کنترل جریان و حلقه‌ها

#include <iostream>
#include <map>
#include <string>
#include <optional>

std::optional<int> find_value(const std::string& key) {
    std::map<std::string, int> data = {{"alpha", 1}, {"beta", 2}, {"gamma", 3}};
    if (auto it = data.find(key); it != data.end()) {  // C++17 if with initializer
        return it->second;
    }
    return std::nullopt;
}

int main() {
    // C++17: if with initializer — the variable's scope is limited to the if/else
    if (auto val = find_value("beta"); val.has_value()) {
        std::cout << "Found: " << *val << '\n';
    } else {
        std::cout << "Not found\n";
    }
    // 'val' is no longer in scope here — cleaner!

    // switch with C++17 initializer
    enum class Color { Red, Green, Blue };
    Color c = Color::Green;

    switch (auto name = "unknown"; c) {  // C++17 switch with initializer
        case Color::Red:   name = "red";   std::cout << name << '\n'; break;
        case Color::Green: name = "green"; std::cout << name << '\n'; break;
        case Color::Blue:  name = "blue";  std::cout << name << '\n'; break;
    }

    // [[fallthrough]] attribute (C++17) — document intentional fallthrough
    int priority = 2;
    switch (priority) {
        case 1:
            std::cout << "Critical: ";
            [[fallthrough]];       // Tells the compiler this is intentional
        case 2:
            std::cout << "Important: ";
            [[fallthrough]];
        case 3:
            std::cout << "Handle it\n";
            break;
        default:
            std::cout << "Unknown priority\n";
    }
}

#include <iostream>
#include <vector>
#include <string>
#include <map>

int main() {
    std::vector<std::string> names = {"Alice", "Bob", "Charlie"};

    // Traditional for loop — use when you need the index
    for (std::size_t i = 0; i < names.size(); ++i) {
        std::cout << i << ": " << names[i] << '\n';
    }

    // Range-based for loop (C++11) — preferred for iteration
    for (const auto& name : names) {        // const auto& avoids copying
        std::cout << name << '\n';
    }

    // Range-based for with structured bindings (C++17)
    std::map<std::string, int> scores = {{"Alice", 95}, {"Bob", 87}, {"Charlie", 92}};
    for (const auto& [name, score] : scores) {   // Destructure the pair
        std::cout << name << " scored " << score << '\n';
    }

    // C++20: range-based for with init-statement
    for (auto v = std::vector{1, 2, 3, 4, 5}; auto elem : v) {
        std::cout << elem << ' ';
    }
    std::cout << '\n';

    // while loop — use when the number of iterations is unknown
    int n = 1;
    while (n <= 1024) {
        std::cout << n << ' ';
        n *= 2;
    }
    std::cout << '\n';

    // do-while — body executes at least once
    int input;
    do {
        std::cout << "Enter a positive number: ";
        input = 42;  // Simulated input for example
    } while (input <= 0);
}

#include <iostream>
#include <type_traits>
#include <string>

template <typename T>
void describe(T value) {
    std::cout << "Value: " << value;

    if constexpr (std::is_integral_v<T>) {
        // This branch only compiles when T is an integer type
        std::cout << " (integer, " << (value % 2 == 0 ? "even" : "odd") << ")";
    } else if constexpr (std::is_floating_point_v<T>) {
        // This branch only compiles when T is a floating-point type
        std::cout << " (floating-point)";
    } else {
        std::cout << " (other type)";
    }
    std::cout << '\n';
}

int main() {
    describe(42);           // Value: 42 (integer, even)
    describe(3.14);         // Value: 3.14 (floating-point)
    describe("hello");      // Value: hello (other type)
}