Pure virtual member and namespaces.

Pure virtual member and namespaces.

[Savas Parastatidis]

Dear all,

I think there is a problem while compiling the following example using the
latest snapshot of egcs:

#include <iostream>

using namespace std;

namespace FooNamespace
{
   class Foo
   {
   public:
      Foo() {}
      virtual void f() = NULL;
   };
}

class MyClass : public FooNamespace::Foo
{
public:
   MyClass() {};
   void f() { cout << "Boom" << endl; }
};

void main()
{
   MyClass* x = new MyClass;
   FooNamespace::Foo* foo = x;

   foo->f();
}

The message I get is:
/tmp/cca11232.s: Assembler messages:
/tmp/cca11232.s:257: Fatal error: Symbol __tfQ212FooNamespace3Foo already
defined.


Any ideas? Am I doing something wrong? Ah, the problem I talked about in a
previous message persists. I do not send this to comp.std.c++ as it is my
understanding that the following example complies with the standard. Please,
comment...

#include <map>

namespace X
{
   class foo1
   {
   public:
      int bar;
      friend bool operator<(const foo1&, const foo1&);
   };
}

bool operator<(const X::foo1& rhs, const X::foo1& lhs)
{
  cout << "using this" << endl;
  return (rhs.bar < lhs.bar);
}

namespace X
{
   class foo2
   {
   public:
      foo2()
      {
         MyMap m;
         foo1  f1, f2;
         m.insert(MyMap::value_type(f1, 1));
         m.insert(MyMap::value_type(f2, 1));
      }

   private:
      typedef std::map<foo1, int> MyMap;
   };
}

int main()
{
   X::foo2 f2;
}

Thank you very much,
--
Savas Parastatidis, PhD. Student, Claremont Bridge 353
Dept. of Computing Science, University of Newcastle, NE1 7RU, UK
e-mail: Savas.Parastatidis@ncl.ac.uk, tel. +44 191 222 8789
URL: http://www.ncl.ac.uk/~n5831124 , fax +44 191 222 8232

Re: Pure virtual member and namespaces.

[Martin von Loewis]

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> I think there is a problem while compiling the following example using the
> latest snapshot of egcs:

Dear Savas,
Thanks for your report.

> The message I get is:
> /tmp/cca11232.s: Assembler messages:
> /tmp/cca11232.s:257: Fatal error: Symbol __tfQ212FooNamespace3Foo already
> defined.

This does not happen with my current sources, so it should be fixed
when I submit them.

> #include <map>
> 
> namespace X
> {
>    class foo1
>    {
>    public:
>       int bar;
>       friend bool operator<(const foo1&, const foo1&);
>    };
> }
> 
> bool operator<(const X::foo1& rhs, const X::foo1& lhs)
> {
>   cout << "using this" << endl;
>   return (rhs.bar < lhs.bar);
> }

I believe the program you present is not a complete C++ program.
In particular, the operator you declare inside X::foo is in fact
X::operator<(const X::foo&, const X::foo&). [namespace.memdef] says

  If a friend declaration in a non­local class first declares a
  class or function 83) the friend class or function is a member of
  the innermost enclosing namespace.

This is not a problem per se. However, you call it later. The call
will find both ::operator< and X::operator<, and determine the call to
be ambiguous. Instead, you need to use a qualifier in the friend
declaration as allowed in [decl.meaning]. This would give

namespace X
{
   class foo1
   {
   public:
      int bar;
      friend bool ::operator<(const foo1&, const foo1&);
   };
}

Unfortunately, scoping a friend with global scope is currently not
supported by g++. Your best bet will be define the operator inside
X.

Regards,
Martin