符号重载时的错误。

代码是书上的源码是从官网上下的。应该没问题吧。在visual C++ 6上运行的。
报出了如下错误。'SAMSCalculator::aConsoleOnlyExternalInterface<float>::<<' : overriding virtual function differs from 'SAMSCalculator::anExternalInte
rface<float>::<<' only by return type or calling convention

代码截取：     //继承关系
               template <class ofType> class aConsoleOnlyExternalInterface: public anExternalInterface<ofType>, public isInstanceCountable
               //父类虚方法
               virtual anExternalInterface &operator << (const char *theText) const = 0;
                //方法重载
               aConsoleOnlyExternalInterface &operator << (const char *theText) const;

//方法实现
  template <class ofType> aConsoleOnlyExternalInterface<ofType> &aConsoleOnlyExternalInterface<ofType>::operator << (const char *theText) const
   {
       Display(theText);
      return const_cast<aConsoleOnlyExternalInterface<ofType> &>(*this);
   };