Re: gcc-4.4.3 failure in rejecting instantiation?

Paul Bibbings <>
Sun, 20 Jun 2010 11:04:00 +0100
"Alf P. Steinbach /Usenet" <> writes:

* Paul Bibbings, on 20.06.2010 00:15:

I've been working on some code (not mine) that relies on the following
motif leading to a compilation failure, with sensible output, at line
17 (below). The trouble is, with gcc-4.4.3, it *doesn't* fail:

    template<typename T>
    struct Error { };

    template<typename T>
    struct A
       static const int i = Error<T>::TempArgMustBeInt; // line: 7

    struct A<int>
    { };

    int main()
       A<int> A_int;
       A<char> A_char; // line: 17

I'm thinking that it probably should (or, I have no immediate
understanding of why it wouldn't); whoever wrote the original code
(probably some while ago) thought it should; and, Comeau seems to

    Comeau C/C++ (Oct 6 2008 11:28:09) for
    Copyright 1988-2008 Comeau Computing. All rights reserved.
    MODE:strict errors C++ noC++0x_extensions

    "ComeauTest.c", line 7: error: class "Error<char>" has no member
      static const int i = Error<T>::TempArgMustBeInt;
           detected during instantiation of class "A<T> [with T=char]" at
           line 17
    // ...

    1 error detected in the compilation of "ComeauTest.c".

What does anyone else think?

Sounds like it incorrectly optimizes the "i" away (perhaps it
optimizes A_char away) before checking whether it exists.

I'd try a typedef instead.

I'm assuming that there is some good reason for the restriction (e.g.,
A used as a template template parameter); otherwise just remove the
templating... :-)

The code that I was looking at, and which used the above motif, is from
a paper by McNamara and Smaragdakis on "Static interfaces in C++" -
(I haven't found the date for this paper, but the latest reference is
Alexandrescu, 2000). The example illustrates static dispatch, along the
lines of:

   enum {
      SET_HASH, // implement using a hash table
      SET_BST, // implement using a binary search tree
      SET_LL, // implement using a linked list
      SET_NONE // no appropriate implementation

   template<class T>
   struct SetDispatch {
      static const bool Hash =
           StaticIsA<T, Hashtable>::valid;
      static const bool LtC =
           StaticIsA<T, LessThanComparable<T> >::valid;
      static const bool EqT =
           StaticIsA<T, EqualityComparable<T> >::valid;
      static const int which = Hash ? SET_HASH
                                    : LtC ? SET_BST
                                          : EqT ? SET_LL
                                                : SET_NONE;

      class T,
      int which = SetDispatch<T>::which
   struct Set;

   template<class T>
   struct Set<T, SET_NONE> {
      static const int x = Error<T>::

   template<class T>
   struct Set<T, SET_LL> { /* ... */ };

   template<class T>
   struct Set<T, SET_BST> { /* ... */ };

   template<class T>
   struct Set<T, SET_HASH> { /* ... */ };

By default the implementation does not accept fundamental types as
LessThanComparable, EqualityComparable, etc., and this is achieved with
appropraite specializations of a local traits type:

   struct Valid {
     static const bool valid = true;
     ~Valid() { }

   struct LessThanComparable<int>::Traits<int> : public Valid { };

Without any such specializations, instantiations such as:

   Set<char> char_set;

should fail with a nice (4-line) error output finishing with:

   error: `Set_only_works_on_Hashtables_or_LTCs_or_ECs' is not a member
      of `Error<char>'

with the advantage that the failure:

   1. signals what is `missing' for the particular type as template
   2. signals the type that failed as template argument (the reason for
      parameterizing Error); and
   3. gives concise error output that doesn't go deep into the bowels of
      the implementation.

However, *without* the traits specialization for char, the
implementation *allows*:

   Set<char> char_set;

To get the code working I can replace:

   template<class T>
   struct Set<T, SET_NONE> {
      static const int x = Error<T>::


   template<class T>
   struct Set<T, SET_NONE> {
      Set() {
         const int x = Error<T>::

but the effect is not necessarily the same in all cases (or, at least, I
would have to think it through to decide whether it is).


Paul Bibbings

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