Multiple Inheritance Problems

[richie-m]

heya!

ive got a problem with this code, i dont understand why there is a double free, it doesnt make sense!

CODE
#pragma once

#include

class Parser {
    public:
        Parser() {
            file = 0;
        }
        virtual ~Parser() {
            Cleanup();
        }
        void Cleanup() {
            delete[] file;
            file = 0;
        }
    private:
        char *file;
};

class ParserTranslator : public Parser,public MPxFileTranslator {
    public:
        ParserTranslator() : Parser() {}
        ~ParserTranslator() {}
        static void *creator() {
            return new ParserTranslator;
        }
    private:
        //
};

*** glibc detected *** /usr/aw/maya/bin/maya.bin: double free or corruption (fasttop): 0x09bfce30 ***

what am i doing wrong?

[richie-m]

ah im still struggling with this!

[richie-m]

what would you think the problem was:

CODE
#pragma once

#include

class Parser {
    public:
        Parser() {
            cout << "Parser::Parser()" << endl;
            file = 0;
            cout << "end Parser::Parser()" << endl;
        }
        virtual ~Parser() {
            cout << "Parser::~Parser()" << endl;
            Cleanup();
            cout << "end Parser::~Parser()" << endl;
        }
        void Cleanup() {
            cout << "Parser::Cleanup()" << endl;
            delete[] file;
            file = 0;
            cout << "end Parser::Cleanup()" << endl;
        }
    private:
        char *file;
};

class ParserTranslator : public Parser,public MPxFileTranslator {
    public:
        ParserTranslator() : Parser() {
            cout << "ParserTranslator::ParserTranslator()" << endl;
            cout << "end ParserTranslator::ParserTranslator()" << endl;
        }
        ~ParserTranslator() {
            cout << "ParserTranslator::~ParserTranslator()" << endl;
            cout << "end ParserTranslator::~ParserTranslator()" << endl;
        }
        static void *creator() {
            return new ParserTranslator;
        }
    private:
        //
};

CODE
Parser::Parser()
end Parser::Parser()
ParserTranslator::ParserTranslator()
end ParserTranslator::ParserTranslator()
ParserTranslator::~ParserTranslator()
end ParserTranslator::~ParserTranslator()
Parser::~Parser()
Parser::Cleanup()
end Parser::Cleanup()
end Parser::~Parser()
*** glibc detected *** /usr/aw/maya/bin/maya.bin: double free or corruption (fasttop): 0x0a7260f0 ***

hope someone can help! im confused!

[rob-bateman]

erm. Have you tried to simply not use multiple inheritance and make the Parser class a member. multiple inheritance is generally a bad thing in my opinion - the only exception being the inheritance of interfaces....

[richie-m]

hey rob! thanks for the reply...

sorry, i didnt see your message at my last post...

well i dont see why it would be a bad idea... thats how i initially did it when i ran into problems with multiple inheritance, but now id like to try find out why this code is a problem...

any ideas?

thanks!

[richie-m]

there must be a reason why this is a problem?

[rob-bateman]

yes. you are calling the dtor twice. once from your so. once from maya. read up on shared objects, allocating across dll bounds, pure virtual interfaces, virtual base classes, malloc/free, memory mapping and writing your own plugin mechanisms in C++. Maya is calling delete on ParserTranslator, which means it is calling delete on MPxFileTranslator (which is fine, it's allowed to, it's defined in it's dll, and it's designed to be safe since it has declared a virtual dtor).

It's also calling delete on Parser - which it is not allowed to do, since you are the one calling new on that object, and maya has not got a clue what exists in the memory alloted to your plugin via the CRT. Thus, maya has caused malloc/free to blow up because maya's version of malloc/free (that exists in a seperately mapped memory location to yours) does not have a clue where the feck that memory resides. so, the CRT crashes.

Anyhow, multiple inheritance is a terrible idea. It's one of the worst programming paradigms ever invented and should be avoided at all cost. If you continue to use it you will very soon start finding the true horror of virtual base classes and all that nastyness.

CODE
class Person {}
class Student : public Person {}
class Musician : public Person {}
class StudentMusician : public Student, public Musician {}

Imo, ParserTranslator is a file translator, that uses a pre-defined working parser class. ergo,

CODE
ParserTranslator : MpxFileTranslator {
   Parser p;
}

Ultimately maya doesn't give a toss about your Parser class, however you are trying to believe that it should.... that's not the way plugins work.

[richie-m]

QUOTE
yes. you are calling the dtor twice.

yeah i figured something similar...

QUOTE
once from your so. once from maya. read up on shared objects, allocating across dll bounds, pure virtual interfaces, virtual base classes, malloc/free, memory mapping and writing your own plugin mechanisms in C++. Maya is calling delete on ParserTranslator, which means it is calling delete on MPxFileTranslator (which is fine, it's allowed to, it's defined in it's dll, and it's designed to be safe since it has declared a virtual dtor).

mmm, thats all very new to me... allocating across dll bounds, so basically im in the wrong memory space? ill look through google and read up some stuff... thanks for pointing that out!

so am i getting this right? when the plugin (so/dll) is loaded it allocates its own memory space, and since it is inherited from the Parser class its trying to delete within the wrong memory space, so basically for this to work i must be mapping the Parser to the dll/so memory space right? i could do this my basically making the Parser class a member of the file translator...

Id still like to be able to get this multiple inheritance working, just for understanding a little more....

thanks for the help!