I/O: Streams, Formatting & std::print

#include <iostream>
#include <string>
#include <limits>    // For numeric_limits

int main() {
    // Output with cout
    std::cout << "Hello, " << "World!" << '\n';      // Chained insertion
    std::cout << "The answer is: " << 42 << '\n';    // Automatic type formatting

    // Error output — cerr is unbuffered (immediate), clog is buffered
    std::cerr << "Error: something went wrong\n";

    // Input with cin
    std::cout << "Enter your name: ";
    std::string name;
    std::getline(std::cin, name);         // Read entire line (including spaces)

    std::cout << "Enter your age: ";
    int age{};
    std::cin >> age;                       // Read a single value (stops at whitespace)

    // Check if input succeeded
    if (std::cin.fail()) {
        std::cerr << "Invalid input!\n";
        std::cin.clear();                  // Reset error flags
        std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');  // Discard bad input
    }

    // The getline-after-cin trap:
    // After cin >> age, a '\n' remains in the buffer.
    // A subsequent getline() would read an empty string!
    // Fix: call cin.ignore() before getline():
    std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

    std::string city;
    std::cout << "Enter your city: ";
    std::getline(std::cin, city);          // Now works correctly

    std::cout << name << " (" << age << ") from " << city << '\n';
}

#include <fstream>
#include <iostream>
#include <string>
#include <vector>

int main() {
    // Writing to a file
    {
        std::ofstream out("scores.txt");  // Opens for writing (creates or truncates)
        if (!out) {
            std::cerr << "Failed to open file for writing\n";
            return 1;
        }
        out << "Alice 95\n";
        out << "Bob 87\n";
        out << "Charlie 92\n";
    }  // File is automatically closed when 'out' goes out of scope (RAII!)

    // Reading from a file
    std::ifstream in("scores.txt");
    if (!in) {
        std::cerr << "Failed to open file for reading\n";
        return 1;
    }

    std::string name;
    int score{};
    std::vector<std::pair<std::string, int>> results;

    while (in >> name >> score) {           // Read until failure (EOF or bad data)
        results.emplace_back(name, score);
    }

    for (const auto& [n, s] : results) {
        std::cout << n << ": " << s << '\n';
    }

    // Appending to a file
    std::ofstream append("scores.txt", std::ios::app);  // Open in append mode
    append << "Diana 98\n";

    // Reading entire file line by line
    std::ifstream in2("scores.txt");
    std::string line;
    while (std::getline(in2, line)) {
        std::cout << "Line: " << line << '\n';
    }
}

#include <format>   // C++20
#include <print>    // C++23
#include <string>
#include <vector>
#include <numbers>  // C++20: mathematical constants

int main() {
    // std::format (C++20) — returns a formatted std::string
    std::string msg = std::format("Hello, {}!", "World");
    // msg == "Hello, World!"

    // Positional arguments
    std::string pos = std::format("{1} comes before {0}", "second", "first");
    // pos == "first comes before second"

    // Formatting specifiers
    std::string nums = std::format(
        "int: {:>10d}\n"    // Right-aligned, width 10, decimal
        "hex: {:#06x}\n"    // '0x' prefix, width 6, zero-padded hex
        "float: {:.3f}\n"   // 3 decimal places, fixed notation
        "sci: {:.2e}\n"     // 2 decimal places, scientific notation
        "pi: {:.10f}",      // 10 decimal places
        42, 255, 3.14159, 12345.6789, std::numbers::pi
    );

    // std::print (C++23) — format + output in one step
    std::print("Name: {}, Age: {}\n", "Alice", 30);
    std::print("Pi is approximately {:.6f}\n", std::numbers::pi);

    // std::println (C++23) — like print but adds a newline
    std::println("Score: {:>5}", 95);       // "Score:    95"
    std::println("Score: {:>5}", 100);      // "Score:   100"

    // Formatting containers — print each element
    std::vector<int> values = {10, 20, 30, 40, 50};
    for (std::size_t i = 0; i < values.size(); ++i) {
        std::println("[{:2}] = {:>4}", i, values[i]);
    }

    // Why prefer std::format/print over iostream?
    // 1. Type-safe (unlike printf)
    // 2. Readable format strings (unlike iostream manipulators)
    // 3. No state leakage (iostream manipulators are sticky!)
    // 4. Better performance in many cases
}