从C++类制作Visual C++ DLL

Question

我有以下C++代码来制作DLL(Visual Studio 2010).

class Shape {
public:
  Shape() {
    nshapes++;
  }
  virtual ~Shape() {
    nshapes--;
  };
  double  x, y;   
  void    move(double dx, double dy);
  virtual double area(void) = 0;
  virtual double perimeter(void) = 0;
  static  int nshapes;
};

class __declspec(dllexport) Circle : public Shape {
private:
  double radius;
public:
  Circle(double r) : radius(r) { };
  virtual double area(void);
  virtual double perimeter(void);
};

class __declspec(dllexport) Square : public Shape {
private:
  double width;
public:
  Square(double w) : width(w) { };
  virtual double area(void);
  virtual double perimeter(void);
};

我有__declspec,

class __declspec(dllexport) Circle

我可以使用以下命令构建一个dll

CL.exe /c example.cxx
link.exe /OUT:"example.dll" /DLL example.obj 

当我试图使用该库时,

Square* square; square->area()

我收到了错误消息.有什么不对或缺失？

example_unittest.obj : error LNK2001: unresolved external symbol "public: virtual double __thiscall
...
Square::area(void)" (?area@Square@@UAENXZ)

添加

在wengseng的回答之后,我修改了头文件,对于DLL C++代码,我添加了

#define XYZLIBRARY_EXPORT

但是,我仍然有错误.

解决了

对于链接example.dll的主程序,我没有链接example.lib.

cl /MD /EHsc gtest_main.cc example_unittest.cc /I"./include" /link /libpath:"./lib" /libpath:"." gtest_md.lib example.lib /out:gtest_md_release.exe

除此之外,一切正常.

Answer 1

在DLL中,我建议添加一个宏,并在预处理器中添加XYZLIBRARY_EXPORT:

#if defined(XYZLIBRARY_EXPORT) // inside DLL
#   define XYZAPI   __declspec(dllexport)
#else // outside DLL
#   define XYZAPI   __declspec(dllimport)
#endif  // XYZLIBRARY_EXPORT

class XYZAPI Circle  

它将导出Circle类.

在EXE中,导入Circle类,而不添加预处理器,因为它将默认导入该类.