c++ - 为 DLL 实现 C++ 类时出现错误 LNK2019

Question

我将 C++ 类实现为 DLL。但是当我连接到 dll 时出现链接器错误。我有正确的设置，但不确定哪里出了问题。研究过，没有找到相关的解决方案，所以提出疑问。

ASM_Lib.h

    #ifdef EXPORT
    #define DLLCLASS __declspec(dllexport)
    #else
    #define DLLCLASS __declspec(dllimport)
    #endif
    class ASM
    {
        public:
            ASM();
            ~ASM();
            int loadData(string path, string ext);
            int landmarkEqualization();

        private:
            vector<PtsData_<CurrentType_>> pts;//this vector size is same as number of images, released after use
            vector<string> files;//file names
            vector<TrainingData_<CurrentType_>> td;//this vector size is same as number of images, released after use
            vector<Mat> images;
    };

    extern "C" /*Important for avoiding Name decoration*/
    {
        DLLCLASS  ASM* _cdecl CreateASMObject();
    };

    // Function Pointer Declaration of CreateASMObject() [Entry Point Function]
    typedef ASM* (*CREATE_ASM) ();

ASM_Lib.cpp

namespace VIDEO_ANALYTICS_PLATFORM{

    DLLCLASS ASM* _cdecl CreateASMObject() {
        return new ASM();
    }

    ASM::ASM()
    {
    }

    ASM::~ASM()
    {
    }

    int ASM::loadData(string path, string ext)
    {

            return FILE_READ_WRITE_ERROR;
    }

    ///*
    //This loop equalize all landmark points to 
    //be equal distances
    //*/
    int ASM::landmarkEqualization()
    {
        //Clear vector
        pts.clear();
        vector<PtsData_<CurrentType_>>().swap(pts);
        return SUCCESS;
    }


}

然后在我的测试程序中，我的接口(interface)为

#include "stdafx.h"
#include <iostream>
#include "ASM_Lib.h"
using namespace VIDEO_ANALYTICS_PLATFORM;

int _tmain(int argc, _TCHAR* argv[])
{
    HINSTANCE hDLL = LoadLibrary(L"ASM_Lib.dll"); 

    if (hDLL == NULL){
        std::cout << "Failed to load library.\n";
    }else{
        CREATE_ASM pEntryFunction = (CREATE_ASM)GetProcAddress(hDLL, "CreateASMObject");
        ASM* pASM = pEntryFunction();
        if (pASM) {                             
            pASM->loadData("C:\\PointsFiles", "pts");                         
        }
        FreeLibrary(hDLL);
    }

    std::cin.get();

    return 0;
}

但我有 LNK2019 链接错误(在函数 wmain 中引用)并且我没有 wmain。可能是什么问题？

test.obj : error LNK2019: unresolved external symbol "public: int __cdecl VIDEO_ANALYTICS_PLATFORM::ASM::loadData(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >)" (?loadData@ASM@VIDEO_ANALYTICS_PLATFORM@@QEAAHV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@0@Z) referenced in function wmain

如果我注释掉这个 API pASM->loadData("C:\\PointsFiles", "pts");，那么它就可以正常加载 dll。

编辑:最后更新

ASM_Lib.h
#ifdef EXPORT
#define DLLCLASS __declspec(dllexport)
#else
#define DLLCLASS __declspec(dllimport)
#endif

namespace VIDEO_ANALYTICS_PLATFORM{

    class i_ASM
    {
       public:
          virtual ~i_ASM(){ ; };
          virtual int loadData(string path, string ext)=0;
          virtual int landmarkEqualization() = 0;
    };

    class ASM : public i_ASM
    {
        public:
            ASM(){ }
            int loadData(string path, string ext);
            int landmarkEqualization();

        private:
            vector<PtsData_<CurrentType_>> pts;//this vector size is same as number of images, released after use
            vector<string> files;//file names
            vector<TrainingData_<CurrentType_>> td;//this vector size is same as number of images, released after use
            vector<Mat> images;
    };

    extern "C" 
    {
        DLLCLASS  i_ASM* _cdecl CreateASMObject();
    };
}

ASM_Lib.cpp
namespace VIDEO_ANALYTICS_PLATFORM{

    DLLCLASS i_ASM* _cdecl CreateASMObject() {
        return new ASM();
    }   

    int ASM::loadData(string path, string ext)
    {
         return 0;
    }

    ///*
    //This loop equalize all landmark points to 
    //be equal distances
    //*/
    int ASM::landmarkEqualization()
    {
        //Clear vector
        pts.clear();
        vector<PtsData_<CurrentType_>>().swap(pts);
        return SUCCESS;
    }
}

Test.cpp
#include "ASM_Lib.h"
using namespace VIDEO_ANALYTICS_PLATFORM;

int _tmain(int argc, _TCHAR* argv[])
{       
     ASM* pASM = ::CreateASMObject();
     if (pASM) {
         pASM->loadData("C:\\PointsFiles", "pts");
         pASM->~ASM();
         pASM = NULL;
     }

    return 0;
}

我认为，我的上次更新应该有效。但是 loadData() 和 ~ASM() 仍然有链接器错误 LNK2019。我在同一解决方案中同时进行了测试项目和 ASM_Lib 项目。有什么问题吗？

Answer 1

DLL 仅导出 CreateASMObject 函数。 ASM::loadData 未导出，但在测试应用程序中使用。我可以提出 2 种修复方法:

1) 使用__declspec(dllexport) 对整个ASM 类或loadData 成员进行属性化，并将ASM_Lib.lib 添加到测试应用程序项目中。

2) 声明纯抽象类(接口(interface))IASM并改变CreateASMObject的返回类型:

class IASM
{
public:
  virtual ~IASM() = 0;
  virtual int loadData(string path, string ext) = 0;
};

extern "C"
{
  DLLCLASS  IASM* _cdecl CreateASMObject();
};

然后从 IASM 派生 ASM 并实现抽象方法(它可能在 CPP 文件中完成)。在这种情况下，链接器不需要 loadData 方法的地址，因为它将在运行时通过 vtable 解析。

附言。您必须确保 DLL 及其客户端使用相同的堆管理器实例(例如，相同版本的 C 运行时 DLL)。否则，为在另一个模块中创建的对象调用 delete 是不安全的。解决方案是添加一个实现删除的方法:

class IASM
{
public:
  virtual void destroy() = 0;
protected:
  ~IMyAPI() = default;
};    

class ASM: public IASM
{
public:
  virtual void destroy() override
  {
    delete this;
  }
};