template parameters as friends

[John Harrison]

Why isn't this legal code and is there any other way to achieve the
effect I want (i.e. make a template parameter the friend of a template
class).
template <class X> class I
{
friend class X;
private:
I() {}
};

class C
{
public:
I<C> f() { return I<C>(); }
};

int main()
{
C c;
c.f();
}

john

[Gianni Mariani]

John Harrison wrote:

Why isn't this legal code and is there any other way to achieve the
effect I want (i.e. make a template parameter the friend of a template
class).
template <class X> class I
{
friend class X;
private:
I() {}
};

class C
{
public:
I<C> f() { return I<C>(); }
};

int main()
{
C c;
c.f();
}

This does not do quite what I think you want it to do but it's what I've
used in some cases.

template <class X> class I;

template <class D>
class I_Accessor
{
protected:
I<D> f() { return I<D>(); }
};

template <class X> class I
{
friend class I_Accessor<X>;
private:
I() {}
};

class C
: I_Accessor<C>
{
public:
I<C> f() { return I_Accessor<C>::f(); }
};

int main()
{
C c;
c.f();
}

[Ian]

John Harrison wrote:

Why isn't this legal code and is there any other way to achieve the
effect I want (i.e. make a template parameter the friend of a template
class).
template <class X> class I
{
friend class X;
private:
I() {}
};

class C
{
public:
I<C> f() { return I<C>(); }
};

int main()
{
C c;
c.f();
}

It should be, shouldn't it? It compiles fine for me.

Could it depend on when the compiler parses I? If this is done when
instantiating C, X (type C) is known. If not, X could be anything, for
example

class D
{
public:
I<int> f() { return I<int>(); }
};

Will fail as an int can't be a friend.

Ian

[John Harrison]

Gianni Mariani wrote:

John Harrison wrote:

Why isn't this legal code and is there any other way to achieve the
effect I want (i.e. make a template parameter the friend of a template
class).
template <class X> class I
{
friend class X;
private:
I() {}
};

class C
{
public:
I<C> f() { return I<C>(); }
};

int main()
{
C c;
c.f();
}

This does not do quite what I think you want it to do but it's what I've
used in some cases.

template <class X> class I;

template <class D>
class I_Accessor
{
protected:
I<D> f() { return I<D>(); }
};

template <class X> class I
{
friend class I_Accessor<X>;
private:
I() {}
};

class C
: I_Accessor<C>
{
public:
I<C> f() { return I_Accessor<C>::f(); }
};

int main()
{
C c;
c.f();
}

That's clever, I can make that work, thanks.

john

[John Harrison]

Ian wrote:

John Harrison wrote:

Why isn't this legal code and is there any other way to achieve the
effect I want (i.e. make a template parameter the friend of a template
class).
template <class X> class I
{
friend class X;
private:
I() {}
};

class C
{
public:
I<C> f() { return I<C>(); }
};

int main()
{
C c;
c.f();
}
It should be, shouldn't it? It compiles fine for me.

Which compiler? I tried VC++ 7.1, which just ignored the friend
declaration, so I got an access error, and g++ 3.4.4 which complained
about the friend declaration.

Could it depend on when the compiler parses I? If this is done when
instantiating C, X (type C) is known. If not, X could be anything, for
example

class D
{
public:
I<int> f() { return I<int>(); }
};

Will fail as an int can't be a friend.

Ian

The compiler has to parse a template twice. Once when it is first read
and once when it is instantiated. But exactly what should be done when
is not something I'm very familar with.

But now I'm thinking my code is probably just a syntax error. X isn't a
class name, its a template parameter so 'friend class X' is just wrong.
I guess I should try looking at the C++ standard, but asking here is
much easier.

john

[Ian]

John Harrison wrote:

It should be, shouldn't it? It compiles fine for me.

Which compiler? I tried VC++ 7.1, which just ignored the friend
declaration, so I got an access error, and g++ 3.4.4 which complained
about the friend declaration.

Sun CC.


The compiler has to parse a template twice. Once when it is first read
and once when it is instantiated. But exactly what should be done when
is not something I'm very familar with.
It depends how much checking it does when it is first read. I know that
gcc does more than the Sun compiler does, I think it parses all member
functions, where as the Sun compiler only does this when they are
instantiated.
But now I'm thinking my code is probably just a syntax error. X isn't a
class name, its a template parameter so 'friend class X' is just wrong.
I guess I should try looking at the C++ standard, but asking here is
much easier.

This could be a good one for comp.std.c++.

Ian