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From: roman@hri.com (Roman Budzianowski)
Newsgroups: comp.lang.c++
Subject: Multiple Inheritance problem
Message-ID: <1990Oct11.194148.14657@hri.com>
Date: 11 Oct 90 19:41:48 GMT
Sender: news@hri.com
Reply-To: roman@hri.com (Roman Budzianowski)
Organization: Horizon Research
Lines: 101


I am having a hard time with multiple inheritance (cfront 2.0 on sparc).

Basically, when in the constructor of a derived class, parts of the
object are not initialized and as a result I get 'no mapping' errors
when I call methods of this object.
I can get it to work, but any minor change brings the problem again.

Here is the class hierarchy:


		class Schema { 
			public:
			 virtual void f() {}
			 ...a lot of virtuals 
			}

class WithSimpleSlot : virtual public Schema | class WithMethod :
virtual public Schema {}
{					     |
	void f() {}			     |
}                                            |


...



class SystemSchema : public WithSimpleSlot, public WithMethod, public
WithNoRelation {}


class SchemaStream : public SystemSchema 
{
...

SchemaStream() { f(); } // here is the problem
}


An instance of SchemaStream is created as a static object.


It appears that the cfront actually creates three copies of Schema, and
the methods called in the constructor of SchemaStream refer to a wrong copy.

Can somebody help me ? Is this a cfront bug, or a problem with the design.
Actually I got it to work, but it broke again when I tryed to make the
destructor for Schema virtual.

Below is an ouput from dbx, the 'no mapping' problem is due to
myNode.ptr being null, which is not.

(dbx) p *this
*`extstream`StreamSchema::StreamSchema`this = {
        slotTable                = 0x1aff20
        [__vptr__14WithSimpleSlot = 0x1a278c]
        PSchema                  = 0x1a22e0
        OWithMethod              = {
                __vptr__10WithMethod = 0x1a25ac
                PSchema              = 0x1a22e0
                OSchema              = {
                        __W68__13SlotOwnerType = '\0'
                        schemaNameID  = (nil)
                        myNode        = {
                                ptr = (nil)
                        }
                        state         = 0
                        lock          = '\0'
                        __vptr__6Schema        = (nil)
                }
        }
        OWithNoRelation          = {
                __vptr__14WithNoRelation = 0x1a2500
                PSchema                  = 0x1a22e0
                OSchema                  = {
                        __W68__13SlotOwnerType = '\0'
                        schemaNameID  = (nil)
                        myNode        = {
                                ptr = (nil)
                        }
                        state         = 0
                        lock          = '\0'
                        __vptr__6Schema        = (nil)
                }
        }
        stream                   = 0x1a521c
        OSchema                  = {
                __W68__13SlotOwnerType = '\0'
                schemaNameID  = 0x1affb8
                myNode        = {
                        ptr = 0x1ad310
                }
                state         = 0
                lock          = '\0'
                __vptr__6Schema        = 0x1a25e0
        }
}


Thanks.