error allocating compiler reports pure virtual function

in main I have

Menu *ptrBaseMenu = new MainMenu(library);

but this gives me the following error:
ain.cpp: In function `int main(int, char**)':

main.cpp:37: cannot allocate an object of type `MainMenu'
main.cpp:37: because the following virtual functions are abstract:
menu.h:12: virtual void Menu::process_option(int) const

but its not abstract here is my header file with the ovriden function from a pure virtual function of the abstract class menu

Code:

---

#ifndef MAINMENU_H
#define MAINMENU_H
#include <map>
using std::map;
#include "book.h"
#include "menu.h"

class MainMenu:public Menu{
    public:
        MainMenu();
        MainMenu(map<string,Book> &);
        virtual void display_menu() const;
               
        virtual bool validate_option(int) const;
        virtual void process_option(int);
        void display_library(map<string,Book> *); 
    private:
        map<string,Book> *ptrLibrary;
};
#endif

---

virtual void process_option(int);
and,
virtual void process_option(int) const;

are probably not being recognized as the same functions, so if you want to override the baseclass's
'virtual void process_option(int) const', you need to prototype it as

virtual void process_option(int) const;

in the derived class.