c++ - Derived class not recognized as covariant -

so creating huffman tree, , having hard time overriding function, , believe due covariance issue. here hierarchy having hard time in code:

class treeinterface { public:      treeinterface() {}     virtual ~treeinterface() {}      virtual nodeinterface * getrootnode() const = 0; };  class tree : treeinterface { public:     tree()      {}     virtual ~tree()      {}      node* getrootnode()     {         return treeroot;     }      private:     node* treeroot; }; 

those work fine, next block has issues.

class huffmaninterface{ public:     huffmaninterface() {}     virtual ~huffmaninterface() {}      virtual bool createtree(string filename) = 0;      virtual string encodemessage(string toencode) = 0;      virtual treeinterface * gettree() = 0;      virtual map<char, string> getencodings() = 0; }; class huffman : huffmaninterface{ public:      huffman() {}     ~huffman() {}  bool huffman::createtree(string filename){ }  string huffman::encodemessage(string toencode){ }  string huffman::decodemessage(string todecode){ }  tree* huffman::gettree(){ }  map<char, string> huffman::getencodings(){ } 

so problem apparently in gettree() function, giving following error

invalid covariant return type ‘virtual tree* huffman::gettree()’: tree * gettree(); 

but far know, tree* should valid covariant of treeinterface*. replacing tree* treeinterface* makes program compile, it's not need in actual program. appreciated!

class tree : treeinterface { ... }; 

is equivalent

class tree : private treeinterface { ... }; 

you need make inheritance public.

class tree : public treeinterface { ... };