This question already has answers here:
friend declaration declares a non-template function [duplicate]
(6 answers)
Closed 4 years ago.
Consider this class example. Simplified version.
#include <iostream>
#include <vector>
#include <iomanip>
using std::setw;
using std::ostream;
using std::istream;
template<class T=int, int size=10>
class Array
{
friend istream& operator>>(istream&, Array<T,size>&);
public:
private:
T arr[size];
};
template<class T, int size>
istream& operator>>(istream& input, Array<T,size> &a)
{
for(int i=0; i<size;++i)
{
input>>a.arr[i];
}
return input;
}
int main()
{
Array<int> x;
std::cin>>x;
}
I am getting:
1>main.obj : error LNK2019: unresolved external symbol "class std::basic_istream<char,struct std::char_traits<char> > & __cdecl operator>>(class std::basic_istream<char,struct std::char_traits<char> > &,class Array<int,10> &)" (??5#YAAAV?$basic_istream#DU?$char_traits#D#std###std##AAV01#AAV?$Array#H$09###Z) referenced in function _main
Can anyone explain why? I think I have implemented the function.
The friend declaration inside class definition declares a non-template function, then you define a template function later, they don't match. The non-template function is perferred in overload resolution, but it's not defined so you got the link error.
You could define it inside the class definition,
template<class T=int, int size=10>
class Array
{
friend istream& operator>>(istream&, Array<T,size>&)
{
for(int i=0; i<size;++i)
{
input>>a.arr[i];
}
return input;
}
public:
private:
T arr[size];
};
Or make the friend declaration refering to the template function.
template<class T=int, int size=10>
class Array;
template<class T, int size>
istream& operator>>(istream& input, Array<T,size> &a);
template<class T, int size>
class Array
{
friend istream& operator>> <>(istream&, Array<T,size>&);
public:
private:
T arr[size];
};
template<class T, int size>
istream& operator>>(istream& input, Array<T,size> &a)
{
for(int i=0; i<size;++i)
{
input>>a.arr[i];
}
return input;
}
Related
This question already has answers here:
C++ templates and friends, linker error
(1 answer)
Friend template overloaded operator <<: unresolved external symbol
(3 answers)
Linker error when operator== is a friend [duplicate]
(1 answer)
overloading friend operator<< for template class
(5 answers)
Closed 4 years ago.
I've encountered the following exercises while studying templates:
#include <iostream>
using namespace std;
template <class T>
class A {
T _v;
public:
A() {}
A(T v) : _v(v) {}
friend ostream & operator<<(ostream & c, const A<T> & v);
};
template <class T>
ostream & operator<<(ostream & c, const A<T> & v){
c << v._v; return c;
}
int main()
{
A<int>a(10);
cout << a << endl;
return 0;
}
This code snippet should produce an error during compilation and it actually does. It's a linker error, but I can't understand it.
I tried to change a few lines of code and the error seems to be caused by the instation of the template operator<<, since removing that template and writing a specific operator make the code work. I also have the feeling that the template gets instantiated multiple times, not only for int.
However, to my limited knowledge, the template definition seems fine. What am I missing?
The exact error (VS 2017) is:
Error LNK2019: unresolved external symbol "class std::basic_ostream > & __cdecl operator<<(class std::basic_ostream > &,class A const &)" (??6#YAAEAV?$basic_ostream#DU?$char_traits#D#std###std##AEAV01#AEBV?$A#H###Z) referenced in function main
You need to declare the friend function as template:
class A {
...
template <class U> // <-- NOTICE ---------------v
friend ostream & operator<<(ostream & c, const A<U> & v);
...
Or better yet, use a safer approach, despite a bit more verbose:
#include <iostream>
using namespace std;
template <class T>
class A;
template <class T>
ostream& operator<<(ostream& c, const A<T>& v);
template <class T>
class A {
T _v;
public:
A() {}
A(T v) : _v(v) {}
friend ostream& operator<< <T>(ostream& c, const A<T>& v);
};
template <class T>
ostream& operator<<(ostream& c, const A<T>& v) {
c << v._v;
return c;
}
int main() {
A<int> a(10);
cout << a << endl;
return 0;
}
See this page for more details.
I'm trying to write the function definition for overloading the operators ">>" and "<<" outside of the class definition, though in the same file, as you can see. I get the following errors:
1>Source.obj : error LNK2019: unresolved external symbol "class std::basic_istream<char,struct std::char_traits<char> > & __cdecl operator>>(class std::basic_istream<char,struct std::char_traits<char> > &,class MyClass<int> &)" (??5#YAAAV?$basic_istream#DU?$char_traits#D#std###std##AAV01#AAV?$MyClass#H###Z) referenced in function _main
1>Source.obj : error LNK2019: unresolved external symbol "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class MyClass<int>)" (??6#YAAAV?$basic_ostream#DU?$char_traits#D#std###std##AAV01#V?$MyClass#H###Z) referenced in function _main
1>Source.obj : error LNK2019: unresolved external symbol "class std::basic_istream<char,struct std::char_traits<char> > & __cdecl operator>>(class std::basic_istream<char,struct std::char_traits<char> > &,class MyClass<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > &)" (??5#YAAAV?$basic_istream#DU?$char_traits#D#std###std##AAV01#AAV?$MyClass#V?$basic_string#DU?$char_traits#D#std##V?$allocator#D#2##std#####Z) referenced in function _main
1>Source.obj : error LNK2019: unresolved external symbol "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class MyClass<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >)" (??6#YAAAV?$basic_ostream#DU?$char_traits#D#std###std##AAV01#V?$MyClass#V?$basic_string#DU?$char_traits#D#std##V?$allocator#D#2##std#####Z) referenced in function _main
I've been up and down the river with this one. The strange thing is that if I write their definitions inside the class definition, it works flawlessly.
#include <iostream>
#include <string>
using namespace std;
template <class MYTYPE>
class MyClass {
MYTYPE *myVector;
int dim;
string name;
public:
MyClass(int, string);
MyClass() {};
friend istream& operator>>(istream&, MyClass<MYTYPE>&);
friend ostream& operator<<(ostream&, MyClass<MYTYPE>);
};
template <class MYTYPE>
MyClass<MYTYPE>::MyClass(int x, string y) {
dim = x;
name = y;
myVector = new MYTYPE[dim];
}
template <class MYTYPE>
istream& operator>>(istream& X, MyClass<MYTYPE>& a){
cout<<"Reading vector: "<<a.name<<endl;
for(int indice = 0; indice < a.dim; indice++){
cout<<a.name<<'['<<indice<<"]= ";
X >> a.myVector[indice];
}
return X;
}
template <class MYTYPE>
ostream& operator<<(ostream& X, MyClass<MYTYPE> a){
X<<"Vector: "<<a.name<<endl;
for(int indice = 0; indice < a.dim; indice++)
X<<a.myVector[indice]<<' ';
X<<endl;
return X;
}
int main() {
MyClass<int> object(4, "Ints vector");
MyClass<string> object2(5, "String vector");
cin >> object;
cin >> object2;
cout << object;
cout << object2;
system("pause");
return 0;
}
In your code, declarations of friend operators within MyClass should look like these:
template<typename T> friend istream& operator>>(istream&, MyClass<T>&);
template<typename T> friend ostream& operator<<(ostream&, MyClass<T>);
That is, they shall have their own template parameters.
A valid definition for MyClass based on the one provided in the question follows:
template <class MYTYPE>
class MyClass {
MYTYPE *myVector;
int dim;
string name;
public:
MyClass(int, string);
MyClass() {}
template<typename T>
friend istream& operator>>(istream&, MyClass<T>&);
template<typename T>
friend ostream& operator<<(ostream&, MyClass<T>);
};
Turning up the warning level of the compiler helps. By using -Wall with g++, I get the following warnings before the linker error.
socc.cc:13:58: warning: friend declaration ‘std::istream& operator>>(std::istream&, MyClass<MYTYPE>& )’ declares a non-template function [-Wnon-template-friend]
friend istream& operator>>(istream&, MyClass<MYTYPE>&);
^
socc.cc:13:58: note: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here)
socc.cc:14:57: warning: friend declaration ‘std::ostream& operator<<(std::ostream&, MyClass<MYTYPE> ’ declares a non-template function [-Wnon-template-friend]
friend ostream& operator<<(ostream&, MyClass<MYTYPE>);
You need to use function templates for the operator>> and operator<< functions. You can declare them before the definition of the class with:
// Forward the class template.
template <class MYTYPE> class MyClass;
// Declare the function templates.
template <class MYTYPE>
std::istream& operator>>(std::istream&, MyClass<MYTYPE>&);
template <class MYTYPE>
std::ostream& operator<<(st::ostream&, MyClass<MYTYPE>);
Then, you'll have to use the friend declaration with the appropriate template parameter.
// This makes sure that operator>><int> is not a friend of MyClass<double>
// Only operator>><double> is a friend of MyClass<double>
friend std::istream& operator>><MYTYPE>(std::istream&, MyClass<MYTYPE>&);
friend std::ostream& operator<<<MYTYPE>(std::ostream&, MyClass<MYTYPE>);
Here's an updated version of your code that builds for me. I haven't tried to run it.
#include <iostream>
#include <string>
using namespace std;
template <class MYTYPE> class MyClass;
template <class MYTYPE>
std::istream& operator>>(std::istream&, MyClass<MYTYPE>&);
template <class MYTYPE>
std::ostream& operator<<(std::ostream&, MyClass<MYTYPE>);
template <class MYTYPE>
class MyClass {
MYTYPE *myVector;
int dim;
string name;
public:
MyClass(int, string);
MyClass() {};
friend std::istream& operator>><MYTYPE>(std::istream&, MyClass<MYTYPE>&);
friend std::ostream& operator<<<MYTYPE>(std::ostream&, MyClass<MYTYPE>);
};
template <class MYTYPE>
MyClass<MYTYPE>::MyClass(int x, string y) {
dim = x;
name = y;
myVector = new MYTYPE[dim];
}
template <class MYTYPE>
std::istream& operator>>(std::istream& X, MyClass<MYTYPE>& a){
cout<<"Reading vector: "<<a.name<<endl;
for(int indice = 0; indice < a.dim; indice++){
cout<<a.name<<'['<<indice<<"]= ";
X >> a.myVector[indice];
}
return X;
}
template <class MYTYPE>
std::ostream& operator<<(std::ostream& X, MyClass<MYTYPE> a){
X<<"Vector: "<<a.name<<endl;
for(int indice = 0; indice < a.dim; indice++)
X<<a.myVector[indice]<<' ';
X<<endl;
return X;
}
int main() {
MyClass<int> object(4, "Ints vector");
MyClass<string> object2(5, "String vector");
cin >> object;
cin >> object2;
cout << object;
cout << object2;
system("pause");
return 0;
}
It is a bit complicated. Actually there are two ways a class template can have a function friend.
For example, take this:
template <typename T>
void fun();
template <typename T>
class A
{
};
Then what do you want?
Or do you want that func<X>() be friend of A<Y> for any other Y?
Do you want that fun<X>() be friend of A<X> but not of A<Y>?
That is, should the function be able to access privater members of any specialization or only to the specialization of the same type.
My guess is that you need option 2.
The trick is that you must make friend of the function specialization so the template function must already exist: you have declare the template function first and then make it friend. Something like:
//first declare both function and class
template <typename T>
class A;
template <typename T>
void fun();
//Then define the class
template <typename T>
class A
{
//A<T> is friend to fun<T>, the specializations must match.
friend void fun<T>();
};
//And define the function
template <typename T>
void fun()
{
}
If what you want is option 1, then there is no need of the advanced declarations. Just:
template <typename T>
class A
{
//A<T> is friend to fun<X>, T and X may be different
template <class X>
friend void fun();
};
That said, your code is a bit tricky to write because of the << and the >>:
template <class MYTYPE>
class MyClass;
template <class MYTYPE>
istream& operator>>(istream& X, MyClass<MYTYPE>& a);
template <class MYTYPE>
ostream& operator<<(ostream&, MyClass<MYTYPE>);
template <class MYTYPE>
class MyClass {
// ....
friend istream& operator>> <MYTYPE>(istream&, MyClass<MYTYPE>&);
friend ostream& operator<< <MYTYPE>(ostream&, MyClass<MYTYPE>);
};
I'm trying to recreat a stack with a forward_list. However, i use friend functions to overload the + and << operator.
#pragma once
#include <forward_list>
template <class T> class Stack;
template <class T>
Stack<T> operator+(const Stack<T> &a, const Stack<T> &b){
//implementation
}
template <class T>
std::ostream &operator<<(std::ostream &output, Stack<T> &s)
{
//implementation
}
template <class T>
class Stack
{
friend Stack<T> operator+(const Stack<T> &a, const Stack<T> &b);
friend std::ostream &operator<<(std::ostream &output, Stack<T> &s);
std::forward_list<T> l;
public:
//Some public functions
};
For both of the friend functions i get an linker Error , when i try to call them in my main like:
int main(){
Stack<int> st;
st.push(4);
Stack<int> st2;
st2.push(8);
cout<<st + st2<<endl;
return 0;
}
And these are the errors:
error LNK2019: unresolved external symbol "class Stack<int> __cdecl operator+(class Stack<int> const &,class Stack<int> const &)" (??H#YA?AV?$Stack#H##ABV0#0#Z) referenced in function _main
error LNK2019: unresolved external symbol "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class Stack<int> &)" (??6#YAAAV?$basic_ostream#DU?$char_traits#D#std###std##AAV01#AAV?$Stack#H###Z) referenced in function _main
Thanks in advance.
Your template friend declarations within the Stack class are not quite correct. You need to declare like this:
template<class T>
friend Stack<T> operator+(const Stack<T> &a, const Stack<T> &b);
template<class T>
friend std::ostream &operator<<(std::ostream &output, Stack<T> &s);
Since you are using MSVC, please see this Microsoft documentation for further reference.
I have a template class called "KeyedCollection" that contains functions to insert data into a vector, as well as stream out the data. The vector is a private member function. I can't seem to figure out how to use the information from this vector in my overloading ostream friend function. Note: I cannot change the general structure of the class and the function arguments, they have to stay as they are. I list all of the class for reference, but the function in question is the last one.
#include "stdafx.h"
#include <iostream>
#include <vector>
#include "Windows.h"
#include <string>
using namespace std;
template <class K, class T>
class KeyedCollection {
public:
// Create an empty collection
KeyedCollection();
// Return the number of objects in the collection
int size() const;
// Insert object of type T with a key of type K into the
// collection using an “ignore duplicates” policy
void insert(const K&, const T&);
// Output data value of objects in the collection,
// one data value per line
friend ostream& operator<<(ostream&,
const KeyedCollection&);
private:
// Insert required members here
int objSize;
vector<T> objects;
};
template<class K, class T>
KeyedCollection<K,T>::KeyedCollection() {
objSize = 0;
vector<T> objects;
}
template<class K, class T>
int KeyedCollection<K,T>::size() const {
objSize = objects.size();
return objSize;
}
template<class K, class T>
void KeyedCollection<K,T>::insert(const K&,const T& c) {
objects.push_back(c);
}
// !!! function i am trying to define !!!
template<class K, class T>
ostream& operator<<(ostream& outstream,const KeyedCollection<K,T>& inst) {
outstream<<inst<<endl;
return outstream;
}
Also, I'm getting an error that says
"fatal error LNK1120: 1 unresolved externals"
and one that says
"error LNK2019: unresolved external symbol "class std::basic_ostream > & __cdecl operator<<(class std::basic_ostream > &,class KeyedCollection const &)" (??6#YAAAV?$basic_ostream#DU?$char_traits#D#std###std##AAV01#ABV?$KeyedCollection#HVCustomer#####Z) referenced in function _main" ...
Just as a side question, any idea what those could be?
cppreference and Johannes Schaub - litb both provide the same method for getting this to work.
You want to make one single instance (called "specialization" in
generic terms) of that template a friend. You do it the following way
[...]
First make a forward declaration before the definition of your class:
template <class K, class T> class KeyedCollection;
template<class K, class T>
ostream& operator<<(ostream& outstream,const KeyedCollection<K,T>& inst);
Because the compiler knows from the parameter list that the template
arguments are T and U, you don't have to put those between <...>, so
they can be left empty.
Then make your friend declaration and be sure to add <> after operator<<:
template <class K, class T>
class KeyedCollection {
public:
// snip
friend ostream& operator<< <> (ostream& outstream,const KeyedCollection<K,T>& inst);
// snip
};
Finally you can define it:
template<class K, class T>
ostream& operator<<(ostream& outstream,const KeyedCollection<K,T>& inst) {
// Just an example
for (const auto& t : inst.objects)
{
std::cout << t << std::endl;
}
return outstream;
}
Live Example
Alternately, do what Yakk suggested.
template <class K, class T>
class KeyedCollection {
public:
// snip
friend ostream& operator<<(ostream& outstream,const KeyedCollection<K,T>& inst) {
for (const auto& t : inst.objects)
{
std::cout << t << std::endl;
}
return outstream;
}
// snip
};
Live Example
//cstack.h
# ifndef _STACK_H__
#define _STACK_H__
#include<iostream>
#include<vector>
template< class Type ,int Size = 3>
class cStack
{
Type *m_array;
int m_Top;
int m_Size;
public:
cStack();
cStack(const Type&);
cStack(const cStack<Type,Size> &);
int GetTop()const;
bool Is_Full()const;
bool Is_Empty()const;
void InsertValue(const Type&);
void RemeoveValue();
void show();
~cStack();
friend std::ostream& operator <<(std::ostream &, const cStack<Type,Size> &);
};
// iam writing only one function defination because linking is because of this function
template< class Type,int Size >
std::ostream& operator << ( std::ostream &os, const cStack<Type,Size> &s)
{
for( int i=0; i<=s.GetTop();i++)
{
os << s.m_array[i];
}
return os;
}
//main.cpp
#include "cStack.h"
#include <string>
#include<iostream>
int main()
{
cStack<int> sobj(1);
std::cout << sobj;
}
When I compile I get the following error:
error LNK2019: unresolved external symbol "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class cStack<int,3> const &)" (??6#YAAAV?$basic_ostream#DU?$char_traits#D#std###std##AAV01#ABV?$cStack#H$02###Z) referenced in function _main
35.16 Why do I get linker errors when I use template friends?
friend std::ostream& operator<< (std::ostream &, const cStack<Type, Size> &);
Mark the function as template functions
friend std::ostream& operator<< <>(std::ostream &, const cStack<Type, Size> &);
And the compiler will be happy.
And put the function definition before the class definition.
template< class Type ,int Size>
class cStack;
template< class Type ,int Size >
std::ostream& operator <<(std::ostream &os, const cStack<Type,Size> &s)
{
for( int i=0; i<=s.GetTop();i++)
{
os << s.m_array[i];
}
return os;
}
template< class Type ,int Size = 3>
class cStack
{
Type *m_array;
int m_Top;
int m_Size;
public:
cStack() {}
//...
friend std::ostream& operator<< <>(std::ostream &, const cStack<Type, Size> &);
};
I think the linker can't find your function. Place all the code in the the header file.
It's compiling, it's just not linking. (I know, that's not a big help.) You don't have a place where the function template is being instantiated. Do you have a main() somewhere that tries to use this stream insertion operator?
[snip]
The following works on windows with Visual Studio 2005
template <class Type, int Size = 3 >
class cStack
{
// ....
template<class Type>
friend std::ostream& operator<< (std::ostream & os, const cStack<Type> &s);
};
template<class Type>
std::ostream& operator << (std::ostream &os, const cStack<Type> &s)
{
for( int i=0; i < s.GetTop();i++)
{
os << s.m_array[i];
}
return os;
}