Unreal Engine 4 问题：使用asio后编译打包报错：unresolved external symbol

落日映苍穹つ 2022-05-14 12:12 201阅读 0赞

当项目中使用asio作为通讯库时，编译打包时总是报如下错误：

error LNK2019: unresolved external symbol "__declspec(dllimport) public: __cdecl std::bad_cast::bad_cast(char const *)" (__imp_??0bad_cast@std@@QEAA@PEBD@Z) referenced in function "public: __cdecl asio::ip::bad_address_cast::bad_address_cast(void)" (??0bad_address_cast@ip@asio@@QEAA@XZ)
    error LNK2001: unresolved external symbol "__declspec(dllimport) public: __cdecl std::bad_cast::bad_cast(char const *)" (__imp_??0bad_cast@std@@QEAA@PEBD@Z)
    error LNK2001: unresolved external symbol "protected: virtual void __cdecl std::bad_cast::_Doraise(void)const " (?_Doraise@bad_cast@std@@MEBAXXZ)
    error LNK2001: unresolved external symbol "protected: virtual void __cdecl std::bad_cast::_Doraise(void)const " (?_Doraise@bad_cast@std@@MEBAXXZ)

查看asio::ip::bad\_address\_cast的源代码，发现bad\_address\_cast继承了<typeinfo>中定义的std::bad\_cast，代码如下：

//
    // ip/bad_address_cast.hpp
    // ~~~~~~~~~~~~~~~~~~~~~~~
    //
    // Copyright (c) 2003-2015 Christopher M. Kohlhoff (chris at kohlhoff dot com)
    //
    // Distributed under the Boost Software License, Version 1.0. (See accompanying
    // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
    //
    
    #ifndef ASIO_IP_BAD_ADDRESS_CAST_HPP
    #define ASIO_IP_BAD_ADDRESS_CAST_HPP
    
    #if defined(_MSC_VER) && (_MSC_VER >= 1200)
    # pragma once
    #endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
    
    #include "asio/detail/config.hpp"
    #include <typeinfo>
    
    #include "asio/detail/push_options.hpp"
    
    namespace asio {
    namespace ip {
    
    /// Thrown to indicate a failed address conversion.
    class bad_address_cast : public std::bad_cast
    {
    public:
      /// Default constructor.
      bad_address_cast() {}
    
      /// Destructor.
      virtual ~bad_address_cast() ASIO_NOEXCEPT_OR_NOTHROW {}
    
      /// Get the message associated with the exception.
      virtual const char* what() const ASIO_NOEXCEPT_OR_NOTHROW
      {
        return "bad address cast";
      }
    };
    
    } // namespace ip
    } // namespace asio
    
    #include "asio/detail/pop_options.hpp"
    
    #endif // ASIO_IP_ADDRESS_HPP

从代码中看到\#include <typeinfo>，应该没有理由找不到std::bad\_cast。再查看VC++中提供的typeinfo的源代码：

// typeinfo standard header
    
    /***
    *typeinfo - Defines the type_info structure and exceptions used for RTTI
    *
    *    Copyright (c) Microsoft Corporation. All rights reserved.
    *    Modified January 1996 by P.J. Plauger
    *
    *Purpose:
    *       Defines the type_info structure and exceptions used for
    *       Runtime Type Identification.
    *
    *       [Public]
    *
    ****/
    
    #pragma once
    
    #ifndef _TYPEINFO_
    #define _TYPEINFO_
    #ifndef RC_INVOKED
    #include <exception>
    
    #pragma pack(push,_CRT_PACKING)
    #pragma warning(push,_STL_WARNING_LEVEL)
    #pragma warning(disable: _STL_DISABLED_WARNINGS)
    _STL_DISABLE_CLANG_WARNINGS
    #pragma push_macro("new")
    #undef new
    
    #pragma warning(disable: 4275)    // non dll-interface class 'X' used as base for dll-interface class 'Y'
    
    #include <vcruntime_typeinfo.h>
    
     _STD_BEGIN
    
    // size in pointers of std::function and std::any (roughly 3 pointers larger than std::string when building debug)
    constexpr int _Small_object_num_ptrs = 6 + 16 / sizeof (void *);
    
     #if !(_HAS_EXCEPTIONS)
    
            // CLASS bad_cast
    class _CRTIMP2_IMPORT bad_cast
        : public exception
        {    // base of all bad cast exceptions
    public:
        bad_cast(const char *_Message = "bad cast") noexcept
            : exception(_Message)
            {    // construct from message string
            }
    
        virtual ~bad_cast() noexcept
            {    // destroy the object
            }
    
    protected:
        virtual void _Doraise() const
            {    // perform class-specific exception handling
            _RAISE(*this);
            }
        };
    
            // CLASS bad_typeid
    class _CRTIMP2_IMPORT bad_typeid
        : public exception
        {    // base of all bad typeid exceptions
    public:
        bad_typeid(const char *_Message = "bad typeid") noexcept
            : exception(_Message)
            {    // construct from message string
            }
    
        virtual ~bad_typeid() noexcept
            {    // destroy the object
            }
    
    protected:
        virtual void _Doraise() const
            {    // perform class-specific exception handling
            _RAISE(*this);
            }
        };
    
    class _CRTIMP2_IMPORT __non_rtti_object
        : public bad_typeid
        {    // report a non RTTI object
    public:
        __non_rtti_object(const char *_Message)
            : bad_typeid(_Message)
            {    // construct from message string
            }
        };
     #endif /* _HAS_EXCEPTIONS */
    
     _STD_END
    
    #pragma pop_macro("new")
    _STL_RESTORE_CLANG_WARNINGS
    #pragma pack(pop)
    #pragma warning(pop)
    #endif /* RC_INVOKED */
    #endif // _TYPEINFO_
    
    /*
     * Copyright (c) Microsoft Corporation.  ALL RIGHTS RESERVED.
     * Modified January 1996 by P.J. Plauger
     * Modified November 1998 by P.J. Plauger
     * Consult your license regarding permissions and restrictions.
    V6.50:0009 */

是class bad\_cast定义前有这么一行：  \#if !(\_HAS\_EXCEPTIONS)  
当在.Build.cs中添加bEnableExceptions = true后，编译时会添加参数：/EHsc，这将导致\_HAS\_EXCEPTIONS=1，\#if !(\_HAS\_EXCEPTIONS)为false，从而最终class bad\_cast不被包含。于是就导致unresolved external symbol。

修改方法也很简单，修改asio::ip::bad\_address\_cast的源代码，将std::bad\_cast替换成std::exception即可：

//
    // ip/bad_address_cast.hpp
    // ~~~~~~~~~~~~~~~~~~~~~~~
    //
    // Copyright (c) 2003-2015 Christopher M. Kohlhoff (chris at kohlhoff dot com)
    //
    // Distributed under the Boost Software License, Version 1.0. (See accompanying
    // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
    //
    
    #ifndef ASIO_IP_BAD_ADDRESS_CAST_HPP
    #define ASIO_IP_BAD_ADDRESS_CAST_HPP
    
    #if defined(_MSC_VER) && (_MSC_VER >= 1200)
    # pragma once
    #endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
    
    #include "asio/detail/config.hpp"
    #include <exception>
    
    #include "asio/detail/push_options.hpp"
    
    namespace asio {
    namespace ip {
    
    /// Thrown to indicate a failed address conversion.
    class bad_address_cast : public std::exception
    {
    public:
      /// Default constructor.
      bad_address_cast() {}
    
      /// Destructor.
      virtual ~bad_address_cast() ASIO_NOEXCEPT_OR_NOTHROW {}
    
      /// Get the message associated with the exception.
      virtual const char* what() const ASIO_NOEXCEPT_OR_NOTHROW
      {
        return "bad address cast";
      }
    };
    
    } // namespace ip
    } // namespace asio
    
    #include "asio/detail/pop_options.hpp"
    
    #endif // ASIO_IP_ADDRESS_HPP