Why would a struct need a friend function?

Question

I\'m trying to build a program whose source I downloaded from the internet. When I try to compile it, I get the error message

friend declaration specifying          


        

Answer 1

It took a while until I got it but finally I found out.

Thereby, I ignore the mis-leading title (which has already been answered by user463035818 IMHO sufficiently) and concentrate on

friend declaration specifying a default argument must be the only declaration

which raised my attention.

Therefore, I don't repeat what has been said about friend and access to public members because I think this error deals with a different issue.

First I tried example of OP on coliru (with a little fix and a main() to test):

#include 

typedef int Var;
struct Lit {
    int     x;
    // Use this as a constructor:
    friend Lit mkLit(Var var, bool sign = false);
    bool operator == (Lit p) const { return x == p.x; }
    bool operator != (Lit p) const { return x != p.x; }
    bool operator <  (Lit p) const { return x < p.x;  }
};
inline  Lit  mkLit(Var var, bool sign) { Lit p; p.x = var + var + (int)sign; return p; }

int main()
{
  Lit lit2 = mkLit(123, false);
  std::cout << "lit2.x: " << lit2.x << '\n';
  return 0;
}

Output:

g++ (GCC) 8.1.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

lit2.x: 246

Live Demo on coliru

Ah, yepp. Runs fine.

Then I did the same on wandbox this time with clang HEAD 8.0.0:

Start

prog.cc:7:16: error: friend declaration specifying a default argument must be a definition
    friend Lit mkLit(Var var, bool sign = false);
               ^
prog.cc:12:14: error: friend declaration specifying a default argument must be the only declaration
inline  Lit  mkLit(Var var, bool sign) { Lit p; p.x = var + var + (int)sign; return p; }
             ^
prog.cc:7:16: note: previous declaration is here
    friend Lit mkLit(Var var, bool sign = false);
               ^
2 errors generated.

1

Finish

Live Demo on wandbox

Here we are.

So, I tried to understand what clang complains about.

Finally, I found that clang finds Lit Lit::mkLit() and this does not match Lit mkLit() defined later.

This can be fixed by a prototype for mkLit() inserted before struct Lit:

typedef int Var;

Lit mkLit(Var var, bool sign = false);

struct Lit {
    int     x;
    // Use this as a constructor:
    friend Lit mkLit(Var var, bool sign);
    bool operator == (Lit p) const { return x == p.x; }
    bool operator != (Lit p) const { return x != p.x; }
    bool operator <  (Lit p) const { return x < p.x;  }
};

Now, I get a new issue: Lit not (yet) known when proto defined for mkLit(). So, I need a forward declaration for Lit and end up with:

#include 

typedef int Var;

struct Lit;
Lit mkLit(Var var, bool sign = false);

struct Lit {
    int     x;
    // Use this as a constructor:
    friend Lit mkLit(Var var, bool sign);
    bool operator == (Lit p) const { return x == p.x; }
    bool operator != (Lit p) const { return x != p.x; }
    bool operator <  (Lit p) const { return x < p.x;  }
};
inline  Lit  mkLit(Var var, bool sign) { Lit p; p.x = var + var + (int)sign; return p; }

int main()
{
  Lit lit2 = mkLit(123, false);
  std::cout << "lit2.x: " << lit2.x << '\n';
  return 0;
}

Output:

Start

lit2.x: 246

0

Finish

Live Demo on wandbox

Problem fixed.

I must admit that I'm not sure whether g++ accepts the original version although it shouldn't or whether clang denies the original version although it shouldn't. My stomache feeling tends to the former (i.e. clang is correct)...

---

After thinking again what happens in g++, I came to the conclusion that it does the following:

1. accept the friend Lit Lit::mkLit() (which is never used)

2. defining another Lit mkLit().

To find out I "converted" struct Lit to class Lit and this brings an error in g++ as well:

#include 

typedef int Var;
class Lit {
    int     x;
    // Use this as a constructor:
    friend Lit mkLit(Var var, bool sign = false);
    bool operator == (Lit p) const { return x == p.x; }
    bool operator != (Lit p) const { return x != p.x; }
    bool operator <  (Lit p) const { return x < p.x;  }
};
inline  Lit  mkLit(Var var, bool sign) { Lit p; p.x = var + var + (int)sign; return p; }

int main()
{
  Lit lit2 = mkLit(123, false);
  std::cout << "lit2.x: " << lit2.x << '\n';
  return 0;
}

Output:

g++ (GCC) 8.1.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

main.cpp: In function 'int main()':
main.cpp:17:35: error: 'int Lit::x' is private within this context
   std::cout << "lit2.x: " << lit2.x << '\n';

Live Demo on coliru

So, the original version of OP just worked in g++ because struct Lit with public int Lit::x simply doesn't need the friend Lit::mkLit().

Now, I'm a bit puzzled. Which one is right g++ or clang? I don't know.