//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;
}
Related
This question already has an answer here:
How do we declare a friend function with a class template into .h file and define them into a .cpp file (not all in one header file)?
(1 answer)
Closed 9 months ago.
so i have this template class:
template<class T = int, unsigned int SIZE =2>
class FixedPoint {
public:
explicit FixedPoint(T dollars = 0);
FixedPoint(T dollars, T cents);
friend std::ostream& operator<<(std::ostream& os ,const FixedPoint& price);
private:
static long digitsAfterDotFactor();
long sizeAfterDot;
T dollars;
T cents;
};
and this is it's definition under the class in the h file
template<class T,unsigned int SIZE>
inline std::ostream& operator<<(std::ostream& os ,const FixedPoint<T,SIZE>& price){
os << price.dollars << "." << price.cents;
return os;
}
the code gives me the following error:
friend declaration ‘std::ostream& operator<<(std::ostream&, const FixedPoint<T, SIZE>&)’ declares a non-template function
i tried adding the template name in the decleration but it doesn't recognise T class so what can i do? should i make specification templates for each type ?
As the error message said, the friend declaration declares a non-template operator<<, but it's defined as a template, they don't match.
You can make the friend declaration referring to the operator template, e.g.
// forward declaration
template<class T = int, unsigned int SIZE =2>
class FixedPoint;
// declaration
template<class T,unsigned int SIZE>
std::ostream& operator<<(std::ostream& os ,const FixedPoint<T,SIZE>& price);
template<class T, unsigned int SIZE>
class FixedPoint {
public:
...
friend std::ostream& operator<< <T, SIZE> (std::ostream& os ,const FixedPoint<T, SIZE>& price);
// or just
// friend std::ostream& operator<< <> (std::ostream& os ,const FixedPoint& price);
...
};
// definition
template<class T,unsigned int SIZE>
inline std::ostream& operator<<(std::ostream& os ,const FixedPoint<T,SIZE>& price){
os << price.dollars << "." << price.cents;
return os;
}
You can just define the friend member function within the class template itself,
template<class T = int, unsigned int SIZE =2>
class FixedPoint {
public:
/* ... */
friend std::ostream& operator<<(std::ostream& os ,const FixedPoint& price)
{
return os << price.dollars << "." << price.cents;
}
/* ... */
};
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>);
};
Why does the following code not compile (linker error, 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 cClass<int> const &)" (??6#YAAEAV?$basic_ostream#DU?$char_traits#D#std###std##AEAV01#AEBV?$cClass#H###Z))
#include <iostream>
template <class Type> class Class{
public:
friend std::ostream& operator<<(std::ostream& Stream, const Class& Op);
};
template <class Type> std::ostream& operator<<(std::ostream& Stream, const Class<Type>& Op){
return(Stream);
}
int main(){
Class<int> A;
std::cout << A;
return(0);
}
This line:
friend std::ostream& operator<<(std::ostream& Stream, const Class& Op);
Should be:
template <class T> friend std::ostream& operator<<(std::ostream& Stream, const Class<T>& Op);
This is my code:
mov.h
#include <iostream>
template< class T>
class Movie {
public:
Movie(T in) {
a = in;
}
friend std::ostream& operator<<(std::ostream& os, const Movie<T>& movie);
private:
T a;
};
template<class T>
std::ostream& operator<<(std::ostream& os, const Movie<T>& movie) {
return os;
}
main.cpp
#include "mov.h"
int main() {
Movie<int> movie1(1);
std::cout << movie1 << std::endl;
return 0;
}
I try to compile this code, and I get the error:
Error 1 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 Movie<int> const &)" (??6#YAAAV?$basic_ostream#DU?$char_traits#D#std###std##AAV01#ABV?$Movie#H###Z) referenced in function _main c:\Users\Adi\documents\visual studio 2013\Projects\bdika01\bdika01\main.obj bdika01
Error 2 error LNK1120: 1 unresolved externals c:\users\adi\documents\visual studio 2013\Projects\bdika01\Debug\bdika01.exe 1 1 bdika01
If I try the code inline like this it's okay:
mov.h
#include <iostream>
template<class T>
class Movie {
public:
Movie(T in) {
a = in;
}
friend std::ostream& operator<<(std::ostream& os, const Movie<T>& movie){
return os;
}
private:
T a;
};
What can I do if I want to separate the defination and declaration?
Thanks.
template <typename T> class Movie;
template<class T>
std::ostream& operator<<(std::ostream& os, const Movie<T>& movie);
template< class T>
class Movie {
friend std::ostream& operator<< <T>(std::ostream& os, const Movie<T>& movie);
};
template<class T>
std::ostream& operator<<(std::ostream& os, const Movie<T>& movie){
return os;
}
In your original code, you befriend a non-template function that just happens to take the right instantiation of Movie<> as a parameter. So, every time a Movie<T> is instantiated, a corresponding non-template operator<< is declared (but not defined) in the enclosing namespace scope. friend declarations are strange this way.
In addition, you declare and define a function template named operator<< (which is not a friend of any instantiation of Movie). However, overload resolution prefers non-templates, other things equal.
The following codes are in file Heap.h
template <typename T>
class Heap {
public:
Heap();
Heap(vector<T> &vec);
void insert(const T &value);
T extract();
T min();
void update(int index, const T &value);
/* for debug */
friend ostream &operator<< (ostream &os, const Heap<T> &heap);
#if 0
{
for (int i = 0; i < heap.vec_.size(); i++) {
os << heap.vec_[i] << " ";
}
return os;
}
#endif
private:
void minHeapify(int index);
int left(int index);
int right(int index);
int parent(int index);
void swap(int, int);
vector<T> vec_;
};
template <typename T>
ostream &operator<<(ostream &os, const Heap<T> &heap)
{
for (int i = 0; i < heap.vec_.size(); i++) {
os << heap.vec_[i];
}
return os;
}
In a testheap.cpp file, I use this template class, when I compile, an undefined reference to the << operator overloading function error occur. Confused with this situation.
When I put the definition of the function in the class, it works.
The OS is Ubuntu, compiler is g++.
The following should work:
template <typename T>
class Heap {
public:
...
template<class U>
friend ostream &operator<<(ostream &os, const Heap<U> &heap);
...
};
template <typename T>
ostream &operator<<(ostream &os, const Heap<T> &heap)
{
...
}
Try this :
template <typename T2>
friend ostream &operator<< (ostream &os, const Heap<T2> &heap);
I had the same problem. Here is the question