boost实现FTP文件下载

Boost库提供了asio网络编程库，可以用它来实现FTP文件下载。以下是一个简单的示例代码，可以下载FTP服务器上的文件：

#include <iostream>
#include <fstream>
#include <boost/asio.hpp>
#include <boost/filesystem.hpp>

using namespace boost::asio;
using namespace boost::asio::ip;
using namespace boost::filesystem;

class ftp_client {
public:
    ftp_client(io_service& ios) : m_sock(ios) {}

    void download(const std::string& url, const std::string& user, const std::string& password, const std::string& remote_path, const std::string& local_path) {
        // 解析URL
        std::string host;
        uint16_t port = 21;
        std::string path;
        if (!parse_url(url, host, port, path)) {
            std::cerr << "Invalid URL: " << url << std::endl;
            return;
        }

        // 连接FTP服务器
        tcp::resolver resolver(m_sock.get_io_service());
        tcp::resolver::query query(host, std::to_string(port));
        tcp::resolver::iterator iter = resolver.resolve(query);
        tcp::endpoint ep = *iter;
        m_sock.connect(ep);

        // 读取欢迎消息
        read_reply();
        if (m_reply_code != 220) {
            std::cerr << "FTP server is not available." << std::endl;
            return;
        }

        // 登录
        send_command("USER " + user);
        read_reply();
        if (m_reply_code == 331) {
            send_command("PASS " + password);
            read_reply();
        }
        if (m_reply_code != 230) {
            std::cerr << "Invalid username or password." << std::endl;
            return;
        }

        // 进入被动模式
        send_command("PASV");
        read_reply();
        if (m_reply_code != 227) {
            std::cerr << "Failed to enter passive mode." << std::endl;
            return;
        }
        uint16_t data_port;
        if (!parse_pasv_reply(m_reply_msg, data_port)) {
            std::cerr << "Failed to parse PASV reply." << std::endl;
            return;
        }

        // 下载文件
        std::string filename = path.substr(path.find_last_of('/') + 1);
        if (!remote_path.empty()) {
            filename = remote_path + "/" + filename;
        }
        send_command("TYPE I");
        read_reply();
        if (m_reply_code != 200) {
            std::cerr << "Failed to set binary mode." << std::endl;
            return;
        }
        send_command("RETR " + filename);
        read_reply();
        if (m_reply_code != 150) {
            std::cerr << "Failed to start download." << std::endl;
            return;
        }
        tcp::socket data_sock(m_sock.get_io_service());
        tcp::endpoint data_ep = tcp::endpoint(m_sock.remote_endpoint().address(), data_port);
        data_sock.connect(data_ep);
        boost::system::error_code ec;
        boost::asio::streambuf buf;
        boost::asio::read(data_sock, buf, ec);
        data_sock.close();
        std::ofstream outfile(local_path, std::ios::out | std::ios::binary);
        outfile.write(boost::asio::buffer_cast<const char*>(buf.data()), buf.size());
        outfile.close();
        read_reply();
        if (m_reply_code != 226) {
            std::cerr << "Failed to complete download." << std::endl;
            return;
        }
    }

private:
    bool parse_url(const std::string& url, std::string& host, uint16_t& port, std::string& path) {
        size_t pos = url.find("://");
        if (pos == std::string::npos) {
            return false;
        }
        std::string protocol = url.substr(0, pos);
        if (protocol != "ftp") {
            return false;
        }
        url = url.substr(pos + 3);
        pos = url.find('/');
        if (pos == std::string::npos) {
            return false;
        }
        host = url.substr(0, pos);
        url = url.substr(pos);
        pos = host.find(':');
        if (pos != std::string::npos) {
            port = std::stoi(host.substr(pos + 1));
            host = host.substr(0, pos);
        }
        path = url;
        return true;
    }

    bool parse_pasv_reply(const std::string& reply, uint16_t& port) {
        size_t pos1 = reply.find('(');
        size_t pos2 = reply.find(')');
        if (pos1 == std::string::npos || pos2 == std::string::npos) {
            return false;
        }
        std::string data_str = reply.substr(pos1 + 1, pos2 - pos1 - 1);
        std::vector<std::string> parts;
        boost::split(parts, data_str, boost::is_any_of(","));
        if (parts.size() != 6) {
            return false;
        }
        uint16_t high_byte = std::stoi(parts[4]);
        uint16_t low_byte = std::stoi(parts[5]);
        port = high_byte * 256 + low_byte;
        return true;
    }

    void send_command(const std::string& cmd) {
        std::cout << "--> " << cmd << std::endl;
        boost::asio::write(m_sock, boost::asio::buffer(cmd + "\r\n"));
    }

    void read_reply() {
        m_reply_msg.clear();
        boost::asio::streambuf buf;
        boost::asio::read_until(m_sock, buf, "\r\n");
        std::string line = boost::asio::buffer_cast<const char*>(buf.data());
        m_reply_msg += line;
        std::cout << "<-- " << line;
        if (line.size() < 3 || line[3] != '-') {
            m_reply_code = std::stoi(line.substr(0, 3));
        }
        else {
            std::string cont;
            do {
                boost::asio::read_until(m_sock, buf, "\r\n");
                line = boost::asio::buffer_cast<const char*>(buf.data());
                m_reply_msg += line;
                std::cout << "<-- " << line;
                cont = line.substr(4);
            } while (line.size() >= 4 && line[3] == '-' && !cont.empty());
            m_reply_code = std::stoi(m_reply_msg.substr(0, 3));
        }
    }

    tcp::socket m_sock;
    int m_reply_code;
    std::string m_reply_msg;
};

int main(int argc, char* argv[]) {
    if (argc < 6) {
        std::cerr << "Usage: " << argv[0] << " URL USERNAME PASSWORD REMOTE_PATH LOCAL_PATH" << std::endl;
        return 1;
    }
    io_service ios;
    ftp_client client(ios);
    client.download(argv[1], argv[2], argv[3], argv[4], argv[5]);
    return 0;
}

使用方式如下：

./ftp_client ftp://ftp.example.com/file.zip username password /path/to/remote/file.zip /path/to/local/file.zip

其中，`ftp://ftp.example.com/file.zip` 是要下载的文件的URL，`username` 和 `password` 是FTP服务器的登录凭据，`/path/to/remote/file.zip` 是文件在FTP服务器上的路径，`/path/to/local/file.zip` 是文件在本地保存的路径。