class: center, middle # CMPE 30: Lecture 14 std::format and std::print --- # What is wrong with this file-writing code? .lc[ ```cpp std::ofstream out; out << "Yo me la paso bien\n"; out.close(); ``` ] .rc[ 1. Missing `#include <fstream>` 1. No filename passed to the constructor 1. `\n` should be `std::endl` 1. Cannot `<<` to an `ofstream` 1. Ben got this wrong ] --- # Learning Objectives - Use `std::format` with `{}` placeholders - Distinguish implicit and indexed argument numbering - Apply format specifiers for width, alignment, fill, sign, base - Control floating-point precision with `{:.Nf}` - Use `std::print` / `std::println` to print in one call --- # Motivation .lc[ ```cpp // old way std::cout << std::fixed << std::setprecision(2) << std::setw(10) << score << "\n"; // new way std::println("{:10.2f}", score); ``` ] .rc[ - Stream manipulators are verbose and sticky - `std::format` (C++20) and `std::println` (C++23) are clean and one-shot ] --- # std::format Basics .lc[ ```cpp #include <format> #include <string> std::string artist = "Santana"; int year = 1999; std::string msg = std::format( "{} --- Smooth ({})", artist, year); // Santana --- Smooth (1999) ``` ] .rc[ - Returns a `std::string` - `{}` placeholders substituted **in order** - The compiler validates format strings ] --- # Indexed Arguments .lc[ ```cpp std::format("{1} --- {0} ({2})", "Santana", "Smooth", 1999); // "Smooth --- Santana (1999)" std::format("{0}, {0}, {0}!", "yeah"); // "yeah, yeah, yeah!" ``` ] .rc[ - Zero-based indices inside the braces - Good for reordering or repeating **Trap:** You **cannot mix** implicit `{}` and indexed `{0}` in the same format string. ] --- # Width and Alignment .lc[ ```cpp std::format("{:>10}", "hola"); // " hola" std::format("{:<10}", "hola"); // "hola " std::format("{:^10}", "hola"); // " hola " ``` ] .rc[ - Format spec: `{index:[[fill]align][sign][#][0][width][.prec][type]}` - `<`, `>`, `^` for left, right, center alignment - Default fill is space ] --- # Fill Characters .lc[ ```cpp std::format("{:*>10}", "hola"); // "******hola" std::format("{:-^20}", "Smooth"); // "-------Smooth-------" ``` ] .rc[ - Fill character comes **before** the alignment - Any single character works - Combine with width for clean tables ] --- # Sign and Alternate Form .lc[ ```cpp std::format("{:+}", 42); // "+42" std::format("{:-}", 42); // "42" std::format("{: }", 42); // " 42" std::format("{:#x}", 255); // "0xff" std::format("{:#b}", 10); // "0b1010" std::format("{:05}", 42); // "00042" ``` ] .rc[ - `+` always shows the sign - Space pads with a space for positive - `#` adds the base prefix - `0` zero-pads to width ] --- # Bases .lc[ ```cpp std::format("{:d}", 42); // "42" std::format("{:x}", 255); // "ff" std::format("{:o}", 8); // "10" std::format("{:b}", 10); // "1010" ``` ] .rc[ - `d` decimal, `x` hex, `o` octal, `b` binary - `X` for uppercase hex - Combine with `#` for the prefix ] --- # Floating-Point Precision .lc[ ```cpp std::format("{:.2f}", 3.14159); // "3.14" std::format("{:.4f}", 2.5); // "2.5000" std::format("{:10.2f}", 3.14); // " 3.14" ``` ] .rc[ - `.N` is the number of decimal places - `f` is fixed-point - Combine with width for aligned columns ] --- # std::print and std::println .lc[ ```cpp #include <print> std::println( "You get what you give"); std::print( "Track {:d}: {}", 1, "You Get What You Give"); std::println(""); double score = 9.5; std::println( "Rating: {:.1f}/10", score); ``` ] .rc[ - `std::print` --- no trailing newline - `std::println` --- adds a newline - One call instead of format + cout **Wut:** Requires C++23. Fall back to `cout << format(...)` if `<print>` is missing. ] --- # Try It --- Formatted Table .lc[ ```cpp std::cout << std::format( "{:<20} {:>5} {:>8}\n", "Song", "Year", "Score"); std::cout << std::format( "{:<20} {:>5} {:>8.1f}\n", "Wonderwall", 1995, 9.5); std::cout << std::format( "{:<20} {:>5} {:>8.1f}\n", "Jumper", 1997, 9.8); ``` ] .rc[ ``` Song Year Score Wonderwall 1995 9.5 Jumper 1997 9.8 ``` - Columns align cleanly - Reads top to bottom ] --- # What does std::format("{:>8.2f}", 3.1) produce? 1. `"3.10"` 1. `" 3.10"` 1. `"3.1 "` 1. `"00003.10"` 1. Ben got this wrong --- # What is wrong with this? .lc[ ```cpp std::string result = std::format( "{} scored {1} points", name, score); ``` ] .rc[ 1. Missing `#include <format>` 1. Cannot mix implicit `{}` and indexed `{1}` 1. `score` must be a string 1. Cannot have a space in a format string 1. Ben got this wrong ] --- # What does std::format("{:*^20}", "Hola") produce? 1. `"****Hola************"` 1. `"********Hola********"` 1. `"Hola****************"` 1. `"********Hola"` 1. Ben got this wrong --- # Key Points - `std::format` returns a `std::string` - `std::print`/`std::println` write directly - Implicit `{}` and indexed `{0}` **cannot be mixed** - Spec: `[[fill]align][sign][#][0][width][.prec][type]` - `{:.Nf}` for floats, `{:x}`/`{:o}`/`{:b}` for bases - Prefer `std::format`/`std::println` over `<<` with `setw`/`setprecision` **Read:** chapter 11 of *Gorgo Starting C++* (Exceptions). **Do:** exercises 1-9.