Indexer on generic argument

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  • christian2.schmidt@gmx.de
    #1

    Indexer on generic argument

    Hi,
    why does the first Console::Write give error "indexer needs array or
    pointer" and the second not?

    generic <typename TList, typename ItemTypewhere TList :
    IList<ItemType>
    ref class MyClass
    {
    public:
    MyClass() {}
    void Method(TList list1)
    {
    Console::Write( "{0}, ", list1[0]); // error C2109 ??
    IList<ItemType> ^ list2 = list1;
    Console::Write( "{0}, ", list2[0]); // ok
    }
    };

    Cheers,
    Christian
    Tags: None
    • Pavel Minaev
      #2
      Re: Indexer on generic argument

      On Aug 5, 9:22 pm, christian2.schm ...@gmx.de wrote:
      why does the first Console::Write give error "indexer needs array or
      pointer" and the second not?
      >
      generic <typename TList, typename ItemTypewhere TList :
      IList<ItemType>
      ref class MyClass
      {
      public:
              MyClass() {}
              void Method(TList list1)
              {
                      Console::Write( "{0}, ", list1[0]); // error C2109 ??
                      IList<ItemType> ^ list2 = list1;
                      Console::Write( "{0}, ", list2[0]); // ok
              }
      >
      };
      I can't figure out why it should be any different, but specifying the
      indexer property explicitly works:

      Console::Write( "{0}, ", list1->default[0]);

      Since there's no real reason why it shouldn't be able to figure out
      the default property when the short form is used (after all, it can do
      it for types which are not generic type parameters just fine!), it's
      probably worth opening a suggestion on VS Connect regarding this.

        Comment

        • Ben Voigt [C++ MVP]
          #3
          Re: Indexer on generic argument

          Pavel Minaev wrote:
          On Aug 5, 9:22 pm, christian2.schm ...@gmx.de wrote:
          >why does the first Console::Write give error "indexer needs array or
          >pointer" and the second not?
          >>
          >generic <typename TList, typename ItemTypewhere TList :
          >IList<ItemType >
          >ref class MyClass
          >{
          >public:
          >MyClass() {}
          >void Method(TList list1)
          >{
          >Console::Write ("{0}, ", list1[0]); // error C2109 ??
          >IList<ItemType >^ list2 = list1;
          This line shouldn't have worked -- list1 has stack semantics (tracking
          reference), list2 is a tracking handle. You need an address-of operator to
          make a handle from a reference.

          Maybe changing the formal parameter to a handle "TList^ list1" would resolve
          your problem?
          >Console::Write ("{0}, ", list2[0]); // ok
          >}
          >>
          >};
          >
          I can't figure out why it should be any different, but specifying the
          indexer property explicitly works:
          >
          Console::Write( "{0}, ", list1->default[0]);
          >
          Since there's no real reason why it shouldn't be able to figure out
          the default property when the short form is used (after all, it can do
          it for types which are not generic type parameters just fine!), it's
          probably worth opening a suggestion on VS Connect regarding this.

            Comment

            • christian2.schmidt@gmx.de
              #4
              Re: Indexer on generic argument

              On 7 Aug., 16:47, "Ben Voigt [C++ MVP]" <r...@nospam.no spamwrote:
              Pavel Minaev wrote:
              On Aug 5, 9:22 pm, christian2.schm ...@gmx.de wrote:
              why does the first Console::Write give error "indexer needs array or
              pointer" and the second not?
              >
              generic <typename TList, typename ItemTypewhere TList :
              IList<ItemType>
              ref class MyClass
              {
              public:
              MyClass() {}
              void Method(TList list1)
              {
              Console::Write( "{0}, ", list1[0]); // error C2109 ??
              IList<ItemType> ^ list2 = list1;
              >
              This line shouldn't have worked -- list1 has stack semantics (tracking
              reference), list2 is a tracking handle. You need an address-of operator to
              make a handle from a reference.
              Isn't this some kind of autocasting magic?
              Maybe changing the formal parameter to a handle "TList^ list1" would resolve
              your problem?
              No, this gives error C3229: dereferencing a generic type parameter is
              not allowed.

              Thanks,
              Christian

                Comment

                • Ben Voigt [C++ MVP]
                  #5
                  Re: Indexer on generic argument

                  christian2.schm idt@gmx.de wrote:
                  On 7 Aug., 16:47, "Ben Voigt [C++ MVP]" <r...@nospam.no spamwrote:
                  >Pavel Minaev wrote:
                  >>On Aug 5, 9:22 pm, christian2.schm ...@gmx.de wrote:
                  >>>why does the first Console::Write give error "indexer needs array
                  >>>or pointer" and the second not?
                  >>
                  >>>generic <typename TList, typename ItemTypewhere TList :
                  >>>IList<ItemTy pe>
                  >>>ref class MyClass
                  >>>{
                  >>>public:
                  >>>MyClass() {}
                  >>>void Method(TList list1)
                  >>>{
                  >>>Console::Wri te("{0}, ", list1[0]); // error C2109 ??
                  >>>IList<ItemTy pe>^ list2 = list1;
                  >>
                  >This line shouldn't have worked -- list1 has stack semantics
                  >(tracking reference), list2 is a tracking handle. You need an
                  >address-of operator to make a handle from a reference.
                  >
                  Isn't this some kind of autocasting magic?
                  >
                  >Maybe changing the formal parameter to a handle "TList^ list1" would
                  >resolve your problem?
                  >
                  No, this gives error C3229: dereferencing a generic type parameter is
                  not allowed.
                  Are you instantiating as MyClass<TListor MyClass<TList^> ?
                  >
                  Thanks,
                  Christian

                    Comment

                    • Pavel Minaev
                      #6
                      Re: Indexer on generic argument

                      On Aug 7, 6:47 pm, "Ben Voigt [C++ MVP]" <r...@nospam.no spamwrote:
                      generic <typename TList, typename ItemTypewhere TList :
                      IList<ItemType>
                      ref class MyClass
                      {
                      public:
                      MyClass() {}
                      void Method(TList list1)
                      {
                      Console::Write( "{0}, ", list1[0]); // error C2109 ??
                      IList<ItemType> ^ list2 = list1;
                      >
                      This line shouldn't have worked -- list1 has stack semantics (tracking
                      reference), list2 is a tracking handle.  You need an address-of operator to
                      make a handle from a reference.
                      It works because it's a generic declaration, not a template
                      declaration. Generic type parameters can only be instantiated with
                      handles for ref types - so, for a specific instantiation, TList would
                      be something like List<T>^, and not just plain List<T>.

                        Comment

                        • Ben Voigt [C++ MVP]
                          #7
                          Re: Indexer on generic argument

                          Pavel Minaev wrote:
                          On Aug 7, 6:47 pm, "Ben Voigt [C++ MVP]" <r...@nospam.no spamwrote:
                          >>>generic <typename TList, typename ItemTypewhere TList :
                          >>>IList<ItemTy pe>
                          >>>ref class MyClass
                          >>>{
                          >>>public:
                          >>>MyClass() {}
                          >>>void Method(TList list1)
                          >>>{
                          >>>Console::Wri te("{0}, ", list1[0]); // error C2109 ??
                          >>>IList<ItemTy pe>^ list2 = list1;
                          >>
                          >This line shouldn't have worked -- list1 has stack semantics
                          >(tracking reference), list2 is a tracking handle. You need an
                          >address-of operator to make a handle from a reference.
                          >
                          It works because it's a generic declaration, not a template
                          declaration. Generic type parameters can only be instantiated with
                          handles for ref types - so, for a specific instantiation, TList would
                          be something like List<T>^, and not just plain List<T>.
                          Hmmm. Then the constraint line makes little sense. A tracking handle
                          doesn't implement an interface, the type of the referenced object does. I
                          realize this is connected to the .NET behavior of following different rules
                          when instantiating generics using ref types vs value types, and there
                          probably is no perfectly consistent syntax which expresses the behavior.


                            Comment

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