C++ 设备层，传输层，应用层demo

C++设备层通常涉及底层硬件交互，如操作系统设备驱动程序的编写。它通常涉及到系统编程，例如使用`<devio.h>`（假设这是一个虚构的头文件）这样的库来进行硬盘读写、网络I/O等操作。下面是一个简单的设备层示例，创建一个模拟设备驱动读取数据：

#include <iostream>
#include <string>

class DeviceDriver {
public:
    virtual ~DeviceDriver() {}
    virtual std::string readData(int buffer_size) = 0;
};

class MockDeviceDriver : public DeviceDriver {
private:
    std::string data;

public:
    MockDeviceDriver(const std::string& init_data) : data(init_data) {}

    std::string readData(int buffer_size) override {
        if (buffer_size >= data.size()) {
            return data;
        } else {
            return data.substr(0, buffer_size);
        }
    }
};

int main() {
    DeviceDriver* driver = new MockDeviceDriver("Hello from device layer!");
    std::string received = driver->readData(10); // 模拟读取10字节
    std::cout << "Received: " << received << std::endl;
    delete driver; // 释放资源
    return 0;
}

传输层（如TCP/IP或UDP）在C++中经常通过`boost.asio`或更现代的`std::network`库来实现。一个基础的TCP服务器示例：

#include <boost/asio.hpp>
using tcp = boost::asio::ip::tcp;

int main() {
    try {
        boost::asio::io_context io;
        tcp::acceptor acceptor(io, tcp::endpoint(tcp::v4(), 12345));

        for (;;) {
            tcp::socket socket(io);
            acceptor.accept(socket);

            boost::array<char, 1024> receive_buffer;
            auto bytes_received = boost::asio::async_read(socket,
                boost::asio::buffer(receive_buffer), 
                [socket](auto ec, std::size_t /* bytes_transferred */) mutable {
                    if (ec == boost::asio::error::eof) {
                        std::cout << "Client disconnected." << std::endl;
                    } else if (!ec) {
                        std::cout << "Received: '" << std::string(receive_buffer.data(), bytes_received) << "'\n";
                    } else {
                        std::cerr << "Error reading: " << ec.message() << std::endl;
                    }
                });
        }
    } catch (std::exception& e) {
        std::cerr << e.what() << std::endl;
    }

    return 0;
}

应用层示例，比如使用HTTP客户端发送GET请求：

#include <boost/beast/core.hpp>
#include <boost/beast/http.hpp>
#include <boost/asio/connect.hpp>
#include <cstdlib>
#include <iostream>

int main() {
    try {
        using beast::net::get;
        using http::verb::get;

        auto const url = u8"https://www.example.com/"; // 替换为实际URL

        beast::flat_buffer buffer;
        http::request<http::string_body> req{ get{} };
        req.set_version(http::version::http_1_1);
        req.target(url);
        req.prepare_payload();

        beast::write(buffer, req);

        beast::streambuf sb;
        beast::async_connect(beast::make_io_service().get_executor(),
            {req.host(), req.port()}, sb,
            [&](auto err) {
                if (err)
                    std::cerr << "Failed to connect: " << err.message() << std::endl;
                else {
                    beast::async_write(sb, buffer, [](auto const& ec, std::size_t) {
                        if (ec)
                            std::cerr << "Failed to write: " << ec.message() << std::endl;
                        else
                            std::cout << "Request sent.\n";
                    });
                }
            });

        beast::io_service().run();
    } catch (std::exception const& e) {
        std::cerr << "Error: " << e.what() << std::endl;
    }

    return 0;
}