How can I fix the friend function error?

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Hello
I wrote a program where friend functions give errors when defined outside the program.
that said " "n" , "A" was not declared in this scope"
and when this is " complex l; l.A[j] " , said: no match for 'operator<' (operand types are "vector" and "vector""
can you help me?

What I have tried:

class vector{
 private:
	  vector *A;
	  int a;
	  int n;
	  public:
	  vector (int);
	  vector(){};
	  void output();
	  friend vector min();
	  friend int search(int s);
<pre>vector min()
{
	vector l;
	vector m;
	vector temp;
	for (int i=0;i<l.n;i++)
	m=l.A[0];
	for (int j=1;j<l.n;j++)
	{
	if (l.A[j]<m)
	{
		temp=l.A[j];
		l.A[j] =m;
		m=temp;
	}
    };
    }

Posted 15-Jun-23 8:34am

sogia

Updated 15-Jun-23 12:18pm

v2

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Richard MacCutchan 15-Jun-23 15:08pm

Where is the other code and where does the error occur?

2 solutions

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k5054 · Accepted Answer · 2023-06-15T12:18:00

A friend function needs to know which instance of the class to operate on. Consider

C++

class C {
  private:
     int x;
  public:
  A(int n) : x(n) {};

  friend void fn(void);
}

void fn()
{
    std::cout << x << '\n';  // Don't know which C instance to use!
}

int main()
{
   C x(1);
   C y(2);
   fn();  // <- which C object does this call to fn() access?
}

A few simple changes correct this:

C++>

class C {
private:
    int x;
public:
    C(int n) : x(n) {}
    friend void fn(const C&);
};

void fn(const C& c) {
    std::cout << c.x << '\n';  // Because we're a friend, we can access c.x!
}

int main()
{
    C x(1);
    C y(2);

    fn(x);
    fn(y);
}

Rick York · Accepted Answer · 2023-06-15T10:34:00

We can't guess with the fragment of code posted. This tutorial section on friend functions might help : cplusplus.com tutorial friendship & inheritance/[^]

ETA : actually it is fairly clear what some of the problems are. Here is the code for the function with indentation added :

C++

vector min()
{
    vector l;
    vector m;
    vector temp;
    for (int i=0;i<l.n;i++)
        m=l.A[0];

    for (int j=1;j<l.n;j++)
    {
        if (l.A[j]<m)
        {
            temp=l.A[j];
            l.A[j] =m;
            m=temp;
        }
    }   // ;  the semicolon here is unnecessary
}

There are several things wrong with this.
- All references to m are bad because it is a vector.
- The vector l is accessed several times but it is never initialized.
- The variable temp is declared as a vector but it should be of the type contained by the vector and that is not clear.
- The function should return a vector but it does not.

How can I fix the friend function error?

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