Friend function not declared in this scope error

Question

Hi I am trying to understand the scope of friend functions and I get a "not declared in scope" error. Here is my code:

//node.h
class Node{

public:

  int id;
  int a;
  int b;

  friend int add(int,int);

  void itsMyLife(int);
  Node();
};

//node.cpp
Node::Node(){
  a=0;
  b=0;
  id=1;
}

void Node::itsMyLife(int x){

  cout<<"In object "<<id<<" add gives "<<add(x,a)<<endl;

}

//routing.cpp
#include "node.h"

int add(int x, int y){

     return x+y;
}

//main.cpp
#include "node.h"

int main(){

return 0;
}

I get the error "add not declared in this scope" in node.cpp. Why do I get this error when I have declared the function in the class scope? Any help will be appreciated. Thanks

AFAIK friend declaration is also a forward declaration. If I'm right there shouldn't be an error. Maybe it sees it as in the class scope (though that doesn't make much sense for a friend function), try adding :: before add in the friend declaration to declare it is in the global scope and not class scope. Maybe it will help. — selalerer
Actually the problem disappears if I define the add function in node.cpp. But I want to define it in a separate file. How can I do that? — user2105632

hetepeperfan hetepeperfan · Accepted Answer · 2013-05-27T20:20:28

Inside your node class you declare a friend function int add (int, int). However, currently the compiler hasn't encountered the function yet and therefore it is unknown.

You could make a separate header and source file for your add function. Then in node.h include you new header. Because in the file where you declare Node the function add is not known currently.

So you might make a add.h and a add.cpp file for example and include add.h before declaring Node. Don't forget to compile add.cpp as well.

Friend function not declared in this scope error

3 Answers