Forward declaration issue on gcc 3.4.3

["dahui dahui" <dahui06 at hotmail dot com>]

Hi, 

I encountered an issue when building our program. The compilation will fail 
when using gcc 3.4.3. However, 
the same program can be compiled successfully with gcc 3.2.3. 

I used the following example to reproduce the issue: 
===== 
a.h 
===== 
#ifndef A_H_ 
#define A_H_ 
class A 
{ 
public: 
       A() { } 
       ~A() { } 
       void FuncA(); 
}; 
#endif 

====== 
a.cpp 
====== 
#include "a.h" 

====== 
b.h 
====== 
#ifndef B_H_ 
#define B_H_ 
class A; 
template class B 
{ 
public: 
       B() { } 
       ~B() { } 
       void FuncB() 
       { 
               A* pa; 
               pa->FuncA(); 
       } 
}; 
#endif 

====== 
b.cpp 
====== 
#include "b.h" 
#include "a.h" 

I used the following commands to compile b.cpp: 
   /usr/bin/g++  -c -g  b.cpp 

I got different result when using different version gcc. 
   =================================================== 
   gcc version 3.2.3 20030502 (Red Hat Linux 3.2.3-42) 
   =================================================== 
   Success. 
   
   =================================================== 
   gcc version 3.4.3 20041212 (Red Hat 3.4.3-9.EL4) 
   =================================================== 
   Fail with errors as below: 
       In file included from b.cpp:1: 
       b.h: In member function `void B::FunB()': 
       b.h:16: error: invalid use of undefined type `struct A' 
       b.h:4: error: forward declaration of `struct A' 

If I change the order of head files in b.cpp to: 
   #include "a.h" 
   #include "b.h" 
The compilation will pass. 

Now my question is: Why there are such difference between the two version 
gcc? Could I compile b.cpp 
by simply changing some compiler options? Hopeful for your quick reply. I 
'm very appeaciate for your 
help. 

Thanks. 
Best Regards 
Jianping 
2006-01-16