text: #ifndef FOO_H_
  #define FOO_H_
  #include <string>
  class Foo2{
  public:
      virtual ~Foo2(){}
      virtual void Print(std::string content)=0;
  };
  class Foo{
  public:
      Foo(Foo2 *cb):cb_(cb){}
  	void Print(std::string content);
  private:
      Foo2 *cb_;
  };
  #endif /* FOO_H_ */
  