Friend functions and VC++

[Wynd]

I put together this code after looking at some other code here:

class Thing
{
private:
    int x;
public:
    Thing();
    Thing(int);
    void set(int);
    friend Thing operator +(const Thing&, const Thing&);
    friend ostream operator <<(const ostream&, const Thing&);
};

I get this error on the bold line:

c:\cpp\oper\thing.h(12) : fatal error C1001: INTERNAL COMPILER ERROR
(compiler file 'msc1.cpp', line 1786)
Please choose the Technical Support command on the Visual C++
Help menu, or open the Technical Support help file for more information
Error executing cl.exe.

What does that mean?

[kedaman]

It means that you are ****ed, because MSVC6 sucks.
You have to copy in blocks of code until you can track down the error, and then post that line here (along with the rest).

[nabeels786]

what does friend do? i saw it somewhere too, but didn't understand what it was for/meant

[kedaman]

friend is a anti OOP thing it allows access of a class' private members to a global function

[parksie]

Originally posted by kedaman
friend is a anti OOP thing it allows access of a class' private members to a global function

Nope, it allows access to private or protected members from any named item.

Frequently used for things like linked lists:

class list {
    class link {
        friend class list;
    };

public:
    void add(...);
};

[CornedBee]

Could it be that you shouldn't specify the return type? The name and the parameters are enough to identify the function, so I think you don't need to specify type.

friend certainly has it's uses. But you can usually avoid it. e.g. a global + operator can be realized by giving the class a += operator.

class MyBadClass
{
  int m_i;
  friend operator +(const MyBadClass&, int);
  friend operator +(int, const MyBadClass&);
};

MyBadClass operator +(int i, const MyBadClass& rbc)
{
  MyBadClass tmp;
  tmp = rbc;
  tmp.m_i += i;
  return tmp;
}

class MyGoodClass
{
public:
  MyGoodClass& operator +=(int i) { m_i += i; return *this;};
};

MyGoodClass operator +(int i, const MyGoodClass& rgc)
{
  MyGoodClass tmp = rgc;
  tmp += i;
  return tmp;
}

That's both the bad and the good way to do it.

[Wynd]

Originally posted by CornedBee

class MyGoodClass
{
public:
  MyGoodClass& operator +=(int i) { m_i += i; return *this;};
};

Kinds off the subject, but can you explain this line to me? Why do you have an ampersand in the return type, and why return *this? (Sorry, I'm new at this and can't find a good explanation anywhere.)

[kedaman]

Originally posted by parksie
Nope, it allows access to private or protected members from any named item.

Frequently used for things like linked lists:

class list {
    class link {
        friend class list;
    };

public:
    void add(...);
};

Come on parksie! I know this stuff!

1. The object in OOP can do two things a) encapsulates contained data and provide access trough accessors and methods, b) delegate other objects trough its accessors.
2. Everything is an Object, there should not be any global functions, in other words, all functions are bound to their first parameter, their object.

Hence these paradoxal facilities:
1. Friend functions and classes
2. Protected inheritance
3. Functors

OOP can't work alone, its too obvious, functional style does.

[kedaman]

Originally posted by Wynd
Kinds off the subject, but can you explain this line to me? Why do you have an ampersand in the return type, and why return *this? (Sorry, I'm new at this and can't find a good explanation anywhere.)

the ampersand before the type means that its a reference to the type, returning a& for a+=b means that you can still use a for further reference, ex: (a+=b)*=c

*this is dereferenced this, and this points to the object of which function you have called

[CornedBee]

Since there is no int class that you can modify, global + operators are necessary if you want to add a predefined type to your own class and want this type as first parameter.
Like:

int i = 4;
MC mc1(2, 3), mc2;
mc2 = mc1 + i;   // this can be handled inside the class
mc2 = i + mc2;  // this requires a global operator

[kedaman]

Yep, but that's faaar from OOP

[CornedBee]

Still I'm happy when I can do "predefined type" + "object" and not only "object" + "predefined type".
Didn't you complain just some days ago that Java is so fixed on objects?

[kedaman]

I did, and I didn't insist OOP was good in any ways
You'll like how SQ handle things