I am receiving the error "declaration of reference variable 'operator' requires an initialization" in my main.cpp.
In the constructor have already tried including:
friend ostream& operater << (ostream& out, Coins& c)
in the header file, however the error still occurs. Also, I tried adding
#include <ostream>
#include <iostream>
in the header file.
This is the function I am using.
myfile.h
ostream& operater << (ostream& out, Coins& c){
c.print(out);
return out;
}
Related
While doing some code practice myself I got some Error
student.h file
#pragma once
class Student {
public:
Student() = default;
Student(int id, const char* name, int score);
Student(const Student& s);
Student(Student&& other);
Student& operator=(Student&& other);
virtual ~Student();
**friend std::ostream & operator<<(std::ostream & os, const Student & rhs);**
void print();
private:
int mId;
char* mName;
int mScore;
size_t mSize;
};
student.cpp file
#include "student.h"
#include <iostream>
#include <string>
... //bunch of constructors, override...
std::ostream & operator<<(std::ostream& os, const Student & rhs)
{
os << rhs.mId << rhs.mName << rhs.mScore ; // Compile Error: Cannot Access Student Member(rhs.mId, rhs.mName...)
return os;
}
The Error i got was i cannot access Student class member despite I declared it friend function in student.h
Strange to me, It compiled successfully if i declare #include<iostream> statement in student.cpp file like this
#include <iostream> //change position <iostream> and "student.h"
#include <string>
#include "student.h"
... //bunch of constructors, override...
std::ostream & operator<<(std::ostream& os, const Student & rhs)
{
os << rhs.mId << rhs.mName << rhs.mScore ; //Compile successfully
return os;
}
So, why #include statement sequences are matters? i thought #include statement just copy-paste to my cpp file.
And it also compiled successfully when i declare #include<iostream> to student.h,
so when i include some #include things the Best-case is declare it to header-file?
Could you give me some advise?
I compiled my program in Visual Studio 2019.
In student.h add an include for iosfwd:
#pragma once
#include <iosfwd>
class Student ...
This will ensure correct resolution of the std::ostream symbol and recognize the ostream operator you defined as the one you declared as a friend.
The iosfwd header contains forward declarations of the symbols in iostream header, and it is defined for exactly this scenario.
In C++ we must declare anything before using it.
So your problem is that you haven't declared std::ostream but you have used it in student.h.
An suitable solution is to #include <iostream> in the beginning of student.h, below #pragma once.
You can also put these before class Stduent:
namespace std
{
class ostream;
}
This is called forward declaration.
I have a project with multiple header and source files, which I have shortened to what I believe is important here (though I could be wrong). They look like so:
A.hpp:
#pragma once
struct date_t {
unsigned int day{ 0 };
unsigned int month{ 0 };
unsigned int year{ 0 };
};
A.cpp:
#include <iostream>
#include "A.hpp"
using namespace std;
ostream& operator<<(ostream& output, const date_t& date) {
output << date.month << "/" << date.day << "/" << date.year;
return output;
}
B.hpp:
#pragma once
#include "A.hpp"
class B {
public:
date_t date;
};
B.cpp:
#include <iostream>
#include "B.hpp"
using namespace std;
ostream& operator<<(ostream& output, B& b) {
output << b.date;
return output;
}
In this particular case, B.cpp gives the error no operator "<<" matches these operands; operand types are: std::basic_ostream<char, std::char_traits<char>> << date_t. I'm not quite sure whether or not I need to change the parameters in the overloaded function or if this is an access problem, so why is this error being thrown here? To follow that up, would class B have a similar issue if it's insertion overload was used too?
You have defined operator<< in A.cpp but you need to declare it in a header file too, so code in other cpp files knows about it.
Just add
std::ostream& operator<<(std::ostream& output, const date_t& date);
to A.hpp (after the definition of struct date_t). You will need to add #include <iostream> to A.hpp as well.
Do the same thing with the other insertion operator.
Any function or operator that you define in one cpp file but wish to use in another cpp file should be declared in a header file.
The aim of this code is to read the file containing the name, dollars in billions and the country which are separated by tabs.
I need to create a class Billionaire and overload the ostream and istream operators to conveniently
read the file into a vector and write the content to the output. And then create a map which maps the country string to a pair. The pair contains a copy of the first
billionaire of every country from the list and a counter to count the number of billionaires per
country. However, I cannot overload stream and stream operators.
I've tried to overload these operators in Billionaire class but I am ending up with errors.
#include <iostream>
#include <vector>
#include <utility>
#include <algorithm>
#include <set>
#include <map>
#include <string>
#include <iterator>
#include <fstream>
#include <istream>
#include <ostream>
using namespace std;
class Billionaire{
//overload the ostream and istream operators to conveniently
//read the file into a vector and write the content to the output
public :
friend ostream &operator<<(ostream &stream, Billionaire o);
friend istream &operator>>(istream &stream, Billionaire &o);
};
int main(){
std::ifstream stream("Forbes2018.txt");
if(!stream){
cout << " WARNING : File not found !" << endl ;
}
vector <Billionaire> billionaires;
copy (istream_iterator<Billionaire>( stream ),
istream_iterator<Billionaire>() , back_inserter( billionaires ));
copy (billionaires.begin () , billionaires.end () ,
ostream_iterator < Billionaire >( cout , "\n"));
map < string , pair < const Billionaire , size_t >> m;
}
I am having 2 errors:
:-1: error: symbol(s) not found for architecture x86_64
:-1: error: linker command failed with exit code 1 (use -v to see invocation)
Your overload attempt is a good start: you have announced to the compiler that there will be an overload:
friend ostream &operator<<(ostream &stream, Billionaire o);
friend istream &operator>>(istream &stream, Billionaire &o);
Unfortunately, something is missing. This is what the linker message says. You still need to tell the compiler how this overload looks like:
ostream &operator<<(ostream &stream, Billionaire o) {
// put your code here
...
return stream;
}
istream &operator>>(istream &stream, Billionaire &o) {
// put your code here
...
return stream;
}
In case you have defined these operators in the Billionaire, the compiler wont be able to use them here : in main you invoke the free standing operator (that you have declared as friend), whereas you would have defined class members that have to be invoked on a Billionaire with the . or -> operator and have a different signature than what you’re using in main.
Header file:
#ifndef CART_H
#define CART_H
#include "Tops.h"
#include <iostream>
#include <vector>
using namespace std;
class Cart
{
public:
Cart();
void addTop(Tops& top);
friend ostream& operator<<(ostream& ostr, const Cart& c);
private:
vector<Tops> tops;
};
#endif
Implementation file:
#include "Cart.h"
#include <iostream>
#include <fstream>
#include <vector>
using namespace std;
Cart::Cart() { }
void Cart::addTop(Tops &top)
{
tops.push_back(top);
}
ostream& operator<<(ostream &ostr, const Cart &c)
{
ostr << "TOPS IN CART:\n-------------\n";
for (auto const top : c.tops) {ostr << top << endl; } // no match for 'operator<<'
return ostr;
}
Problem: I keep getting a "no match for operator<<" error and I have no idea why, and I also have no idea what it means. When I google this error, what has caused the error in other people's codes does not apply to mine.
In your declaration you have stated that the Cart argument will be const:
friend ostream& operator<<(ostream& ostr, const Cart& cart);
but your definition does not:
ostream& operator<<(ostream &ostr, Cart &c)
They need to match (either both const or both not - both const would be right here) for the friend declaration to be of any use.
Hello I'm in the process of learning about operator overloading and friend functions.
I've declared the operator<< function as a friend of my class in a .h file but I still cant access the private member variables from the function definition in a .cpp file
My code is as follows:
Test.h
class Test
{
private:
int size;
public:
friend ostream& operator<< (ostream &out, Test& test);
};
Test.cpp
#include "Test.h"
#include <iostream>
using namespace std;
ostream& operator<< (ostream& out, Test& test)
{
out << test.size; //member size is inaccessible!
}
Apparently size is inaccessible although I've already made the operator<< function a friend of my class. I've Googled a bit and haven't found anything so can anyone help me out? Thanks.
Note: If I move the class definition to the .cpp file everyone works so I assume my problem has something to do with multiple files.
In c++ the scope of the declaration go from top to bottom. So if you include first Test.h and after that <iostream> the friend declaration has does not know about the type std::ostream.
The solution:
Test.h:
#include <iostream>
class Test
{
private:
int size;
public:
friend std::ostream& operator<< (std::ostream &out,const Test& test);
};
Test.cpp:
#include "Test.h"
std::ostream& operator<< (std::ostream& out,const Test& test)
{
out << test.size;
return (*out);
}
Note that the #include <iostream> has been moved from Test.cpp to Test.h and the argument of the global operator << takes const Test& test. The const makes the operator work for rvalues.