C++20協程實戰：高效網絡庫、手機終端、多媒體開發開發指南

基于C++協程和事件循環的網絡庫

以下是基于C++協程和事件循環的網絡庫實例，涵蓋常見場景和功能實現。示例基于libuv、Boost.Asio或自定義事件循環，結合C++20協程（如std::coroutine）或其他協程庫（如cppcoro）實現。

基礎TCP服務器

#include <cppcoro/task.hpp>
#include <cppcoro/io_service.hpp>
#include <cppcoro/socket.hpp>cppcoro::task<> handle_connection(cppcoro::socket client) {char buffer[1024];auto bytes_read = co_await client.recv(buffer, sizeof(buffer));co_await client.send(buffer, bytes_read);client.close();
}cppcoro::task<> tcp_server(cppcoro::io_service& io_service, uint16_t port) {auto listen_socket = cppcoro::socket::create_tcpv4(io_service);listen_socket.bind(cppcoro::ipv4_endpoint{ {}, port });listen_socket.listen();while (true) {auto client = co_await listen_socket.accept();handle_connection(std::move(client));}
}

異步HTTP客戶端

#include <cppcoro/http/client.hpp>cppcoro::task<> fetch_data() {cppcoro::http::client client;auto response = co_await client.request("GET", "https://example.com");std::string body = co_await response.body();std::cout << "Response: " << body << std::endl;
}

定時器任務

#include <cppcoro/task.hpp>
#include <cppcoro/io_service.hpp>
#include <cppcoro/sync_wait.hpp>cppcoro::task<> delayed_task(cppcoro::io_service& io_service) {co_await io_service.schedule_after(std::chrono::seconds(1));std::cout << "Task executed after 1 second" << std::endl;
}

文件異步讀取

#include <cppcoro/io_service.hpp>
#include <cppcoro/read_only_file.hpp>cppcoro::task<> read_file(cppcoro::io_service& io_service) {auto file = cppcoro::read_only_file::open(io_service, "example.txt");char buffer[1024];auto bytes_read = co_await file.read(0, buffer, sizeof(buffer));std::cout << "File content: " << std::string(buffer, bytes_read) << std::endl;
}

UDP廣播服務器

#include <cppcoro/socket.hpp>cppcoro::task<> udp_broadcaster(cppcoro::io_service& io_service) {auto socket = cppcoro::socket::create_udpv4(io_service);socket.bind(cppcoro::ipv4_endpoint{ {}, 12345 });const char* message = "Broadcast message";co_await socket.send_to(cppcoro::ipv4_endpoint{ INADDR_BROADCAST, 12345 }, message, strlen(message));
}

協程并行任務

#include <cppcoro/when_all.hpp>cppcoro::task<int> task1() { co_return 42; }
cppcoro::task<std::string> task2() { co_return "hello"; }cppcoro::task<> run_parallel() {auto [num, str] = co_await cppcoro::when_all(task1(), task2());std::cout << num << ", " << str << std::endl;
}

WebSocket服務器

#include <boost/beast/core.hpp>
#include <boost/beast/websocket.hpp>boost::asio::awaitable<void> ws_session(boost::beast::websocket::stream<boost::beast::tcp_stream> ws) {co_await ws.async_accept();while (true) {boost::beast::flat_buffer buffer;co_await ws.async_read(buffer);co_await ws.async_write(buffer.data());}
}

SSL/TLS加密連接

#include <boost/asio/ssl.hpp>boost::asio::awaitable<void> ssl_client(boost::asio::ssl::stream<boost::asio::ip::tcp::socket> ssl_socket) {co_await ssl_socket.async_handshake(boost::asio::ssl::stream_base::client);co_await ssl_socket.async_write_some(boost::asio::buffer("Hello SSL"));
}

協程任務取消

#include <cppcoro/cancellation_token.hpp>cppcoro::task<> cancellable_task(cppcoro::cancellation_token token) {while (!token.is_cancellation_requested()) {co_await std::chrono::seconds(1);std::cout << "Working..." << std::endl;}std::cout << "Cancelled!" << std::endl;
}

異步DNS解析

#include <cppcoro/net/ip_address.hpp>cppcoro::task<> resolve_hostname(cppcoro::io_service& io_service) {auto addresses = co_await cppcoro::dns::resolve(io_service, "example.com");for (const auto& addr : addresses) {std::cout << "Resolved IP: " << addr.to_string() << std::endl;}
}

基于C++ `auto`關鍵字的實例

以下是基于C++ auto關鍵字的實用示例，涵蓋不同場景下的應用，包括容器遍歷、Lambda表達式、類型推導等。示例均遵循現代C++（C++11及以上）標準編寫。

基礎類型推導

auto x = 42;            // 推導為int
auto y = 3.14;          // 推導為double
auto z = "Hello";       // 推導為const char*
auto flag = true;       // 推導為bool

容器遍歷

std::vector<int> vec = {1, 2, 3};
for (auto num : vec) {  // 自動推導為intstd::cout << num << " ";
}

STL迭代器簡化

std::map<std::string, int> m = {{"a", 1}, {"b", 2}};
for (auto it = m.begin(); it != m.end(); ++it) {std::cout << it->first << ": " << it->second << "\n";
}

Lambda表達式

auto add = [](auto a, auto b) { return a + b; };  // C++14泛型Lambda
std::cout << add(5, 3.2);  // 輸出8.2

函數返回類型推導（C++14）

auto multiply(double a, double b) {return a * b;  // 返回類型推導為double
}

結構化綁定（C++17）

std::tuple<int, std::string> t = {1, "Alice"};
auto [id, name] = t;  // 自動解包

模板函數簡化

template<typename T>
auto process(T container) -> decltype(container.size()) {return container.size();
}

類型別名簡化

auto ptr = std::make_shared<int>(10);  // 推導為std::shared_ptr<int>

避免冗長的類型聲明

std::vector<std::map<std::string, std::list<int>>> complex;
for (auto& outer : complex) {  // 避免重復復雜類型for (auto& inner : outer) {for (auto num : inner.second) {/* ... */}}
}

類型安全的NULL指針

auto nullPtr = nullptr;  // 推導為std::nullptr_t

范圍循環與引用

std::vector<std::string> names = {"Bob", "Eve"};
for (auto& name : names) {  // 避免拷貝name += "!";
}

函數指針簡化

auto funcPtr = &multiply;  // 推導為double(*)(double, double)

類型推導與const

const auto pi = 3.14159;  // 推導為const double
auto& ref = pi;           // 推導為const double&

初始化列表

auto list = {1, 2, 3};    // 推導為std::initializer_list<int>

類型萃取輔助

std::vector<int> v = {1, 2, 3};
auto size = v.size();      // 推導為size_t（避免有符號/無符號問題）

多類型容器遍歷

std::unordered_map<std::string, float> prices = {{"apple", 1.2f}};
for (auto& [key, value] : prices) {  // C++17結構化綁定std::cout << key << ": " << value << "\n";
}

配合decltype使用

int a = 10;
decltype(auto) b = a;  // 推導為int（保留引用和const）

自動推導數組長度

int arr[] = {1, 2, 3};
auto len = std::size(arr);  // C++17，推導為size_t

簡化復雜模板實例

auto matrix = std::vector<std::vector<double>>(3, std::vector<double>(3, 0.0));

類型安全的宏替代

#define MAX(a, b) ({ auto _a = (a); auto _b = (b); _a > _b ? _a : _b; })

配合智能指針

auto uptr = std::make_unique<int>(42);  // 推導為std::unique_ptr<int>

避免臨時對象

auto result = std::find(vec.begin(), vec.end(), 2);  // 推導為std::vector<int>::iterator

類型推導與移動語義

auto str = std::string("Hello");
auto movedStr = std::move(str);  // 推導為std::string&&

泛型編程輔助

template<typename T>
auto printTypeInfo(const T& val) {std::cout << typeid(val).name() << "\n";
}

自動推導函數對象

auto greaterThan = [threshold=10](int x) { return x > threshold; };
std::cout << greaterThan(15);  // 輸出1（true）

配合類型轉換

auto intValue = static_cast<int>(3.14);  // 顯式轉換后推導為int

多返回值處理

auto [min, max] = std::minmax({5, 2, 8, 3});  // 推導為std::pair<int, int>

避免類型截斷

auto largeNumber = 10000000000LL;  // 推導為long long

動態類型檢查輔助

std::any data = 42;
if (auto ptr = std::any_cast<int>(&data)) {std::cout << *ptr;  // 安全類型檢查
}

這些示例覆蓋了auto在變量聲明、循環、函數返回類型、Lambda表達式等場景的應用，能有效提升代碼簡潔性和可維護性。注意合理使用以避免降低可讀性。

基于C++的移動開發實例

以下是基于C++的移動開發實例，涵蓋基礎功能、網絡通信、傳感器交互等場景，適用于Android/iOS平臺開發（通常通過NDK或跨平臺框架實現）。每個例子均提供核心思路和代碼片段：

基礎UI交互

例子1：調用系統通知欄

// Android NDK示例
#include <jni.h>
#include <string>extern "C" JNIEXPORT void JNICALL
Java_com_example_app_MainActivity_showNotification(JNIEnv* env, jobject thiz, jstring title) {const char *nativeTitle = env->GetStringUTFChars(title, nullptr);// 通過JNI調用Java的NotificationManagerjclass cls = env->FindClass("android/app/NotificationManager");// 實際代碼需補充完整構造邏輯env->ReleaseStringUTFChars(title, nativeTitle);
}

例子2：創建本地對話框

// Qt Quick示例（跨平臺）
#include <QMessageBox>void showDialog() {QMessageBox::information(nullptr, "Title", "C++ triggered dialog");
}

硬件訪問

例子3：獲取電池狀態

// Android NDK
extern "C" JNIEXPORT jint JNICALL
Java_com_example_app_BatteryMonitor_getLevel(JNIEnv* env, jobject thiz) {jclass cls = env->GetObjectClass(thiz);jmethodID mid = env->GetMethodID(cls, "getSystemService", "(Ljava/lang/String;)Ljava/lang/Object;");// 實際需通過Java層BatteryManager獲取return 75; // 示例值
}

例子4：控制閃光燈

// iOS通過AVFoundation
#import <AVFoundation/AVFoundation>void toggleTorch(bool on) {AVCaptureDevice *device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];if ([device hasTorch]) {[device lockForConfiguration:nil];[device setTorchMode: on ? AVCaptureTorchModeOn : AVCaptureTorchModeOff];[device unlockForConfiguration];}
}
</