I have the following error:
IntelliSense: return type is not identical to nor covariant with return type "Counter" of overridden virtual function "Counter::operator++"
Here are the headers of my project.
counter.h
/* Header file of Counter Class*/
#pragma once
#include <iostream>
using namespace std;
//Class definition
class Counter {
friend ostream &operator<<(ostream &out, const Counter &c);
public:
Counter(int n0 = 0);
virtual Counter &operator++();
virtual Counter operator++(int);
void reset();
int getCount() const;
private:
int count;
};
LimitedCounter.h
#pragma once
#include "counter.h"
class LimitedCounter : public Counter{
friend ostream &operator<<(ostream &out, const LimitedCounter &c);
public:
LimitedCounter(int low, int up);
void reset();
LimitedCounter& operator++();
LimitedCounter operator++(int); // error here
operator int() { return getCount(); };
int getCount() const { return Counter::getCount(); };
private:
int upper;
};
And the implementation
counter.cpp
/* Implementation of Counter Class*/
#include "counter.h"
#include <iostream>
Counter:: Counter(int n0) {
count = n0;
}
Counter& Counter::operator++() {
count++;
return *this;
}
Counter Counter::operator++(int) {
Counter old = *this;;
count++;
return old;
}
void Counter::reset(){
count = 0;
}
int Counter::getCount() const{
return count;
}
ostream &operator<<(ostream & out, const Counter &c) {
out << "\nCounter value is now " << c.count ;
return out;
}
LimitedCounter.cpp
#include "LimitedCounter.h"
LimitedCounter::LimitedCounter(int low, int up) : Counter(low), upper(up) {}
LimitedCounter& LimitedCounter::operator++() {
if (getCount() < upper) {
Counter::operator++();
}
return *this;
}
LimitedCounter LimitedCounter::operator++(int) {
if (getCount() < upper) {
LimitedCounter old = *this;
Counter::operator++(0); // question?
return old;
}
else {
return *this;
}
}
void LimitedCounter::reset() {
Counter::reset();
}
//friend function
ostream &operator<<(ostream &out, const LimitedCounter &c) {
out << c.getCount() << endl;
return out;
}
I get the error:
error C2555: 'LimitedCounter::operator ++': overriding virtual function return type differs and is not covariant from 'Counter::operator ++'
When I remove virtual in the post-increment of counter.h then there is no error at all. So everything works fine with the pre-increment . So I don't know is it because how I implement the post-increment ? And also when I override the post-increment ( operator++(int) ), is it right that I write like this:
Counter::operator++(0);
Thanks for helping me.
The problem is simple: you are returning an object by value, so you have the following situation
virtual Counter Counter::operator++(int)
LimitedCounter LimitedCounter::operator++(int) override
Now, since the method is virtual
, the correct implementation is chosen at runtime according to the vtable of the object you are calling it on.
This means that the compiler can't know a priori the type returned by operator++
but he needs to know at least its size since it's a value (and not just a pointer).
Indeed if you had the following situation would be accepted:
virtual Counter* Counter::operator++(int)
LimitedCounter* LimitedCounter::operator++(int) override
because the runtime implementation would in any case return a pointer so the compiler would be able to cope with it correctly.
The standard specifies what's allowed and considered covariant in §10.3 (C++11):
The return type of an overriding function shall be either identical to the return type of the overridden function or covariant with the classes of the functions. If a function D::f
overrides a function B::f
, the return types of the functions are covariant if they satisfy the following criteria:
both are pointers to classes, both are lvalue references to classes, or both are rvalue references to classes
the class in the return type of B::f
is the same class as the class in the return type of D::f
, or is an unambiguous and accessible direct or indirect base class of the class in the return type of D::f
both pointers or references have the same cv-qualification and the class type in the return type of D::f has the same cv-qualification as or less cv-qualification than the class type in the return type of B::f.
It is some different ways to solve problem like this
Just keep all things in one class. Do you really need 2 counter class instead of one that can be parametrized with counter strategy (that can be from simple enum
to some complicated class )
enum class CountinngStretegy { Unlimited, Limited} class Counter { public: Counter(CountinngStretegy strategy); //... }
Make Counter
independent from LimitedCounter
. Then if you want parametrize on counter type, use templates for that:
template <typename SomeCounter> void do_smth_with_counter(SomeCounter counter);
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