c++ - invalid covariant return type error -

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• in c++, possible forward declare class inheriting class? 4 answers

i getting below error on overriding base class function in derived class.

./inc/rbtree.h:16:18: error: invalid covariant return type ‘virtual rbnode* rbtree::get_root()’ ./inc/tree.h:25:24: error:   overriding ‘virtual node* tree::get_root()’ 

rbtree.h

class rbnode;  class rbtree: public tree {     protected:     public:         rbtree();         rbnode *root;         rbnode * get_root();         void insert_into_tree();         //void delete_from_tree(); }; 

and tree.h below

class node;  class tree {     protected:         node * root;         list<int> treedata;     public:         tree();         /* gives error */         virtual node * get_root();         void set_root(node *root_node);         void insert_into_tree();         void delete_from_tree();         void print_tree(); }; 

more info: rbnode derived node. , read article , says alright override base class function return derived type instace.

i searched on well, questions regarding didn't much. why failing if allowed in c++.

it appears forward-declaring rbnode. when compiler sees rbtree, doesn't yet know rbnode node.