I’m writing a simple class for list of products and there I need to overload both extraction >> operator and insertion << operator to write into file and read from file
student_projectV1/list.h
#ifndef STUDENT_PROJECTV1_LIST
#define STUDENT_PROJECTV1_LIST
#include <fstream>
namespace list {
class list {
private:
string name;
int price;
short quantity;
public:
ofstream ofs;
ifstream ifs;
// file_mutators
void set_created_file () noexcept(false) ;
void set_readable_file ();
// constructors
list() noexcept ( noexcept ( set_created_file() ) ) ;
list ( string , int , short ) noexcept(false) ;
list ( class list &) noexcept ( noexcept ( set_created_file() ) ) ;
// initialization to cover after construction of an obj
void initialize ( string , int , short ) noexcept(false) ;
// mutators
void set_name ( string ) noexcept(false);
void set_price ( int ) noexcept(false) ;
void set_quantity ( short ) noexcept(false) ;
// accessors
string get_name ( ) const noexcept;
int get_price () const noexcept;
int get_quantity () const noexcept;
// Enqueries
bool check_created_file () noexcept;
bool check_opened_file();
// destructor
~list();
// friend global functions
// overloaded operators
friend ofstream & friend_global_funcs :: operator << ( ofstream & , class list::list &) ;
friend ifstream & friend_global_funcs :: operator >> ( ifstream & , class list::list &) ;
};
}
#endif
now I plan to place the definition of these two overloaded functions in another friend_global_funcs.h file inside a namespace
friend ofstream & friend_global_funcs :: operator << ( ofstream & , class list::list &) ;
friend ifstream & friend_global_funcs :: operator >> ( ifstream & , class list::list &) ;
friend_global_funcs.h
//
// Created by solo-l-ub on 2/27/22.
//
#ifndef STUDENT_PROJECTV1_FRIEND_GLOBAL_FUNCS_H
#define STUDENT_PROJECTV1_FRIEND_GLOBAL_FUNCS_H
namespace friend_global_funcs {
ofstream & operator<< (ofstream &ofs, class list::list &l) {
if (!l.check_created_file())
throw new file_missing::file_missing(
"can not write info to file something wrong with acquiring file in constructor of obj \n");
ofs << l.name() << "\t" << l.price << "\t" << l.quantity << "\n";
return ofs;
}
ifstream & operator>>(ifstream &ifs, list :: list &l) {
l.set_readable_file();
if (!l.check_opened_file())
throw new file_missing::file_missing(
"can't retrieve data cuz file is not associated with obj currently I'm in operated >> overloaded fuc \n");
ifs >> l.name >> l.price >> l.quantity;
return ifs;
}
}
#endif //STUDENT_PROJECTV1_FRIEND_GLOBAL_FUNCS_H
now My scenario of the order of including files in main.cpp file I’ve included first the class list.h and then the friend_global_funcs.h file
main.cpp
#include <iostream>
using namespace std;
// using namespaces
// classes
#include "file_missing.h"
#include "empty.h"
#include "list.h"
// libraries
#include "friend_global_funcs.h"
int main() {
//////////////////////////////////////
return 0;
}
now when I try to compile in terminal using g++
g++ main.cpp -o out
I get the error that overloaded functions in list.h are not declared even though I’ve used the scope resolution operator to tell compiler to exactly where to look for the function friend_global_funcs :: operator <<
g++ terminal error
In file included from main.cpp:33:
list.h:64:31: error: ‘friend_global_funcs’ has not been declared
64 | friend ofstream & friend_global_funcs :: operator << ( ofstream & , class list::list &) ;
now I’ve uploaded my project which is a very very light project just to practice writing and reading to files from inside classes to github in case you want to take a look at it and guide me the proper sequence to use the friendship of a function which is defined in another .h file in a class
github src https://github.com/moji2013/student_projectV1.git
The problem is that in your current list.h, you have 2 friend declarations that uses the scope resolution operator :: to refer to operator<< and operator>> inside the namespace friend_global_funcs. And at the point of these friend declaration the compiler doesn't know about the namespace friend_global_funcs.
So to solve this you need to tell the compiler that there is a namespace called friend_global_funcs that has the declarations for operator<< and operator>> as shown below(Working Demo):
list.h
#ifndef STUDENT_PROJECTV1_LIST
#define STUDENT_PROJECTV1_LIST
#include <fstream>
#include <string>
namespace list {
class list;
}
namespace friend_global_funcs {
std::ofstream & operator << ( std::ofstream & , const list::list &) ; //const added here and keyword class removed from second parameter
std::ifstream & operator >> ( std::ifstream & , list::list &) ;//keyword class removed from second parameter
}
namespace list {
class list {
private:
std::string name;
int price;
short quantity;
public:
std::ofstream ofs;
std::ifstream ifs;
// file_mutators
void set_created_file () noexcept(false) ;
void set_readable_file ();
// constructors
list() noexcept ( noexcept ( set_created_file() ) ) ;
list ( std::string , int , short ) noexcept(false) ;
list ( class list &) noexcept ( noexcept ( set_created_file() ) ) ;
// initialization to cover after construction of an obj
void initialize ( std::string , int , short ) noexcept(false) ;
// mutators
void set_name ( std::string ) noexcept(false);
void set_price ( int ) noexcept(false) ;
void set_quantity ( short ) noexcept(false) ;
// accessors
std::string get_name ( ) const noexcept;
int get_price () const noexcept;
int get_quantity () const noexcept;
// Enqueries
bool check_created_file () const noexcept;//const added here
bool check_opened_file();
// destructor
~list();
// friend global functions
// overloaded operators
friend std::ofstream & friend_global_funcs :: operator << ( std::ofstream & , const list &) ;//const added here
friend std::ifstream & friend_global_funcs :: operator >> ( std::ifstream & , list &) ;
};
}
#endif
list.cpp
#include "list.h"
#include"empty.h"
#include "file_missing.h"
namespace list{
// constructors
list :: list () noexcept ( noexcept ( set_created_file() ) ) {
// give 'em a valid obj
// ,and a most common one
name = "not_set" , price = 1 , quantity = 1;
// not sure weather to call the
set_created_file();
// or simply call this
// ofs.open("list.txt", ios::app);
}
list :: list ( std::string name , int price , short quantity ) noexcept(false) {
set_name ( name );
set_price ( price );
set_quantity ( quantity );
set_created_file();
}
list :: list ( class list &r ) noexcept ( noexcept ( set_created_file() ) ) {
name = r.name;
price = r.price;
quantity = r.quantity;
// how to copy file location then?
// ofs = r.ofs;
set_created_file();
}
////
// initialization to cover after construction of an obj
void list :: initialize ( std::string name , int price , short quantity ) {
set_name ( name );
set_price ( price );
set_quantity ( quantity );
set_created_file();
}
////
// mutators
void list :: set_name ( std::string name ) noexcept(false) {
if ( name.empty() )
throw new empty::empty ( "name can not be left out enter something \n");
(*this).name = name;
}
void list :: set_price ( int price ) noexcept(false) {
if ( !price )
throw new empty :: empty ( "price can not be zero \n" );
(*this).price = price;
}
void list :: set_quantity ( short quantity ) noexcept(false) {
if ( !quantity )
throw new empty :: empty ( "quantity can not be zero \n" );
(*this).quantity = quantity;
}
/////
// file mutators
void list :: set_created_file () noexcept(false) {
if ( !ofs.is_open() )
ofs.open("student_list_file.txt", std::ios::app);
if ( !ofs.is_open() )
throw new file_missing :: file_missing ( "file couldn't be created or opened \n" );
}
void list :: set_readable_file () {
if ( !ifs.is_open() )
ifs.open ( "student_list_file.txt" );
if ( !ifs.is_open() )
throw new file_missing :: file_missing ( "file couldn't be opened by set_readable_file function \n" );
}
////
// accessors
std::string list :: get_name () const noexcept {
return name;
}
int list :: get_price () const noexcept {
return price;
}
int list :: get_quantity () const noexcept {
return quantity;
}
///
// enqueries
bool list :: check_created_file () const noexcept{
return ofs.is_open();
}
bool list :: check_opened_file (){
return ifs.is_open();
}
// destructive
list :: ~list() {
// release resources
// close file
// close connection
// release heap memory
ofs.close();
ifs.close();
}
}
friend_global_funcs.cpp
#include "list.h"
#include "file_missing.h"
#include "empty.h"
namespace friend_global_funcs {
std::ofstream & operator<< (std::ofstream &ofs, const list::list &l) { //const added here
if (!l.check_created_file())
throw new file_missing::file_missing(
"can not write info to file something wrong with acquiring file in constructor of obj \n");
ofs << l.name << "\t" << l.price << "\t" << l.quantity << "\n"; //changed l.name() to l.name
return ofs;
}
std::ifstream & operator>>(std::ifstream &ifs, list :: list &l) {
l.set_readable_file();
if (!l.check_opened_file())
throw new file_missing::file_missing(
"can't retrieve data cuz file is not associated with obj currently I'm in operated >> overloaded fuc \n");
ifs >> l.name >> l.price >> l.quantity;
return ifs;
}
}
Some of the changes that i made include:
friend_global_funcs inside file list.h. This namespace contains the forward declarations for the operator<< and operator>>.operator<< inside files list.h and friend_global_funcs.cpp.friend_global_funcs.h to friend_global.funcs.cpp since it contains implementation.PS: It took me around 30 minutes to create a working example out of your given github repo. There may be other logical errors in your program that i haven't checked because the repo is quite big. I was focusing on the problem at hand(which was to overload operator<< and operator>>).