c++ - How to initialize an iterator to something invalid if the container is not known? -

i want use visitor given more complex function should inflict changes on container via visitor of has general knowledge. example reduced minimum. real visitor has more functions , not handle container @ all.

class intersectionhandler{ public:     virtual void advance()=0;   virtual void treatintersection()=0; };  class overlapdisabler: public intersectionhandler{ private:   std::vector<segment>  mempty;   std::vector<segment>* mplinestosearch;   std::vector<segment>::iterator mlineiter;  public:   insidecollector():mempty(),mplinestosearch(&mempty),mlineiter(mempty.end(){}    void setlinecontainer(std::vector<segment>* lines){     mplinestosearch = lines;     mlineiter = mplinestosearch->begin();                                                               }    void advance(){     if(mlineiter != mplinestosearch->end()) ++mlineiter;   }   void treatintersection(){     if(mlineiter != mplinestosearch->end()) mlineiter->disable();   }     }; 

to intitailize iterator use dummy vector, must keep class member in case. initialise iterator globally invalid value such 0 pointer.

unfortunately 0 not valid value iterator many compilers , seems little odd use. there cleaner way accomplish trying without resorting empty dummy container?

the answer

you can't, there no value standard iterators puts them "invalid state".

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proposed solution

instead of relying on value of mlineiter, default-initialize mlineiter , initialize mplinestosearch nullptr in constructor.

this way mlineiter isn't valid iterator if mplinestosearch still nullptr.

sample implementation

class overlapdisabler: public intersectionhandler{ private:   std::vector<segment>* mplinestosearch;   std::vector<segment>::iterator mlineiter;  public:   insidecollector() : mplinestosearch (nullptr), mlineiter() { }    ...    void advance(){     if (mplinestosearch == nullptr)       return; // no iterator increment      if(mlineiter != mplinestosearch->end()) ++mlineiter;   }   ...         void treatintersection(){     if(mplinestosearch == nullptr)       return; // iterator doesn't refer element      if(mlineiter != mplinestosearch->end()) mlineiter->disable();   }     }; 

( note: if compiling source c++03, nullptr isn't available; if case use null or 0 initialize pointer )