Thread: Circular template reference no longer compiles (g++)

  1. #1
    Registered User
    Join Date
    Oct 2008

    Circular template reference no longer compiles (g++)

    The following compiles on g++ version 3.2.3 on Red Hat and HP-UX, but fails on 3.4.4 (cygming special). Since that appears to be newer I'm assuming it's my code that's wrong.

    I've cut it down to this example

    #include <vector>
    #include <iterator>
    #include <memory>
    template <typename T1> class Foo;
    template<typename T1>
    class FooItr
       typedef std::auto_ptr< FooItr<T1> > Ptr;
       FooItr<T1>(Foo<T1>* pFoo) :
          m_itr(pFoo->m_list.begin()) { }
       virtual ~FooItr() { }
       typename Foo<T1>::FooList::const_iterator m_itr;
    template<typename T1>
    class Foo
       virtual typename FooItr<T1>::Ptr getItr()
           return FooItr<T1>::Ptr(new FooItr<T1>(this));
       friend class FooItr<T1>;
       typedef std::vector<T1> FooList;
       FooList m_list;
    int main()
       Foo<int>* p = new Foo<int>();
       delete p;
       return 0;
    The error I get is
    t.C: In instantiation of `FooItr<int>':
    t.C:29:   instantiated from `Foo<int>'
    t.C:42:   instantiated from here
    t.C:20: error: no type named `FooList' in `class Foo<int>'
    If I remove the use of FooItr::Ptr from Foo it compiles, so I assume it's circular dependency of FooItr using Foo::FooList and Foo using FooItr::Ptr that's the problem.

    EDIT: Sorry should have been more clear. I realise I can solve this with a "typedef template" (not sure of the proper term) i.e. FooItrPtr<T1>::Type instead of FooItr<T1>::Ptr, but I'm interested in what I'm doing wrong.


    Last edited by stumcd; 09-23-2009 at 04:54 PM. Reason: More info

  2. #2
    Registered User
    Join Date
    Sep 2004
    Yeah, the problem is that FooItr is using Foo::FooList, but that type has not been declared. It is declared in the Foo class, but FooItr has no visibility of this.

    Usually, the best way to do this type of design is to declare the iterator as an inner class of Foo.
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