由引用集/映射迭代器传递的迭代器不可取消引用

Question

I am having some trouble with passing an iterator by reference. In the program, I have three classes: the class A that contains a std::map> as a private member. The class B holding the class A as a private member. And the class C that receives in its constructor the class B. In the class C, I am trying to retrieve an iterator pointing to the map as follows:

class A(){
   private: 
        std::map<std::vector<>> theMap;
   public:
        void theMap_access() {std::string to_access , std::map<std::vector<>>::iterator &it){
        it =  theMap.find(to_access);
        it-> first; //OK
   }         
};

class B(){
   private: 
        A a;
   public: 
        A A_access(){return a;}
};

class C(B &b){
   public: 
       std::map<std::vector<>>::iterator it;
       std::string to_access = "test";
       B.A_access().theMap_access(to_access, it);
       it-> first; //KO
};

When I execute this code, I am sure "test" is in the map. So when I dereference it in class A, the code runs fine and I get it->first = "test". But after passing it by reference back to class C, I get this error: set/map iterator not dereferencable. I assume the iterator once passed back is not pointing to what it was pointing to in class A. Could you please explain me why and how I can fix that? Many thanks for your help.

Answer 1

I get this error: set/map iterator not dereferencable.

That is because the A object that is being used here in the return of B.A_access :

B.A_access().theMap_access(to_access, it);

no longer exists after the above line is executed. So the iterator you set is referring to a non-existing map , since that map also has gone away.

The reason why is that B.A_access() returns a copy of the A object, and not the actual A object that contains the std::map you're trying to use.

If you want to use the actual A object that contains the std::map you want to manipulate, then A_access should return a reference to A , not a copy of A .

So the fix should be:

 A& A_access(){return a;}