Two-stage name lookup problem

Two-stage name lookup problem

[Jakub Pawlewicz]

I have problem with templates and I need a solution. I want to use
inner class B with a member of outer class A type. Then I wand class A
to use template class B in function f. Here is code I want to change
in order to compile with gcc 3.4. It compiles fine with gcc 3.3.

struct A {
 int x;
 A(int x) : x(x) { }

 template <int X>
 struct B {
   A a;
   B() : a(X) { }
   void f() { }
 };

 template <int X>
 void f()
 {
   B<X> b;
   b.f();
 }
};

int main()
{
 A a(0);
 a.f<0>();
}

Compilation fails with a message
a.cpp:7: error: field `a' has incomplete type
a.cpp: In constructor `A::B<X>::B()':
a.cpp:8: error: class `A::B<X>' does not have any field named `a'

Is there any workaround?

Thanks in advance

Jakub Pawlewicz

Re: Two-stage name lookup problem

[John Love-Jensen]

Hi Jakub,

Your problem is not with templates.

Here's your problem in a nutshell...

struct A
{
    int x;

    struct B
    {
        A a;
    };
};


You cannot use A within B, because at that point A is incomplete.

You can pull B out of A.  If you want B to be in the A namespace, change
'struct A' to 'namespace A', and make 'struct A_impl', fully declared, and
then use that within 'struct B'.

HTH,
--Eljay

Re: Two-stage name lookup problem

[Jakub Pawlewicz]

> Your problem is not with templates.
>
> Here's your problem in a nutshell...
>
> struct A
> {
>     int x;
>
>     struct B
>     {
>         A a;
>     };
> };
>
>
> You cannot use A within B, because at that point A is incomplete.
>
> You can pull B out of A.  If you want B to be in the A namespace, change
> 'struct A' to 'namespace A', and make 'struct A_impl', fully declared, and
> then use that within 'struct B'.

But what about function f(), the member of struct A? I want to declare
variable with type B in the body of the function:

struct A {
    ...
    void f()
    {
        B b;
        ...
    }
};

I am aware that kind of code may be ill formed, but when using
templates, there should be something like deferred instantiation.
Struct B is a template and also function A::f() is a template. Struct
B is needed only when function A::f() is used. Struct B should be able
to construct member of type A and Struct B do not use function A::f().
I want all functions be inline. Any solutions?

Re: Two-stage name lookup problem

[corey taylor]

Jakub,

  The declaration of A a; in struct B because it requires an object
size.  Using a pointer to an incomplete type is allowed.  You could
create a new A in the B constructor and assign it to a pointer.

  The other errors center around this.  The question about using B b
has nothing to do with the error above.

Corey

On 10/26/05, Jakub Pawlewicz <jakub.pawlewicz@gmail.com> wrote:
> > Your problem is not with templates.
> >
> > Here's your problem in a nutshell...
> >
> > struct A
> > {
> >     int x;
> >
> >     struct B
> >     {
> >         A a;
> >     };
> > };
> >
> >
> > You cannot use A within B, because at that point A is incomplete.
> >
> > You can pull B out of A.  If you want B to be in the A namespace, change
> > 'struct A' to 'namespace A', and make 'struct A_impl', fully declared, and
> > then use that within 'struct B'.
>
> But what about function f(), the member of struct A? I want to declare
> variable with type B in the body of the function:
>
> struct A {
>     ...
>     void f()
>     {
>         B b;
>         ...
>     }
> };
>
> I am aware that kind of code may be ill formed, but when using
> templates, there should be something like deferred instantiation.
> Struct B is a template and also function A::f() is a template. Struct
> B is needed only when function A::f() is used. Struct B should be able
> to construct member of type A and Struct B do not use function A::f().
> I want all functions be inline. Any solutions?
>

Re: Two-stage name lookup problem

[Jakub Pawlewicz]

I'm confused. I do not want use any pointers and memory allocation. I
wrote a program which is very fast because I was using many inline
templates and no pointers. The program worked find unless I had to
switch to gcc 3.4. In my first post it is important that there are
templates and that I am using non-pointer variable declaration of type
B in function A::f(). Please go back to the original problem.

I don't know template terminology, but I try my best to describe what
the problem is. So please be gentle if I make something wrong.

The declaration of A a; in struct B should be allowed if we are able
to postpone instantiation of struct B, because it is a template
struct. The function A::f() is template also, so maybe we could better
postpone the instantiation of the function body. I don't know how can
I do such things. Before two-stage names lookup there was no problem
with such hacks and now I need a workaround..

2005/10/26, corey taylor <corey.taylor@gmail.com>:
> Jakub,
>
>   The declaration of A a; in struct B because it requires an object
> size.  Using a pointer to an incomplete type is allowed.  You could
> create a new A in the B constructor and assign it to a pointer.
>
>   The other errors center around this.  The question about using B b
> has nothing to do with the error above.
>
> Corey
>
> On 10/26/05, Jakub Pawlewicz <jakub.pawlewicz@gmail.com> wrote:
> > > Your problem is not with templates.
> > >
> > > Here's your problem in a nutshell...
> > >
> > > struct A
> > > {
> > >     int x;
> > >
> > >     struct B
> > >     {
> > >         A a;
> > >     };
> > > };
> > >
> > >
> > > You cannot use A within B, because at that point A is incomplete.
> > >
> > > You can pull B out of A.  If you want B to be in the A namespace, change
> > > 'struct A' to 'namespace A', and make 'struct A_impl', fully declared, and
> > > then use that within 'struct B'.
> >
> > But what about function f(), the member of struct A? I want to declare
> > variable with type B in the body of the function:
> >
> > struct A {
> >     ...
> >     void f()
> >     {
> >         B b;
> >         ...
> >     }
> > };
> >
> > I am aware that kind of code may be ill formed, but when using
> > templates, there should be something like deferred instantiation.
> > Struct B is a template and also function A::f() is a template. Struct
> > B is needed only when function A::f() is used. Struct B should be able
> > to construct member of type A and Struct B do not use function A::f().
> > I want all functions be inline. Any solutions?
> >
>

Re: Two-stage name lookup problem

[corey taylor]

Jakub,

  I understand your dilemna.  The best thing I can do for you right
now is to show the C++ specification paragraph that rules in the
compiler are generated from:

Knowing which names are type names allows the syntax of every template
definition to be checked. No
diagnostic shall be issued for a template definition for which a valid
specialization can be generated. If no
valid specialization can be generated for a template definition, and
that template is not instantiated, the template
definition is ill-formed, no diagnostic required. If a type used in a
non-dependent name is incomplete
at the point at which a template is defined but is complete at the
point at which an instantiation is done, and
if the completeness of that type affects whether or not the program is
well-formed or affects the semantics
of the program, the program is ill-formed; no diagnostic is required.
[Note: if a template is instantiated,
errors will be diagnosed according to the other rules in this
Standard. Exactly when these errors are diagnosed
is a quality of implementation issue. ] [Example:
int j;
template<class T> class X {
// ...
void f(T t, int i, char* p)
{
t = i; // diagnosed if X::f is instantiated
// and the assignment to t is an error
p = i; // may be diagnosed even if X::f is
// not instantiated
p = j; // may be diagnosed even if X::f is
// not instantiated
}
void g(T t) {
+; //may be diagnosed even if X::g is
// not instantiated
}
};
—end example]

corey

On 10/26/05, Jakub Pawlewicz <jakub.pawlewicz@gmail.com> wrote:
> I'm confused. I do not want use any pointers and memory allocation. I
> wrote a program which is very fast because I was using many inline
> templates and no pointers. The program worked find unless I had to
> switch to gcc 3.4. In my first post it is important that there are
> templates and that I am using non-pointer variable declaration of type
> B in function A::f(). Please go back to the original problem.
>
> I don't know template terminology, but I try my best to describe what
> the problem is. So please be gentle if I make something wrong.
>
> The declaration of A a; in struct B should be allowed if we are able
> to postpone instantiation of struct B, because it is a template
> struct. The function A::f() is template also, so maybe we could better
> postpone the instantiation of the function body. I don't know how can
> I do such things. Before two-stage names lookup there was no problem
> with such hacks and now I need a workaround..
>
> 2005/10/26, corey taylor <corey.taylor@gmail.com>:
> > Jakub,
> >
> >   The declaration of A a; in struct B because it requires an object
> > size.  Using a pointer to an incomplete type is allowed.  You could
> > create a new A in the B constructor and assign it to a pointer.
> >
> >   The other errors center around this.  The question about using B b
> > has nothing to do with the error above.
> >
> > Corey
> >
> > On 10/26/05, Jakub Pawlewicz <jakub.pawlewicz@gmail.com> wrote:
> > > > Your problem is not with templates.
> > > >
> > > > Here's your problem in a nutshell...
> > > >
> > > > struct A
> > > > {
> > > >     int x;
> > > >
> > > >     struct B
> > > >     {
> > > >         A a;
> > > >     };
> > > > };
> > > >
> > > >
> > > > You cannot use A within B, because at that point A is incomplete.
> > > >
> > > > You can pull B out of A.  If you want B to be in the A namespace, change
> > > > 'struct A' to 'namespace A', and make 'struct A_impl', fully declared, and
> > > > then use that within 'struct B'.
> > >
> > > But what about function f(), the member of struct A? I want to declare
> > > variable with type B in the body of the function:
> > >
> > > struct A {
> > >     ...
> > >     void f()
> > >     {
> > >         B b;
> > >         ...
> > >     }
> > > };
> > >
> > > I am aware that kind of code may be ill formed, but when using
> > > templates, there should be something like deferred instantiation.
> > > Struct B is a template and also function A::f() is a template. Struct
> > > B is needed only when function A::f() is used. Struct B should be able
> > > to construct member of type A and Struct B do not use function A::f().
> > > I want all functions be inline. Any solutions?
> > >
> >
>

Re: Two-stage name lookup problem

[Jakub Pawlewicz]

I found a workaround. It looks ugly, but it works. I used dummy
template arguments to overcome instantiation. I hope somebody will
find a nicer solution. Here is a hacked code:

template <typename Dummy> struct A_impl;

typedef A_impl<void> A;

template <typename Dummy>
struct A_impl {
 int x;
 A_impl(int x) : x(x) { }

  template <int X, typename Dummy2>
  struct B {
    A_impl<Dummy2> a;
    B() : a(X) { }
    void f() { }
  };

  template <int X>
   void f()
   {
     B<X, Dummy> b;
     b.f();
   }
};

int main()
{
  A a(0);
  a.f<0>();
}


2005/10/26, Jakub Pawlewicz <jakub.pawlewicz@gmail.com>:
> I have problem with templates and I need a solution. I want to use
> inner class B with a member of outer class A type. Then I wand class A
> to use template class B in function f. Here is code I want to change
> in order to compile with gcc 3.4. It compiles fine with gcc 3.3.
>
> struct A {
>  int x;
>  A(int x) : x(x) { }
>
>  template <int X>
>  struct B {
>    A a;
>    B() : a(X) { }
>    void f() { }
>  };
>
>  template <int X>
>  void f()
>  {
>    B<X> b;
>    b.f();
>  }
> };
>
> int main()
> {
>  A a(0);
>  a.f<0>();
> }
>
> Compilation fails with a message
> a.cpp:7: error: field `a' has incomplete type
> a.cpp: In constructor `A::B<X>::B()':
> a.cpp:8: error: class `A::B<X>' does not have any field named `a'
>
> Is there any workaround?
>
> Thanks in advance
>
> Jakub Pawlewicz
>

Re: Two-stage name lookup problem

[Nathan Sidwell]

John Love-Jensen wrote:
> Hi Jakub,
> 
> Your problem is not with templates.
> 
> Here's your problem in a nutshell...
> 
> struct A
> {
>     int x;
> 
>     struct B
>     {
>         A a;
>     };
> };

write this as
struct A { int x; struct B;};
struct A::B { A a;};

nathan

-- 
Nathan Sidwell    ::   http://www.codesourcery.com   ::     CodeSourcery LLC
nathan@codesourcery.com    ::     http://www.planetfall.pwp.blueyonder.co.uk

Re: Two-stage name lookup problem

[Brian T. Brunner]

The a of type A contains a B, which includes another a of type A which in turn includes a B...
I can't imagine this not being an infinite recursion at the data object creation level.

If B contains a pointer to an object of type A, then we should be OK.

Brian Brunner
brian.t.brunner@gai-tronics.com
(610)796-5838

>>> John Love-Jensen <eljay@adobe.com> 10/26/05 08:47AM >>>
Hi Jakub,

Your problem is not with templates.

Here's your problem in a nutshell...

struct A
{
    int x;

    struct B
    {
        A a;
    };
};


You cannot use A within B, because at that point A is incomplete.

You can pull B out of A.  If you want B to be in the A namespace, change
'struct A' to 'namespace A', and make 'struct A_impl', fully declared, and
then use that within 'struct B'.

HTH,
--Eljay


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