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To: Matt Austern <austern@apple.com>
Cc: gcc@gcc.gnu.org
Subject: Re: When to emit the vtable for a class template
References: <E2FAA71C-DC94-11D6-80E4-00039390D9E0@apple.com>
From: Gabriel Dos Reis <gdr@integrable-solutions.net>
In-Reply-To: Matt Austern's message of "Thu, 10 Oct 2002 14:11:47 -0700"
Organization: Integrable Solutions
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Date: Thu, 10 Oct 2002 15:24:00 -0000
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X-SW-Source: 2002-10/txt/msg00566.txt.bz2

Matt Austern <austern@apple.com> writes:

[...]

| Consider the following translation unit:
| 
|      template<class T> struct X {
|        virtual void foo();
|        virtual ~X();
|      };
| 
|      template <class T> void X<T>::foo() { }
| 
| 
|      X<int>* xyzzy(X<int>* p) {
|        return p + 1;
|      }
| 
| Clearly X<int> must be instantiated.

Hmm, if "instantiated" is used as the C++ standard uses it, then I'm
not clear why X<int> ought to be instantiated.  AFAIK, only the
template-id 'X<int>' needs to be "instantiated". 

Consider

   struct A;

   A* foo(X* p) { return p + 1; }

That is a valid construct that does not need completeness of type A and
the situation is not different from the X<int> case.

| However, compiling this
| with -S and looking at the assembly code, I'm not seeing
| X<int>'s vtable.

I fear the vtable for X<int> does not need to be emitted.

| So, my question stands: under what circumstances does the
| compiler emit a vtable for a polymorphic class template? 

I'm however unable to give an informed answer to this question.

| Is
| the answer the same as what the ABI specification says it
| should be?

I would expect so.

-- Gaby