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From: skip@taygeta.oc.nps.navy.mil (Skip Carter)
Newsgroups: comp.lang.c++
Subject: Inheritance  & type conversion
Message-ID: <2348@taurus.cs.nps.navy.mil>
Date: 5 Jun 91 22:34:56 GMT
Sender: news@taurus.cs.nps.navy.mil
Reply-To: skip@taygeta.oc.nps.navy.mil (Skip Carter)
Lines: 141


	I have been programming C++ for the past few months, using Turbo C++
        V1.0 and Zortech C++ V2.1.  In the process of writing an application
        I came across the following behaviour, which I don't understand:


        There are two classes, Base and Derived.
        
        Derived is derived from the Base class and has a method to do a
        	type conversion from Derived TO Base.

	Base has friends functions, report1 and report2.  Report1 takes a
        	reference to a Base, report2 takes the value of a Base,
                as the parameter.

	The following:                
                
        	Derived dtype;

        	report1( dtype );
               
      	causes a conversion to Base then report1 is invoked with Turbo C++,
        but for Zortech C++ NO CONVERSION occurs.
        
        
        BUT:
                  
        	Derived dtype;

        	report2( dtype );
               

	cases a conversion to Base with Zortech C++, BUT NOT for Turbo C++     
          



	I am too much of a novice to be able to figure out which is the
        correct behavior.  Can anyone give me some insight ?  I have attached
        a copy of the simple program.
        

	Here is the program output for Turbo C++ V1.0:
        
		Type conversion from Derived to Base invoked
		report1 reporting
		report2 reporting


	Here is the program output for Zortech C++ V2.1:

		report1 reporting
		Type conversion from Derived to Base invoked
		report2 reporting
                
                
        (Note I have abstracted the classes from the relatively complicated
         classes in the actual application in order to show the behaviour).
         

 
	The code:


        
// inhtest.cpp		tests the interaction of type conversion
//			and inheritance


// Note:
//	 report1( Derived )	causes a conversion to Base
//				then a call report1( Base )
//                              with TURBO C++ V1.0, with Zortech C++ V2.1
//				no conversion is made

//	 report2( Derived )	causes the OPPOSITE to happen
//


#ifdef __ZTC__
#include <stream.hpp>
#else
#include <iostream.h>
#endif

class Base {

	public:
           Base() {};
           ~Base() {};
        friend void report1(Base& x);
        friend void report2(Base  x);
};

void report1(Base& x)
{
	 cout << "report1 reporting\n";
}

void report2(Base x)
{
	 cout << "report2 reporting\n";
}



class Derived : public Base {

	public:
        	Derived() {};
                ~Derived() {};
                operator Base();	// cast to Base
};


// a type cast from a Derived directly to a Base
Derived::operator Base()		// Base m = Derived a
{
	Base dummy;
        
	cout << "Type conversion from Derived to Base invoked\n";
        
        return dummy;
}


void main()
{
        Derived dtype;

        report1( dtype );
               
        report2( dtype );        
}



   Everett (Skip) Carter		Phone:  408-646-3318
   Naval Postgraduate School	        INTERNET: skip@taygeta.oc.nps.navy.mil
   Dept. of Oceanography, Code OC/CR    UUCP:     ...!uunet!taygeta!skip
   Monterey, CA. 93934                  TELEMAIL: s.carter/omnet