

def trace(func):
  def wrapper(*args, **kwargs):
    print("Calling function:", func.__name__)
    result = func(*args, **kwargs)
    print("Returned value:", result)
    return result
  return wrapperCode language: C++ (cpp)

You can use the trace() decorator to decorate any function. For example, the following code defines a function called add():

def add(a, b):
  return a + bCode language: C++ (cpp)

You can decorate the add() function with the trace() decorator:

@trace
def add(a, b):
  return a + bCode language: C++ (cpp)

Now, when you call the add() function, the trace() decorator will print the name of the function and the return value. For example:

>>> add(1, 2)
Calling function: add
Returned value: 3Code language: C++ (cpp)

Decorators are a powerful tool for adding functionality to functions. They can be used to implement many different patterns, such as logging, caching, and memoization.

Error Handling in C++20 Format Library

The C++20 Format Library can throw two types of errors:

Format errors – Format errors occur when the format string is invalid. For example, if the format string contains a placeholder that is not valid for the type of argument being passed, a format error will be thrown.

Argument errors – Argument errors occur when an argument is not of the correct type. For example, if the format string specifies that an argument is a string, but a number is passed, an argument error will be thrown.

Error handling

Error handling in the C++20 Format Library is done using exceptions. When an error occurs, the Format Library will throw an exception of the appropriate type. The caller of the Format Library must catch the exception and handle it appropriately.

Here is an example of how to handle errors in the C++20 Format Library:

#include <iostream>
#include <format>

int main() {
  try {
    std::cout << std::format("The number is {0}", 42.0);
  } catch(std::format_error& e) {
    std::cerr << "Error: " << e.what() << std::endl;
  }

  return 0;
}Code language: C++ (cpp)

This code will print the following output:

Error: invalid format argument '42.0'Code language: JavaScript (javascript)

The std::format_error exception contains information about the error that occurred. The caller of the std::format() function can use this information to display a more informative error message to the user.

Performance Implications

The C++20 Format Library is a powerful tool for formatting text. However, it is important to be aware of the performance implications of using the Format Library.

The Format Library is more expensive than traditional methods of formatting text, such as using std::cout <<. This is because the Format Library must parse the format string at runtime, which can be a costly operation.

In addition, the Format Library can be slower than traditional methods if the format string is complex. This is because the Format Library must perform a variety of checks on the format string to ensure that it is valid.

For these reasons, it is important to use the Format Library judiciously. If you are formatting text that is not performance-critical, then the Format Library is a good choice. However, if you are formatting text that is performance-critical, then you should consider using a traditional method of formatting text.

Here is a comparison of the performance of the Format Library with traditional methods of formatting text:

Method	Cost
Format Library	More expensive
`std::cout <<`	Less expensive

Here is an example of how to use the Format Library to format text:

#include <iostream>
#include <format>

int main() {
  std::cout << std::format("The number is {0}", 42.0);

  return 0;
}Code language: C++ (cpp)

This code will print the following output:

The number is 42.0Code language: CSS (css)

Here is an example of how to use std::cout << to format text:

#include <iostream>

int main() {
  std::cout << "The number is " << 42.0 << std::endl;

  return 0;
}Code language: C++ (cpp)

This code will print the following output:

The number is 42.0Code language: CSS (css)

As you can see, the Format Library is more expensive than std::cout <<. However, the Format Library is also more powerful. The Format Library can be used to format text in a variety of ways, while std::cout << is limited to formatting text in a simple way.

Ultimately, the decision of whether to use the Format Library or std::cout << depends on your specific needs. If you need to format text in a variety of ways, then the Format Library is a good choice. However, if you need to format text in a simple way, then std::cout << is a good choice.

Real-World Use Cases

The C++20 Format Library can be used in a variety of real-world scenarios and projects. Here are a few examples:

Logging

The Format Library can be used to format log messages. This can be useful for making log messages more readable and informative. For example, the following code formats a log message using the Format Library:

#include <iostream>
#include <format>

int main() {
  std::cout << std::format("Error: {0} occurred on line {1}", "File not found", 123);

  return 0;
}Code language: C++ (cpp)

This code will print the following output:

Error: File not found occurred on line 123Code language: JavaScript (javascript)

Generating output

The Format Library can be used to generate output for a variety of purposes. For example, the following code generates a formatted table of data:

#include <iostream>
#include <format>

int main() {
  std::cout << std::format("{0:^20} | {1:^20} | {2:^20}", "Name", "Age", "Occupation");
  std::cout << std::endl;
  std::cout << std::format("{0:^20} | {1:^20} | {2:^20}", "John Doe", "30", "Software Engineer");
  std::cout << std::endl;
  std::cout << std::format("{0:^20} | {1:^20} | {2:^20}", "Jane Doe", "25", "Doctor");

  return 0;
}Code language: C++ (cpp)

This code will print the following output:

Name                 | Age                | Occupation
-------------------- | ------------------ | ----------------
John Doe             | 30                 | Software Engineer
Jane Doe             | 25                 | Doctor

Creating user interfaces

The Format Library can be used to create user interfaces. For example, the following code creates a simple text-based user interface:

#include <iostream>
#include <format>

int main() {
  std::cout << "Welcome to the Format Library User Interface!" << std::endl;
  std::cout << "What would you like to do?" << std::endl;
  std::cout << std::format("1. Format a string\n2. Generate output\n3. Quit\n");
  int choice;
  std::cin >> choice;
  switch (choice) {
    case 1:
      std::cout << "Enter a string to format: ";
      std::string str;
      std::cin >> str;
      std::cout << std::format("The formatted string is: {0}", str);
      break;
    case 2:
      std::cout << "Enter the type of output to generate: ";
      std::string type;
      std::cin >> type;
      if (type == "table") {
        std::cout << "Enter the data for the table: ";
        std::string data;
        std::cin >> data;
        std::cout << std::format("The table is: {0}", data);
      } else {
        std::cout << "Invalid output type." << std::endl;
      }
      break;
    case 3:
      std::cout << "Goodbye!" << std::endl;
      break;
    default:
      std::cout << "Invalid choice." << std::endl;
      break;
  }

  return 0;
}Code language: C++ (cpp)

This code will print the following output:

Welcome to the Format Library User Interface!
What would you like to do?
1. Format a string
2. Generate output
3. Quit
1
Enter a string to format: Hello, world!
The formatted string is: Hello, world!Code language: JavaScript (javascript)

These are just a few examples of how the C++20 Format Library can be used in real-world scenarios and projects. The Format Library is a powerful tool that can be used to format text in a variety of ways.

Best Practices for Using the Format Library

Here are some best practices for using the C++20 Format Library:

Use the correct format specifiers for the type of argument being passed. The Format Library supports a variety of format specifiers for different types of arguments. For example, the %d format specifier is used for integers, the %s format specifier is used for strings, and the %f format specifier is used for floating-point numbers.
Use the {} braces to specify the arguments to be formatted. The {} braces are used to specify the arguments to be formatted. For example, the following code formats the string “Hello, world!” using the %s format specifier:

std::cout << std::format("Hello, world!");Code language: C++ (cpp)

Use the := operator to specify the format specifiers. The := operator is used to specify the format specifiers. For example, the following code formats the integer 42 using the %d format specifier:

std::cout << std::format("{0:d}", 42);Code language: C++ (cpp)

Use the width and precision modifiers to control the formatting of the output. The width and precision modifiers can be used to control the formatting of the output. For example, the following code formats the integer 42 with a width of 10 and a precision of 2:

std::cout << std::format("{0:10.2f}", 42.0);Code language: C++ (cpp)

Use the fill modifier to specify the fill character. The fill modifier can be used to specify the fill character. For example, the following code formats the integer 42 with a fill character of *:

std::cout << std::format("{0:*^10}", 42);Code language: C++ (cpp)

Use the align modifier to specify the alignment of the output. The align modifier can be used to specify the alignment of the output. For example, the following code formats the integer 42 with a right alignment:

std::cout << std::format("{0:>10}", 42);Code language: C++ (cpp)

Use the locale modifier to specify the locale. The locale modifier can be used to specify the locale. For example, the following code formats the integer 42 using the French locale:

std::cout << std::format("{0:d}", 42, std::locale("fr_FR.utf8"));Code language: C++ (cpp)

Use the named format specifiers to specify the format specifiers by name. The named format specifiers can be used to specify the format specifiers by name. For example, the following code formats the integer 42 using the d named format specifier:

std::cout << std::format("{0:d}", 42);Code language: C++ (cpp)

Use the argument_index modifier to specify the argument index. The argument_index modifier can be used to specify the argument index. For example, the following code formats the first argument using the %s format specifier:

std::cout << std::format("{1:s}", "Hello, world!");Code language: C++ (cpp)

Use the format_args variadic template to format a variable number of arguments. The format_args variadic template can be used to format a variable number of arguments. For example, the following code formats the string “Hello, world!” and the integer 42:

std::cout << std::format("Hello, {0}! You are {1} years old.", "world", 42);Code language: C++ (cpp)

Use the format_to function to format text to a string. The format_to function can be used to format text to a string. For example, the following code formats the string “Hello, world!” to a string:

std::string str = std::format("Hello, world!");Code language: C++ (cpp)

Use the format_to_stream function to format text to a stream. The format_to_stream function can be used to format text to a stream. For example, the following code formats the string “Hello, world!” to the standard output stream:

std::format_to(std::cout, "Hello, world!");Code language: C++ (cpp)