错误例子（我写的）
module;
#include "httplib.h"
export module mcp.compat.httplib;
export namespace mcp {
using HttpClient = httplib::Client;
using DataSink = httplib::DataSink;
using Headers = httplib::Headers;
using HttpRequest = httplib::Request;
using HttpResponse = httplib::Response;
using HttpServer = httplib::Server;
#ifdef MCP_SSL
using SslServer = httplib::SSLServer;
#endif
}
正确例子（官方库提供的）
module;
/*
* Headers
*/
#ifdef _WIN32
#ifndef _CRT_SECURE_NO_WARNINGS
#define _CRT_SECURE_NO_WARNINGS
#endif //_CRT_SECURE_NO_WARNINGS
#ifndef _CRT_NONSTDC_NO_DEPRECATE
#define _CRT_NONSTDC_NO_DEPRECATE
#endif //_CRT_NONSTDC_NO_DEPRECATE
#if defined(_MSC_VER)
#if _MSC_VER < 1900
#error Sorry, Visual Studio versions prior to 2015 are not supported
#endif
#pragma comment(lib, "ws2_32.lib")
#ifndef _SSIZE_T_DEFINED
#define _SSIZE_T_DEFINED
#endif
#endif // _MSC_VER
#ifndef S_ISREG
#define S_ISREG(m) (((m) & S_IFREG) == S_IFREG)
#endif // S_ISREG
#ifndef S_ISDIR
#define S_ISDIR(m) (((m) & S_IFDIR) == S_IFDIR)
#endif // S_ISDIR
#ifndef NOMINMAX
#define NOMINMAX
#endif // NOMINMAX
#include <io.h>
#include <winsock2.h>
#include <ws2tcpip.h>
#if defined(__has_include)
#if __has_include(<afunix.h>)
// afunix.h uses types declared in winsock2.h, so has to be included after it.
#include <afunix.h>
#define CPPHTTPLIB_HAVE_AFUNIX_H 1
#endif
#endif
#ifndef WSA_FLAG_NO_HANDLE_INHERIT
#define WSA_FLAG_NO_HANDLE_INHERIT 0x80
#endif
#else // not _WIN32
#include <arpa/inet.h>
#if !defined(_AIX) && !defined(__MVS__)
#include <ifaddrs.h>
#endif
#ifdef __MVS__
#include <strings.h>
#ifndef NI_MAXHOST
#define NI_MAXHOST 1025
#endif
#endif
#include <net/if.h>
#include <netdb.h>
#include <netinet/in.h>
#ifdef __linux__
#include <resolv.h>
#undef _res // Undefine _res macro to avoid conflicts with user code (#2278)
#endif
#include <csignal>
#include <netinet/tcp.h>
#include <poll.h>
#include <pthread.h>
#include <sys/mman.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <unistd.h>
#ifndef INVALID_SOCKET
#define INVALID_SOCKET (-1)
#endif
#endif //_WIN32
#if defined(__APPLE__)
#include <TargetConditionals.h>
#endif
#include <algorithm>
#include <array>
#include <atomic>
#include <cassert>
#include <cctype>
#include <chrono>
#include <climits>
#include <condition_variable>
#include <cstdlib>
#include <cstring>
#include <errno.h>
#include <exception>
#include <fcntl.h>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <memory>
#include <mutex>
#include <random>
#include <regex>
#include <set>
#include <sstream>
#include <string>
#include <sys/stat.h>
#include <system_error>
#include <thread>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#if defined(CPPHTTPLIB_USE_NON_BLOCKING_GETADDRINFO) || \
defined(CPPHTTPLIB_USE_CERTS_FROM_MACOSX_KEYCHAIN)
#if TARGET_OS_MAC
#include <CFNetwork/CFHost.h>
#include <CoreFoundation/CoreFoundation.h>
#endif
#endif // CPPHTTPLIB_USE_NON_BLOCKING_GETADDRINFO or
// CPPHTTPLIB_USE_CERTS_FROM_MACOSX_KEYCHAIN
#ifdef CPPHTTPLIB_OPENSSL_SUPPORT
#ifdef _WIN32
#include <wincrypt.h>
// these are defined in wincrypt.h and it breaks compilation if BoringSSL is
// used
#undef X509_NAME
#undef X509_CERT_PAIR
#undef X509_EXTENSIONS
#undef PKCS7_SIGNER_INFO
#ifdef _MSC_VER
#pragma comment(lib, "crypt32.lib")
#endif
#endif // _WIN32
#if defined(CPPHTTPLIB_USE_CERTS_FROM_MACOSX_KEYCHAIN)
#if TARGET_OS_MAC
#include <Security/Security.h>
#endif
#endif // CPPHTTPLIB_USE_NON_BLOCKING_GETADDRINFO
#include <openssl/err.h>
#include <openssl/evp.h>
#include <openssl/ssl.h>
#include <openssl/x509v3.h>
#if defined(_WIN32) && defined(OPENSSL_USE_APPLINK)
#include <openssl/applink.c>
#endif
#include <iostream>
#include <sstream>
#if defined(OPENSSL_IS_BORINGSSL) || defined(LIBRESSL_VERSION_NUMBER)
#if OPENSSL_VERSION_NUMBER < 0x1010107f
#error Please use OpenSSL or a current version of BoringSSL
#endif
#define SSL_get1_peer_certificate SSL_get_peer_certificate
#elif OPENSSL_VERSION_NUMBER < 0x30000000L
#error Sorry, OpenSSL versions prior to 3.0.0 are not supported
#endif
#endif // CPPHTTPLIB_OPENSSL_SUPPORT
#ifdef CPPHTTPLIB_MBEDTLS_SUPPORT
#include <mbedtls/ctr_drbg.h>
#include <mbedtls/entropy.h>
#include <mbedtls/error.h>
#include <mbedtls/md5.h>
#include <mbedtls/net_sockets.h>
#include <mbedtls/oid.h>
#include <mbedtls/pk.h>
#include <mbedtls/sha1.h>
#include <mbedtls/sha256.h>
#include <mbedtls/sha512.h>
#include <mbedtls/ssl.h>
#include <mbedtls/x509_crt.h>
#ifdef _WIN32
#include <wincrypt.h>
#ifdef _MSC_VER
#pragma comment(lib, "crypt32.lib")
#endif
#endif // _WIN32
#if defined(CPPHTTPLIB_USE_CERTS_FROM_MACOSX_KEYCHAIN)
#if TARGET_OS_MAC
#include <Security/Security.h>
#endif
#endif // CPPHTTPLIB_USE_CERTS_FROM_MACOSX_KEYCHAIN
// Mbed TLS 3.x API compatibility
#if MBEDTLS_VERSION_MAJOR >= 3
#define CPPHTTPLIB_MBEDTLS_V3
#endif
#endif // CPPHTTPLIB_MBEDTLS_SUPPORT
// Define CPPHTTPLIB_SSL_ENABLED if any SSL backend is available
// This simplifies conditional compilation when adding new backends (e.g.,
// wolfSSL)
#if defined(CPPHTTPLIB_OPENSSL_SUPPORT) || defined(CPPHTTPLIB_MBEDTLS_SUPPORT)
#define CPPHTTPLIB_SSL_ENABLED
#endif
#ifdef CPPHTTPLIB_ZLIB_SUPPORT
#include <zlib.h>
#endif
#ifdef CPPHTTPLIB_BROTLI_SUPPORT
#include <brotli/decode.h>
#include <brotli/encode.h>
#endif
#ifdef CPPHTTPLIB_ZSTD_SUPPORT
#include <zstd.h>
#endif
export module httplib;
export extern "C++" {
#include "httplib.h"
}
当你试图将一个非模块化的第三方库（尤其是像 httplib 这种带全局状态或初始化逻辑的库）封装进模块时：
不要尝试只导出部分类型：除非你非常确定该库没有全局初始化逻辑（Constructor static guards）。
使用 export extern "C++"：这是将传统头文件“模块化”的最标准、最稳妥做法。
环境对齐：在 module; 之后，务必把该库依赖的所有系统宏（如 _WIN32, WIN32_LEAN_AND_MEAN 等）都写清楚。
