Hi!

I'm having problems with defining recursive types in C++. While trying
to solve (or rather to understand) them I encountered with the text
(ARM, section 7.1.3, page 107):

"A 'typedef-name' that names a class is a 'class-name' (9.1). The synonym
may not be used after a 'class', 'struct' or 'union' prefix ... etc.
For example:

     struct S {
   S();
   ~S();
     };

     typedef struct S T;

     S a = T();      // ok
     struct T * p;   // error

"

I wonder what is the motivation behind this, I mean the prohibiting
to use the synonym after 'struct', 'class' or 'union'?
Could someone who knows explain this?

In GNU g++ the feature is implemented inconsistently. The code below
compiles okay without an error

struct S {};
typedef struct S T;
struct T * p;

while the following fails to compile

struct T * p;
struct S {};
typedef struct S T;

t.cc:3: conflicting types for `struct S'
t.cc:1: previous declaration as `struct T'

This same error occured when I tried to define a type
which refers to a list of instances of itself and class template List
was used (Is it a good idea to have class template for generic list?):

class Filter {
       ...
       class FilterList   *p;
       ...
};

typedef List<Filter>   FilterList;

This forces me to write something like

class FilterList : public List<Filter> {
 .....
  // ctors and dtors are not inherited   :-(

};


Maybe there is some sort of ambiguity which will arise if one was to
allowed to write 'class' before typedef-ed names? Or maybe this all
is just g++ bugs? What do you think?

Thanks,

Sergey Ostashenko
KIAE, Moscow, Russia
email: osv@kiae.su

osv@newcom.kiae.su (Sergey Ostashenko ) wrote:
To answer a part of your question:
><snip>
>
> In GNU g++ the feature is implemented inconsistently. The code below
> compiles okay without an error
>
> struct S {};
> typedef struct S T;
> struct T * p;
>
> while the following fails to compile
>
> struct T * p;
> struct S {};
> typedef struct S T;
>
> t.cc:3: conflicting types for `struct S'
> t.cc:1: previous declaration as `struct T'
>
This should error always as the name T is already used and re-declaring
as a typedef should fail as it is in the same namespace.

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