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Pure abstract class using operators.

matty3269 (26)
Hi all.

I'm trying to create a pure abstract numeric class so that I can have an implementation of different number systems but they all are derived from the base class. So I have

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class numberBase
{
public:
        virtual void assign(unsigned int numerator, unsigned int denominator) = 0;
        // Assignment Operators
        virtual numberBase & operator=(const numberBase& other) = 0;
        virtual numberBase & operator+=(const numberBase& other) = 0;
        virtual numberBase & operator-=(const numberBase& other) = 0;
        virtual numberBase & operator*=(const numberBase& other) = 0;

        //Arithmetic operators.
        virtual numberBase operator+(const numberBase& other) = 0;
        virtual numberBase operator-(const numberBase& other) = 0;
        virtual numberBase operator*(const numberBase& other) = 0;

        // Conditional Operators.
        virtual bool operator==(const numberBase& other) = 0;
        virtual bool operator!=(const numberBase& other) = 0;
        virtual bool operator>(const numberBase& other) = 0;
        virtual bool operator<(const numberBase& other) = 0;
        virtual bool operator>=(const numberBase& other) = 0;
        virtual bool operator<=(const numberBase& other) = 0;
};


but when I try to compile this with a derived class, I get

In file included from numbers/probFP.h:2,
from numbers/probFP.cpp:1:
numbers/numberBase.h:16: error: invalid abstract return type for member function 'virtual numberBase numberBase::operator+(const numberBase&)'
numbers/numberBase.h:6: note: because the following virtual functions are pure within 'numberBase':
numbers/numberBase.h:8: note: virtual void numberBase::assign(unsigned int, unsigned int)
numbers/numberBase.h:10: note: virtual numberBase& numberBase::operator=(const numberBase&)
numbers/numberBase.h:11: note: virtual numberBase& numberBase::operator+=(const numberBase&)
numbers/numberBase.h:12: note: virtual numberBase& numberBase::operator-=(const numberBase&)
numbers/numberBase.h:13: note: virtual numberBase& numberBase::operator*=(const numberBase&)
numbers/numberBase.h:16: note: virtual numberBase numberBase::operator+(const numberBase&)
numbers/numberBase.h:17: note: virtual numberBase numberBase::operator-(const numberBase&)
numbers/numberBase.h:18: note: virtual numberBase numberBase::operator*(const numberBase&)
numbers/numberBase.h:21: note: virtual bool numberBase::operator==(const numberBase&)
numbers/numberBase.h:22: note: virtual bool numberBase::operator!=(const numberBase&)
numbers/numberBase.h:23: note: virtual bool numberBase::operator>(const numberBase&)
numbers/numberBase.h:24: note: virtual bool numberBase::operator<(const numberBase&)
numbers/numberBase.h:25: note: virtual bool numberBase::operator>=(const numberBase&)
numbers/numberBase.h:26: note: virtual bool numberBase::operator<=(const numberBase&)
numbers/numberBase.h:17: error: invalid abstract return type for member function 'virtual numberBase numberBase::operator-(const numberBase&)'
numbers/numberBase.h:6: note: since type 'numberBase' has pure virtual functions
numbers/numberBase.h:18: error: invalid abstract return type for member function 'virtual numberBase numberBase::operator*(const numberBase&)'
numbers/numberBase.h:6: note: since type 'numberBase' has pure virtual functions
In file included from numbers/probFP.cpp:1:
numbers/probFP.h:19: error: invalid abstract return type for member function 'probFP probFP::operator+(const probFP&)'
numbers/probFP.h:7: note: because the following virtual functions are pure within 'probFP':
numbers/numberBase.h:10: note: virtual numberBase& numberBase::operator=(const numberBase&)
numbers/numberBase.h:11: note: virtual numberBase& numberBase::operator+=(const numberBase&)
numbers/numberBase.h:12: note: virtual numberBase& numberBase::operator-=(const numberBase&)
numbers/numberBase.h:13: note: virtual numberBase& numberBase::operator*=(const numberBase&)
numbers/numberBase.h:16: note: virtual numberBase numberBase::operator+(const numberBase&)
numbers/numberBase.h:17: note: virtual numberBase numberBase::operator-(const numberBase&)
numbers/numberBase.h:18: note: virtual numberBase numberBase::operator*(const numberBase&)
numbers/numberBase.h:21: note: virtual bool numberBase::operator==(const numberBase&)
numbers/numberBase.h:22: note: virtual bool numberBase::operator!=(const numberBase&)
numbers/numberBase.h:23: note: virtual bool numberBase::operator>(const numberBase&)
numbers/numberBase.h:24: note: virtual bool numberBase::operator<(const numberBase&)
numbers/numberBase.h:25: note: virtual bool numberBase::operator>=(const numberBase&)
numbers/numberBase.h:26: note: virtual bool numberBase::operator<=(const numberBase&)
numbers/probFP.h:20: error: invalid abstract return type for member function 'probFP probFP::operator-(const probFP&)'
numbers/probFP.h:7: note: since type 'probFP' has pure virtual functions
numbers/probFP.h:21: error: invalid abstract return type for member function 'probFP probFP::operator*(const probFP&)'
numbers/probFP.h:7: note: since type 'probFP' has pure virtual functions
numbers/probFP.cpp: In member function 'probFP probFP::operator+(const probFP&)':
numbers/probFP.cpp:93: error: invalid abstract return type for member function 'probFP probFP::operator+(const probFP&)'
numbers/probFP.h:7: note: since type 'probFP' has pure virtual functions
numbers/probFP.cpp:95: error: cannot allocate an object of abstract type 'probFP'
numbers/probFP.h:7: note: since type 'probFP' has pure virtual functions
numbers/probFP.cpp:95: error: cannot declare variable 'result' to be of abstract type 'probFP'
numbers/probFP.h:7: note: since type 'probFP' has pure virtual functions
numbers/probFP.cpp:97: error: cannot allocate an object of abstract type 'probFP'
numbers/probFP.h:7: note: since type 'probFP' has pure virtual functions
numbers/probFP.cpp: In member function 'probFP probFP::operator-(const probFP&)':
numbers/probFP.cpp:100: error: invalid abstract return type for member function 'probFP probFP::operator-(const probFP&)'
numbers/probFP.h:7: note: since type 'probFP' has pure virtual functions
numbers/probFP.cpp:102: error: cannot allocate an object of abstract type 'probFP'
numbers/probFP.h:7: note: since type 'probFP' has pure virtual functions
numbers/probFP.cpp:102: error: cannot declare variable 'result' to be of abstract type 'probFP'
numbers/probFP.h:7: note: since type 'probFP' has pure virtual functions
numbers/probFP.cpp:104: error: cannot allocate an object of abstract type 'probFP'
numbers/probFP.h:7: note: since type 'probFP' has pure virtual functions
numbers/probFP.cpp: In member function 'probFP probFP::operator*(const probFP&)':
numbers/probFP.cpp:107: error: invalid abstract return type for member function 'probFP probFP::operator*(const probFP&)'
numbers/probFP.h:7: note: since type 'probFP' has pure virtual functions
numbers/probFP.cpp:109: error: cannot allocate an object of abstract type 'probFP'
numbers/probFP.h:7: note: since type 'probFP' has pure virtual functions
numbers/probFP.cpp:109: error: cannot declare variable 'result' to be of abstract type 'probFP'
numbers/probFP.h:7: note: since type 'probFP' has pure virtual functions
numbers/probFP.cpp:111: error: cannot allocate an object of abstract type 'probFP'
numbers/probFP.h:7: note: since type 'probFP' has pure virtual functions
... etc,

So a numeric operator in C++ needs a reference to an objcet to be returned to allow chaining, but it can't do that as you can't create an instance of a pure abstract class (which will actually never be created). Is there a nicer way around this? Or should I just not use Pure virtual functions?
LB (13399)
You should return a reference to your derived class. The return type is not part of a function's signature, it should work. Also you must make sure you define all the pure virtual functions the base declares or your derived class will also be pure virtual.
matty3269 (26)
Hi LB,

Thanks for the reply... I'm not sure what you mean by 'return a reference to your derived class'. I will be having multipule derived classes from this base. The derived class declaration I have is:
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#include <iostream>
#include "numberBase.h"

using namespace std;

class probFP : numberBase
{
public:
	probFP(unsigned int numberBits);
	virtual ~probFP();
	void assign(unsigned int numerator, unsigned int denominator);
	// Assignment Operators
	probFP & operator=(const probFP& other);
	probFP & operator+=(const probFP& other);
	probFP & operator-=(const probFP& other);
	probFP & operator*=(const probFP& other);

	//Arithmetic operators.
	probFP operator+(const probFP& other);
	probFP operator-(const probFP& other);
	probFP operator*(const probFP& other);

	// Conditional Operators.
	bool operator==(const probFP& other);
	bool operator!=(const probFP& other);
	bool operator>(const probFP& other);
	bool operator<(const probFP& other);
	bool operator>=(const probFP& other);
	bool operator<=(const probFP& other);

	void print();
private:
	friend ostream &operator<<(ostream &out, const probFP& other);
	int numBits;
	unsigned long currentNumber;
};


Could you maybe provide code as to the base class declaration?

Thanks
Matt
LB (13399)
It looks correct...but you are right, it does not like the operators that are not compound assignment operators...

This code will not compile:
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struct A
{
	virtual A operator+(const A&) = 0;
};
struct B: A
{
	B operator+(const B&);
};
B B::operator+(const B &with)
{
	return(*this);
}
However, if I inline the functions like this it compiles fine:
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struct A
{
	virtual A operator+(const A&) = 0;
};
struct B: A
{
	B operator+(const B&)
	{
		return(*this);
	}
};
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