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From: tuck@jason.cs.unc.edu (Russ Tuck)
Newsgroups: comp.lang.c++
Subject: user-defined conversion--bug or error?
Keywords: user-defined conversion, cfront 2.0, g++ 1.36.1
Message-ID: <10986@thorin.cs.unc.edu>
Date: 6 Dec 89 17:03:37 GMT
Sender: news@thorin.cs.unc.edu
Reply-To: tuck@jason.cs.unc.edu (Russ Tuck)
Organization: University Of North Carolina, Chapel Hill
Lines: 47

The example program below defines classes A and B.  A has operator+, and
B has a user-defined conversion to A.  The question is whether it is legal
to use A's + on B's objects, via the B->A conversion.

GNU g++ 1.36.1 says yes, but AT&T cfront 2.0 says no.  Which is right?
I think the code is legal C++.  Am I wrong?  If so, please explain why.
 
tuck@piglet> cat type2ab.C
class A {
 public:
  friend const A operator+ (const A& lsrc, const A& rsrc);
};

class B {
 public:
  B();
  operator const A&();
};

main()
{
  A a0;
  B b0, b1;

  a0 = b0 + b1;   // legal???
}

tuck@piglet> g++ -v -c type2ab.C
gcc version 1.36.1 (based on GCC 1.36)
 /usr/softlab/contrib/lib/m68k_sunos/gcc-cpp -+ -v -undef -D__GNUC__ -D__GNUG__ -D__cplusplus -Dmc68000 -Dsun -Dunix -D__mc68000__ -D__sun__ -D__unix__ -Dmc68010 type2ab.C /usr/tmp/cca07794.cpp
GNU CPP version 1.36
 /usr/softlab/contrib/lib/m68k_sunos/gcc-cc1plus /usr/tmp/cca07794.cpp -quiet -dumpbase type2ab.C -version -o /usr/tmp/cca07794.s
GNU C++ version 1.36.1 (based on GCC 1.36) (68k, MIT syntax) compiled by GNU C version 1.36.
default target switches:
 /usr/softlab/contrib/lib/m68k_sunos/gcc-as -mc68010 -o type2ab.o /usr/tmp/cca07794.s
tuck@piglet> CC.new -c type2ab.C
CC  type2ab.C:
"type2ab.C", line 17: error: ambiguous use of overloaded +:  B  and  B 
1 error
tuck@piglet> 

piglet is a Sun-3/60 running SunOS 4.0.3.

Russ Tuck		               tuck@cs.unc.edu
UNC Dept. of Computer Science          ...!mcnc!unc!tuck
CB 3175, Sitterson Hall
Chapel Hill, NC 27599-3175, USA        (919) 962-1755 or 962-1700