// c++cpp20_demo.cpp

#include <algorithm>

#include <chrono>

#include <format>

#include <iostream>

#include <numbers>

#include <ranges>

#include <span>

#include <vector>

// ---------- Feature 1: concepts ----------

template<typename T>

concept Numeric = std::is_arithmetic_v<T>;

template<Numeric T>

T average(const std::span<T> data)

{

T sum = 0;

for (T val : data)

sum += val;

return sum / static_cast<T>(data.size());

}

void demo_concepts()

{

std::cout << "\n=== concepts ===\n";

std::vector<double> doubles{1.1, 2.2, 3.3};

std::cout << std::format("Average: {:.2f}\n",

average(std::span(doubles))); // compile error if non-numeric!

// Fails at compile: average(std::vector<std::string>{"a", "b"})

}

// ---------- Feature 2: ranges (views, filter, transform) ----------

void demo_ranges()

{

std::cout << "\n=== ranges ===\n";

std::vector<int> vec{1, 2, 3, 4, 5, 6};

// Lazy composable views (no copies!)

auto evens = vec | std::views::filter([](int n) { return n % 2 == 0; })

| std::views::transform([](int n) { return n * n; });

for (int sq : evens) {

std::cout << sq << " "; // 4 16 36

}

std::cout << "\n";

// reverse order print

for (const int n : vec | std::views::reverse) {

std::cout << n << " "; // 6 5 4 3 2 1

}

}

// ---------- Feature 3: std::format ----------

void demo_format()

{

std::cout << "\n=== std::format ===\n";

std::cout << std::vformat("Pi ≈ {:.3f}, e ≈ {:.3f}\n",

std::make_format_args(std::numbers::pi_v<double>,

std::numbers::e_v<double>));

int x = 42, y = 1234567;

std::cout << std::format("x={:>5}, y={}\n", x, y);

}

// ---------- Feature 4: std::span (non-owning view) ----------

void print_span(std::span<const int> data)

{

std::cout << "Span size=" << data.size() << ": [";

for (size_t i = 0; i < data.size(); ++i) {

std::cout << data[i] << (i + 1 < data.size() ? ", " : "");

}

std::cout << "]\n";

}

void demo_span()

{

std::cout << "\n=== std::span ===\n";

std::vector<int> vec{10, 20, 30, 40};

print_span(vec); // full

print_span(std::span{vec}.subspan(1, 2)); // [20, 30]

}

// ---------- Feature 5: chrono calendar (date literals) ----------

void demo_chrono()

{

std::cout << "\n=== chrono calendar ===\n";

using namespace std::chrono;

auto now = sys_days{2026y / January / 29} + 15h + 30min; // C++20 literals

std::cout << std::format("Vancouver time: {} ({})\n",

now,

year_month_day{

floor<days>(now)}); // use day-precision for year_month_day

}

// ---------- Feature 6: constinit & consteval ----------

constinit int global_counter = 0; // guaranteed init before main (thread-safe)

consteval int square(int x)

{ // always compile-time

return x * x;

}

constexpr int arr[] = {square(3), square(4)}; // consteval used here

void demo_const()

{

std::cout << "\n=== constinit & consteval ===\n";

std::cout << "arr[0]=" << arr[0] << ", arr[1]=" << arr[1] << "\n";

std::cout << "global_counter=" << ++global_counter << "\n";

}

// ---------- main ----------

int main()

{

std::cout << "C++20 feature tour\n";

demo_concepts();

demo_ranges();

demo_format();

demo_span();

demo_chrono();

demo_const();

std::cout << "\nDone.\n";

return 0;

}