重载虚拟＆lt;＆lt; operator c ++

Question

我在io.h中有以下代码：

=INDEX(Dict!$A$2:$AK$500, SMALL(IF(COUNTIF($K$1, Dict!$F$2:$F$500)*(COUNTIF($J$1, Dict!$G$2:$G$500)), ROW(Dict!$A$2:$AK$500)-MIN(ROW(Dict!$A$2:$AK$500))+1), ROW(F1)), COLUMN(F1))

和shape.h：

  #include <iostream>
  #include "shape.h"

  class IODevice { // Interface for displaying CAD objects  
  public:
     virtual void operator << (const Circle&  c) = 0;
     virtual void operator << (const Line&  c) = 0;
  };

  class CmdIODevice : public IODevice {
      void operator << (const Circle&  c);
      void operator << (const Line& c);
  };

并在IODevice中声明虚拟运算符时出现以下错误： 错误C4430缺少类型说明符 - 假定为int。注意：C ++不支持default-int

显然编译器看不到Circle，我不明白，请帮忙。