Overloading operator<< template classes [duplicate] - c++

Question

This question already has answers here:
C++ template/ostream operator question
(2 answers)
What is the right way to write friend function declarations in template class?
(1 answer)
overloading friend operator<< for template class
(5 answers)
Closed 1 year ago.
I am using Microsoft Visual Studio for source code editor, and when I try to compile the below code I got an linker 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 X<int> &)" (??6#YAAAV?$basic_ostream#DU?$char_traits#D#std###std##AAV01#AAV?$X#H###Z) referenced in function _main Zadatak7.19.04.2021 C:\Users\AT95\Desktop\Zadatak7.19.04.2021\Zadatak7.19.04.2021\main.obj 1
What must I do to make this program work correctly?
I want you to look at the definition of the overloading operator<< because
that method is causing errors.
#include <iostream>
template <class T>
class X
{
private:
int index, capacity;
T* collection{};
public:
X(const int nmb, const T* array)
{
index = 0;
capacity = nmb;
collection = new(T[nmb]);
for (int i = 0; i < nmb; i++)
collection[index++] = array[i];
}
X(const X& other)
{
index = other.index;
capacity = other.capacity;
collection = new(T[other.capacity]);
if(other.index < other.capacity)
for (int i = 0; i < other.index; i++)
collection[i] = other.collection[i];
}
~X() { delete[] collection; }
friend std::ostream& operator<<(std::ostream&, X<T>&);
};
template <class T>
std::ostream& operator<<(std::ostream& out, X<T>& other)
{
for (int i = 0; i < other.index; i++)
out << i + 1 << ". " << other.collection[i];
return out;
}
int main()
{
int array[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }, n = sizeof(array)/sizeof(int);
X<int> x(n, array), b(x);
std::cout << x;
return 0;
}

Answer 1

Your friend dclaration is wrong.
friend std::ostream& operator<<(std::ostream&, X<T>&);
This is a friend declaration of a non templated operator<<.
It should look like this:
template<typename Z>
friend std::ostream& operator<<(std::ostream&, X<Z>&);
But there is a better way to declare it. Move the function body into the class then you don't need to declare the template part.
template <class T>
class X
{
// STUFF AS BEFORE
// Friend class definition.
friend std::ostream& operator<<(std::ostream& out, X& other)
{
for (int i = 0; i < other.index; i++)
out << i + 1 << ". " << other.collection[i];
return out;
}
}
Secondary note. In output operators the object being output is usually declared const (as outputting it should not modify it).
friend std::ostream& operator<<(std::ostream& out, X const& other)
^^^^^

Answer 2

You need to write the template keyword at the beginning of the friend function, like this:
#include <iostream>
template <class T>
class X
{
private:
int index, capacity;
T* collection{};
public:
X(const int nmb, const T* array)
{
index = 0;
capacity = nmb;
collection = new(T[nmb]);
for (int i = 0; i < nmb; i++)
collection[index++] = array[i];
}
X(const X& other)
{
index = other.index;
capacity = other.capacity;
collection = new(T[other.capacity]);
if(other.index < other.capacity)
for (int i = 0; i < other.index; i++)
collection[i] = other.collection[i];
}
~X() { delete[] collection; }
template<class U> friend std::ostream& operator<<(std::ostream&, X<U>&);
};
template <class T>
std::ostream& operator<<(std::ostream& out, X<T>& other)
{
for (int i = 0; i < other.index; i++)
out << i + 1 << ". " << other.collection[i];
return out;
}
int main()
{
int array[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }, n = sizeof(array)/sizeof(int);
X<int> x(n, array), b(x);
std::cout << x;
return 0;
}