不使用curl库该如何实现？

如果不使用 curl 库，也可以使用 C++ 标准库中的 `ftp` 头文件实现 FTP 文件下载。以下是一个使用 `ftp` 头文件实现模糊匹配下载文件的 C++ 代码示例：

#include <iostream>
#include <fstream>
#include <string>
#include <regex>
#include <experimental/filesystem>
#include <experimental/net>
#include <chrono>

namespace fs = std::experimental::filesystem;
namespace net = std::experimental::net;

int main()
{
    try
    {
        net::io_context io_context;
        net::ip::tcp::resolver resolver(io_context);
        net::ip::tcp::socket socket(io_context);
        net::connect(socket, resolver.resolve("example.com", "ftp"));

        // 登录
        std::string username = "username";
        std::string password = "password";
        std::string user_command = "USER " + username + "\r\n";
        std::string pass_command = "PASS " + password + "\r\n";
        net::write(socket, net::buffer(user_command));
        net::write(socket, net::buffer(pass_command));

        // 切换到被动模式
        std::string pasv_command = "PASV\r\n";
        net::write(socket, net::buffer(pasv_command));
        std::string response;
        net::read_until(socket, net::dynamic_buffer(response), "\r\n");
        std::regex pasv_regex("\\((\\d+),(\\d+),(\\d+),(\\d+),(\\d+),(\\d+)\\)");
        std::smatch pasv_match;
        if (std::regex_search(response, pasv_match, pasv_regex))
        {
            std::string ip = pasv_match[1].str() + "." + pasv_match[2].str() + "." + pasv_match[3].str() + "." + pasv_match[4].str();
            int port = std::stoi(pasv_match[5].str()) * 256 + std::stoi(pasv_match[6].str());
            net::ip::tcp::socket data_socket(io_context);
            net::ip::tcp::endpoint data_endpoint(net::ip::make_address(ip), port);
            data_socket.connect(data_endpoint);
            std::cout << "Data socket connected to " << data_endpoint << std::endl;
        }
        else
        {
            std::cerr << "Failed to parse PASV response: " << response << std::endl;
            return 1;
        }

        // 切换到指定目录
        std::string cwd_command = "CWD /files/\r\n";
        net::write(socket, net::buffer(cwd_command));
        net::read_until(socket, net::dynamic_buffer(response), "\r\n");
        if (!response.starts_with("250"))
        {
            std::cerr << "Failed to change directory: " << response << std::endl;
            return 1;
        }

        // 列出目录下所有文件
        std::string list_command = "NLST\r\n";
        net::write(socket, net::buffer(list_command));
        std::string list_response;
        net::read_until(socket, net::dynamic_buffer(list_response), "\r\n");
        if (!list_response.starts_with("150"))
        {
            std::cerr << "Failed to list directory: " << list_response << std::endl;
            return 1;
        }
        std::string file_list;
        net::read_until(data_socket, net::dynamic_buffer(file_list), "\r\n");
        std::cout << "File list: " << file_list;

        // 匹配文件名
        std::string filename = "file*.txt"; // 匹配模式
        std::regex filename_regex(filename);
        std::vector<std::string> matched_files;
        std::istringstream iss(file_list);
        std::string line;
        while (std::getline(iss, line))
        {
            if (std::regex_match(line, filename_regex))
            {
                matched_files.push_back(line);
            }
        }

        // 下载匹配的文件
        std::string localpath = "/path/to/local/folder/"; // 本地文件夹路径
        for (const auto& file : matched_files)
        {
            std::string retr_command = "RETR " + file + "\r\n";
            net::write(socket, net::buffer(retr_command));
            std::string retr_response;
            net::read_until(socket, net::dynamic_buffer(retr_response), "\r\n");
            if (!retr_response.starts_with("150"))
            {
                std::cerr << "Failed to download file: " << retr_response << std::endl;
                continue;
            }
            std::string filepath = localpath + file;
            std::ofstream outfile(filepath, std::ios::binary | std::ios::trunc);
            if (!outfile.is_open())
            {
                std::cerr << "Failed to open file " << filepath << std::endl;
                continue;
            }
            auto start_time = std::chrono::high_resolution_clock::now();
            net::streambuf download_buffer;
            net::read(data_socket, download_buffer);
            outfile << &download_buffer;
            auto end_time = std::chrono::high_resolution_clock::now();
            std::chrono::duration<double> elapsed_time = end_time - start_time;
            std::cout << "Downloaded file " << file << " in " << elapsed_time.count() << " seconds" << std::endl;
            outfile.close();
        }

        // 关闭数据连接和控制连接
        std::string quit_command = "QUIT\r\n";
        net::write(socket, net::buffer(quit_command));
        std::string quit_response;
        net::read_until(socket, net::dynamic_buffer(quit_response), "\r\n");
        data_socket.close();
        socket.close();
    }
    catch (std::exception& e)
    {
        std::cerr << "Exception: " << e.what() << std::endl;
        return 1;
    }

    return 0;
}

注意：这里使用了 C++17 中的实验性文件系统库和实验性网络库，需要编译时添加 `-lstdc++fs -lboost_system` 库文件。