postfix increment semantics implemented incorrectly

[hayward@loup.net]

>Submitter-Id:	net
>Originator:	Mike Hayward
>Organization:	Loup Technologies, Inc.
>Confidential:	no
>Synopsis:	postfix increment semantics implemented incorrectly
>Severity:	serious
>Priority:	medium
>Category:	c++
>Class:		wrong-code
>Release:	gcc (GCC) 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)
>Environment:
System: Linux dbox 2.6.18-3-686 #1 SMP Sun Dec 10 19:37:06 UTC 2006 i686 GNU/Linux
Architecture: i686

	
host: i486-pc-linux-gnu
build: i486-pc-linux-gnu
target: i486-pc-linux-gnu
configured with: ../src/configure -v --enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr --enable-shared --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --enable-nls --program-suffix=-4.1 --enable-__cxa_atexit --enable-clocale=gnu --enable-libstdcxx-debug --enable-mpfr --with-tune=i686 --enable-checking=release i486-linux-gnu
>Description:
g++ parses the code correctly and calls the correct overloaded
increment operators, but in the wrong postfix order.  The semantics of
postfix are to take the rvalue before invoking the method.  Note this
is not related to multiple reference ordering between sequence points
as the object is only referenced once.  Although I have not checked
postfix decrement, it may have the same semantic problem and it is
trivial to adapt the supplied code.
>How-To-Repeat:
cat > a.cpp
#include <iostream>

class C
{
public:
  int v;

  C( int v ) : v( v ) {}

  C & operator++( void ) { v++;      return *this; }
  C & operator++( int )  { v += 100; return *this; }
};

main()
{
  C a( 0 );
  C b( 0 );
  
  std::cout << "a:" << a.v << std::endl;  // 0
  std::cout << "b:" << b.v << std::endl;  // 0

  b = ++a;

  std::cout << "a:" << a.v << std::endl;  // 1
  std::cout << "b:" << b.v << std::endl;  // 1

  b = a++;

  std::cout << "a:" << a.v << std::endl;  // 101
  std::cout << "b:" << b.v << std::endl;  // should be 1, not 101!
}
^D
> g++ a.cpp ; ./a.out
a:0
b:0
a:1
b:1
a:101
b:101
>Fix:
Execute postfix methods *after* evaluating the expression they are in.