boost实现FTP文件下载

Boost库提供了一个非常方便的网络编程模块，包括FTP协议的实现。下面是一个使用Boost库实现FTP文件下载的示例代码：

#include <iostream>
#include <fstream>
#include <boost/asio.hpp>
#include <boost/filesystem.hpp>
#include <boost/algorithm/string.hpp>
#include <boost/asio/read_until.hpp>
#include <boost/asio/streambuf.hpp>
#include <boost/asio/write.hpp>

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

class ftp_client {
public:
    ftp_client(const std::string& host, const std::string& username, const std::string& password)
        : m_host(host)
        , m_username(username)
        , m_password(password)
        , m_socket(m_io_service) {}

    void download_file(const std::string& remote_file_path, const std::string& local_file_path) {
        try {
            connect();
            login();
            set_binary_mode();

            std::string command = "RETR " + remote_file_path;
            tcp::iostream data_socket = send_command(command);

            std::ofstream local_file(local_file_path, std::ios::out | std::ios::binary);
            if (!local_file) {
                std::cout << "Failed to create local file: " << local_file_path << std::endl;
                return;
            }

            char buffer[1024];
            while (data_socket.read(buffer, sizeof(buffer)).gcount() > 0) {
                local_file.write(buffer, data_socket.gcount());
            }
            data_socket.close();

            std::cout << "Successfully downloaded file: " << remote_file_path << std::endl;
        }
        catch (std::exception& e) {
            std::cout << "Error: " << e.what() << std::endl;
        }
    }

private:
    void connect() {
        tcp::resolver resolver(m_io_service);
        tcp::resolver::query query(m_host, "21");
        tcp::resolver::iterator endpoint_iterator = resolver.resolve(query);

        asio::connect(m_socket, endpoint_iterator);

        std::cout << "Connected to FTP server: " << m_host << std::endl;
    }

    void login() {
        std::string response = receive_response();

        std::string command = "USER " + m_username;
        send_command(command);

        command = "PASS " + m_password;
        send_command(command);

        std::cout << "Logged in as: " << m_username << std::endl;
    }

    void set_binary_mode() {
        send_command("TYPE I");

        std::cout << "Set binary mode" << std::endl;
    }

    tcp::iostream send_command(const std::string& command) {
        m_socket << command << "\r\n";
        std::string response = receive_response();

        std::cout << "Sent command: " << command << std::endl;
        std::cout << "Received response: " << response << std::endl;

        std::vector<std::string> response_tokens;
        boost::split(response_tokens, response, boost::is_any_of(" "));

        if (response_tokens[0][0] == '4' || response_tokens[0][0] == '5') {
            throw std::runtime_error("Failed to execute command: " + command);
        }

        std::string data_port_text = extract_data_port(response);
        tcp::iostream data_socket = connect_to_data_socket(data_port_text);

        return data_socket;
    }

    std::string receive_response() {
        asio::streambuf response_buffer;
        asio::read_until(m_socket, response_buffer, "\r\n");

        std::string response((std::istreambuf_iterator<char>(&response_buffer)), std::istreambuf_iterator<char>());
        boost::trim_right(response);

        return response;
    }

    std::string extract_data_port(const std::string& response) {
        int start = response.find("(") + 1;
        int end = response.find(")");

        std::string data_port_text = response.substr(start, end - start);
        std::replace(data_port_text.begin(), data_port_text.end(), ',', '.');

        return data_port_text;
    }

    tcp::iostream connect_to_data_socket(const std::string& data_port_text) {
        std::vector<std::string> data_port_tokens;
        boost::split(data_port_tokens, data_port_text, boost::is_any_of("."));

        unsigned short port = std::stoi(data_port_tokens.back());
        data_port_tokens.pop_back();
        port += std::stoi(data_port_tokens.back()) * 256;
        data_port_tokens.pop_back();
        port += std::stoi(data_port_tokens.back()) * 256 * 256;
        data_port_tokens.pop_back();
        port += std::stoi(data_port_tokens.back()) * 256 * 256 * 256;

        tcp::resolver resolver(m_io_service);
        tcp::resolver::query query(m_host, std::to_string(port));
        tcp::resolver::iterator endpoint_iterator = resolver.resolve(query);

        tcp::iostream data_socket;
        asio::connect(data_socket, endpoint_iterator);

        std::cout << "Connected to data socket: " << data_port_text << std::endl;

        return data_socket;
    }

    std::string m_host;
    std::string m_username;
    std::string m_password;
    asio::io_service m_io_service;
    tcp::iostream m_socket;
};

int main(int argc, char* argv[]) {
    if (argc < 4) {
        std::cerr << "Usage: " << argv[0] << " <host> <username> <password> <remote_file_path> <local_file_path>" << std::endl;
        return 1;
    }

    std::string host = argv[1];
    std::string username = argv[2];
    std::string password = argv[3];
    std::string remote_file_path = argv[4];
    std::string local_file_path = argc > 5 ? argv[5] : path(remote_file_path).filename().string();

    ftp_client client(host, username, password);
    client.download_file(remote_file_path, local_file_path);

    return 0;
}

这个示例代码中，ftp_client 类封装了一些FTP协议相关的操作，如连接 FTP 服务器、登录、设置二进制模式、发送命令、接收响应等。其中，download_file() 方法实现了文件下载的功能，通过 RETR 命令获取文件数据，写入本地文件。

使用该示例代码下载文件只需要将 FTP 服务器的地址、用户名、密码、远程文件路径和本地文件路径作为命令行参数传入即可。