写接口导出函数时提示的问题

error C2259: 'CImpMath' : cannot instantiate abstract class due to following members:
..\comcmp\cominterface\impmath.h(14) : see declaration of 'CImpMath'

编译这里的时候提示上面的错误；

//导出函数
IUnknown * CreateInstance()
{
//创建对象
CImpMath pMath = new CImpMath;
//获取接口
IUnknown * piUnknown = NULL;
pMath ->QueryInterface( IID_IUnknown,(LPVOID *)&piUnknown); //LPVOID void
//返回接口*/
return piUnknown;
}

额，请将问题描述完整

CImpMath创建的方式不对吧，看一下CImpMath类的构造函数是不是没有无参的呢