Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!tcs!nujoizey!gwu
From: gwu@nujoizey.tcs.com (George Wu)
Newsgroups: comp.lang.c++
Subject: Re: porting tips for c++
Message-ID: <943@tcs.tcs.com>
Date: 8 Aug 90 00:32:36 GMT
References: <1990Aug6.133901.25745@swbatl.sbc.com>
Sender: usenet@tcs.com
Reply-To: gwu@nujoizey.tcs.com (George Wu)
Organization: Teknekron Communications Systems
Lines: 118


     Here's something I wrote up for our people when we switched from GNU
C++ 1.36.2 to Sun C++ (AT&T Cfront 2.0).

                                                        George


                                MEMORANDUM

FROM:           George Wu

TO:             User Interface Team

DATE:           June 27, 1990

SUBJECT:        Switching from GNU C++ to Sun C++

_______________________________________________________________________________


     This memorandum summarizes the necessary procedures used to switch from
GNU's C++ compiler to Sun's C++ compiler (AT&T Cfront 2.0).  Two areas are
addressed: general code changes; and environmental changes specific to the IMS
project.  There are several attachments to this memorandum: a description of CC
and various C++ tools; a list of compiler implementation specific behavior; and
a list compiler "not implemented" error messages with translations to English
(known to be incomplete).


                        GENERAL C++ CODE MODIFICATIONS
                        ==============================

The following discoveries were made in porting IMS UI code the Sun C++:

        o Constructors cannot have default argument values.  The compiler does
          not always complain about this problem.  Consider the following
          example:

                class C {
                public:
                    C(int x = 0);
                    C(int x, int y = 0);
                };

          In order to compile this code with Sun's C++ compiler, the following
          re-arrangement may be necessary:

                class C {
                public:
                    C(int x);
                    C(int y, int y);
                private:
                    init(int x = 0, int y = 0);
                };

                C::C(int x) { init(x); }
                C::C(int x, int y) { init(x, y); }

        o Inline functions cannot contain loops.  The fix is obvious: make
          the function an ordinary one.

        o Default argument values can only be declared in one place.  Declare
          default argument values in the class definition, not in the function
          definition.

        o Values of type void* can only be assigned to variables of type void*.
          The C++ permits any pointer value to be automatically cast to a
          void* where necessary, but this relationship is not symmetrical.
          Expressions of type void* must therefore be explicitly cast to the
          appropriate pointer type.  For example, given char* s and void* p,
          s = p becomes s = (char *) p.


        o The name of a function is not automatically converted to the address
          of the function when assigned to a function pointer.  Explicitly use
          the address of operator (&).  The Sun C++ compiler appears to only
          complain about this under certain, indeterminate conditions.  When
          it doesn't complain, the Sun C++ compiler sometimes generates bad
          code.

        o The type of a pointer to a function includes the function argument
          types, ie. the function prototyping must also be specified when
          using function pointer.

        o When a pointer to a class member function is used, explicitly
          resolve the class scope.  Even in cases where there is no possibilty
          of confusion, C++ requires the scope.  Therefore, instead of:

                fp = &init;

          you must state:

                fp = &C::init;

          Failure to specify the class will result in the C++ compiler
          generating bad code, as evidenced by a core dump when the function
          pointer is dereferenced.

        o Default argument values which require run-time calculations may
          cause the C compiler (to which Cfront outputs C code) to complain
          about undefined temporary variables.  For instance:

                class C {
                public:
                        C(int size = round(0.5 * inches));
                };

          will cause the C compiler to generate an error message complaining
          about an undefined variable.  The variable name will not only be
          mangled, but will fail to unmangle to any C++ variable.




----
George J Wu                           | gwu@tcs.com or ucbcad!tcs!gwu
Software Engineer                     | 2121 Allston Way, Berkeley, CA, 94704
Teknekron Communications Systems, Inc.| (415) 649-3752