C++20 核心特性速览

#include <iostream>
#include <concepts>
#include <vector>
#include <type_traits>

// 定义 Concept：只接受数值类型
template<typename T>
concept Numeric = std::is_arithmetic_v<T>;

// 受约束的矩阵类
template<Numeric T>
class Matrix {
public:
    Matrix(size_t rows, size_t cols)
        : rows_(rows), cols_(cols), data_(rows * cols) {}

    T& at(size_t r, size_t c) { return data_[r * cols_ + c]; }
    const T& at(size_t r, size_t c) const { return data_[r * cols_ + c]; }

    // Concepts 直接写在参数上
    void fill(Numeric auto val) {
        std::fill(data_.begin(), data_.end(), val);
    }

    size_t rows() const { return rows_; }
    size_t cols() const { return cols_; }

    void print(const char* name) const {
        std::cout << name << " (" << rows_ << "x" << cols_ << "):\n";
        for (size_t i = 0; i < rows_; ++i) {
            std::cout << "  ";
            for (size_t j = 0; j < cols_; ++j)
                std::cout << at(i, j) << " ";
            std::cout << "\n";
        }
    }

private:
    size_t rows_, cols_;
    std::vector<T> data_;
};

// 仅对 Numeric 类型生效
template<Numeric T>
Matrix<T> addMatrices(const Matrix<T>& a, const Matrix<T>& b) {
    Matrix<T> result(a.rows(), a.cols());
    for (size_t i = 0; i < a.rows(); ++i)
        for (size_t j = 0; j < a.cols(); ++j)
            result.at(i, j) = a.at(i, j) + b.at(i, j);
    return result;
}

// 三路比较运算符：自动生成 ==, !=, <, <=, >, >=
struct ServerLoad {
    double cpu;
    double memory;
    int activeConnections;

    auto operator<=>(const ServerLoad&) const = default;
};

int main() {
    std::cout << "=== 飞翔科技 C++20 特性展示 ===\n\n";

    // 1. Concepts
    Matrix<int> a(2, 3);
    a.fill(1);
    a.at(0, 1) = 3;

    Matrix<int> b(2, 3);
    b.fill(2);
    b.at(1, 2) = 5;

    a.print("矩阵 A");
    b.print("矩阵 B");

    auto c = addMatrices(a, b);
    c.print("A + B");

    // 以下将编译失败（Concept 约束）
    // Matrix<std::string> strMat(2, 2);  // ❌ 不满足 Numeric concept

    // 2. 三路比较 <=>
    std::cout << "\n--- 三路比较 ---\n";
    ServerLoad s1{75.0, 60.0, 1000};
    ServerLoad s2{80.0, 55.0, 1200};
    ServerLoad s3{75.0, 60.0, 1000};

    std::cout << std::boolalpha;
    std::cout << "s1 == s3: " << (s1 == s3) << "\n";
    std::cout << "s1 != s2: " << (s1 != s2) << "\n";
    std::cout << "s1 <  s2: " << (s1 < s2) << "\n";
    std::cout << "s2 >  s1: " << (s2 > s1) << "\n";
    // <=> 比较规则：先 cpu，再 memory，最后 activeConnections

    // 3. span：安全数组视图
    std::cout << "\n--- span 安全视图 ---\n";
    std::vector<int> loads{10, 25, 50, 75, 90, 55, 30};

    auto printSpan = [](std::span<const int> s, const char* label) {
        std::cout << label << ": ";
        for (auto v : s) std::cout << v << " ";
        std::cout << " (size=" << s.size() << ")\n";
    };

    printSpan(loads, "完整视图");

    std::span<int> mid = std::span(loads).subspan(2, 3);
    mid[0] = 99;  // 修改视图即修改原数据
    printSpan(loads, "修改后  ");

    // 4. Ranges 管道式操作
    std::cout << "\n--- Ranges 管道 ---\n";
    std::vector<int> data{5, 12, 3, 8, 15, 7, 20, 1, 9};

    auto evenSquares = data
        | std::views::filter([](int n) { return n % 2 == 0; })
        | std::views::transform([](int n) { return n * n; })
        | std::views::take(3);

    std::cout << "偶数平方 (前3个): ";
    for (auto v : evenSquares) std::cout << v << " ";
    std::cout << "\n";

    // Ranges 排序
    std::ranges::sort(data);
    std::cout << "排序后: ";
    for (auto v : data) std::cout << v << " ";
    std::cout << "\n";
}

#include <iostream>
#include <format>
#include <string>
#include <chrono>
#include <vector>

struct ApiMetric {
    std::string endpoint;
    int statusCode;
    double latencyMs;
    size_t bytesTransferred;
};

int main() {
    std::cout << "=== 飞翔科技格式化日志 (C++20 format) ===\n\n";

    // format 替代 ostringstream
    std::string title = std::format("=== {} v{}.{}.{} ===",
        "飞翔-API-Gateway", 2, 1, 0);
    std::cout << title << "\n\n";

    // 精确控制格式
    double hitRate = 95.237;
    std::cout << std::format("缓存命中率: {:.2f}%\n", hitRate);  // 保留2位
    std::cout << std::format("十六进制: {:#x}\n", 255);           // 0xff
    std::cout << std::format("前导零: {:08d}\n", 42);             // 00000042

    // 表格对齐
    std::cout << "\n";
    std::cout << std::format("{:<20} {:>6} {:>10} {:>12}\n",
        "接口", "状态", "延迟(ms)", "传输字节");
    std::cout << std::string(52, '-') << "\n";

    std::vector<ApiMetric> metrics{
        {"/api/v1/users",    200, 12.5, 1024},
        {"/api/v1/orders",   200, 45.8, 8192},
        {"/api/v1/products", 500, 0.0,  0},
    };

    for (const auto& m : metrics) {
        std::cout << std::format("{:<20} {:>6} {:>10.1f} {:>12}\n",
            m.endpoint, m.statusCode, m.latencyMs, m.bytesTransferred);
    }

    // 编译期 format 检查（C++20 编译期字符串）
    // std::format("{:d}", "oops");  // ❌ 编译错误！类型不匹配
}

std::span<int> makeSpan() {
    std::vector<int> v{1, 2, 3};
    return std::span(v);  // ❌ v 析构后 span 悬挂
}

auto v = data | std::views::filter(pred);
// 实际过滤在遍历时才执行
// 修改源数据后再遍历，看到的是最新数据

// operator<=> 不自动生成 operator==（C++20 行为）
// 需要显式 default 或单独定义
// [注：此细节在不同实现中可能有差异]