[comp.lang.c++] Multiple Inheritance ordering

zmact60@doc.ic.ac.uk (I S Roberts) (06/13/90)

What (if any) difference does the ordering of the base classes for
a derived class make?

eg, How could

class A : public B, public C {...

be different from

class A : public C, public B {...

B and C have pure virtual functions, but all function names are unqiue.

I am using a fairly complex class structure with multiple inheritance
to implement an extened syntax tree for a program.  I am using g++
1.37.0.  With the base classes in one order the program works fine.
Reverse the order of the base classes and I either get compilation
errors or (apparently) wrong virtual functions called.

Any suggestions?

Ian.

zmact60@doc.ic.ac.uk (I S Roberts) (06/13/90)

Further to my previous posting, with a little help from Duncan White I
have isolated the problem.  Try compiling the following code;

1) As is,

2) With the comment commenting out the line after the one that is
already commented out, removing the // that is alrady there (Phew...)
Anyway, you get the idea,

As is g++ (1.37.0) compiles it OK,
after (2) I get

g++ -c t.cc
t.cc: In function int main ():
t.cc:25: cannot allocate an object of type `Category'
t.cc:25: since the following virtual functions are abstract:
t.cc:4: void Dependant::Perform (int)

------------------------- cut here ----------------------
class Dependant {
public:
	virtual void Perform(int) = 0 ;
} ;

class ReferenceCounter : public Dependant {
public:
	virtual void Perform(int) = 0 ;
} ;

class DatabaseObj {
public:
	virtual void rootDb() { } ;
} ;

//class Category : public DatabaseObj, public ReferenceCounter {
class Category : public ReferenceCounter, public DatabaseObj {
public:
	virtual void Perform(int) ;
} ;

main()
{
	Category* bar = new Category() ;
}

-------------------- cut again ------------------------------

Ian.

#include Disclaimer.