C++ 代码中的错误“未定义对运算符 << 的引用”

Question

I have written a simple operator overloaded function, which is working fine if I define it within the class (block commented), but getting error, if define outside the class. Any idea where is the mistake?

#include<iostream>

using namespace std;


template<class T>
class vector{
    
    int d;
public:
    vector(){
        d = 0;
    }
    vector(T data){
        d = data;
    }
    
    friend void operator << (vector &v, T data);
    /*{
        cout << data << endl;
    }*/
    
};

template<class T>
void operator << (vector<T> &v, T data){
    
    cout <<  data;
}


int main(){
    
    vector<int> v1;
    v1 << 10;
    
    return 0;
}

Answer 1

This friend function declaration within the class template definition

friend void operator << (vector &v, T data);

is not a template function (though it is a templated entity if it is defined within the class).

On the other hand, this declaration outside the class template definition

template<class T>
void operator << (vector<T> &v, T data){
    
    cout <<  data;
}

defines a template function.

Either define the friend function in the class definition. And in this case the compiler will generate the definition of the function for each used specialization of the class template. Or for each used class specialization you have to define (provide) a separate non-template friend function explicitly yourself.

Here is a demonstrative program

#include<iostream>

template<class T>
class vector{
    
    int d;
public:
    vector(){
        d = 0;
    }
    vector(T data){
        d = data;
    }
    
    friend void operator << (vector &v, T data);
    /*{
        cout << data << endl;
    }*/
    
};

template<class T>
void operator << (vector<T> &v, T data)
{    
    std::cout <<  data;
}

void operator << (vector<int> &v, int data)
{    
    std::cout <<  data;
}

int main()
{
    
    vector<int> v1;
    v1 << 10;
    
    return 0;
}

The program output is

10

In the program there are two overloaded functions operator << . The first one is a friend non-template function declared in the class definition and the second one is a template function that is not a friend function of the class. For the class specialization vector<int> used in main you have to provide the non-template friend function definition outside the class.

Or you could define the friend function within the class.

Answer 2

Problem is how to friend a template to other template.

Without friendship your code works (you do no access private parts).

To fix it you have to friend a template and not to a regular function. Since function you are defining is a template friendship statement have to express that.

template<class T>
class vector{
    
    int d;
public:
    ...

    template<typename U>
    friend void operator<<(vector<U> &v, U data);
};

https://godbolt.org/z/oxcojb

Offtopic :
Personally I avoid friendships (I do not remember to ever use it in my code). Your code I would do like this .

Answer 3

if you are using namespace std and then defining a class called vector, which is a name from the std namespace, the compiler will not know which one to use and this will result in undefined behaviour, or so I think.

also, you are getting the error because you need to define that function inside the class because you are using a template.