Question

I was trying to learn writing stl like iterators, for that I wrote a simple circular array and added an iterator in it. Please look at the bottom of the code to see the problem.

template<typename T, int N>
class RingQueue{
    T * _marray;
    int _mbegin;
    int _msize;
public:
    RingQueue(){
        _marray = new T[N];
        _mbegin = 0;
        _msize= 0;
    }   
    void push_back(const T& val){
        if(_msize!=N){
            _marray[(_mbegin+_msize)%N] = val;
            _msize++;
        }
        else
            throw "Queue Full";
    }   
    T pop_front(){
        if(_msize!=0){
            T&val = _marray[_mbegin];
            _mbegin = (_mbegin+1)%N;
            _msize--;
            return val;
        }
        else
            throw "Queue Empty";
    }

    class iterator{
        RingQueue<T,N>* _container;
        int _idx;
        public:
        iterator(RingQueue<T,N>* container,int idx):_container(container){
            _idx = idx;
        }

        bool operator==(iterator &rhs){
            return (this->_container==rhs._container && this->_idx == rhs._idx);
        }
        bool operator!=(iterator &rhs){
            return !(*this==rhs);
        }   
        T operator*(){
            if(_container->_msize>0&&_idx<_container->_msize){
                return _container->_marray[(_container->_mbegin+_idx)%N];
            }
        }

        iterator& operator++(){
            if(_container->_msize ==0){
                *this = _container->end();
                return *this;
            }
            if(_idx==_container->_msize){   
                *this = _container->end();
                return *this;
            }
            _idx++;
            return *this;
        }
    };
    iterator begin(){
        return iterator(this,0);
    }
    iterator end(){
        return iterator(this,_msize);
    }
};
int current=0;
int gen(){
    return current++;
}

int curr_op=0;
int operation(){
    return 2*(curr_op++&1)-1;
}
int main(){
    RingQueue<int,10> ring;
    vector<int> v(9),op(9);
    generate(v.begin(),v.end(),gen);
    random_shuffle(v.begin(),v.end());
    copy(v.begin(),v.end(),ostream_iterator<int>(cout," "));    
    cout<<endl;

    generate(op.begin(),op.end(),operation);        
    random_shuffle(op.begin(),op.end());
    // copy(op.begin(),op.end(),ostream_iterator<int>(cout," "));   
    cout<<endl;

    for(vector<int>::iterator itv  = v.begin();itv!=v.end();itv++){
        try{
            ring.push_back(*itv);   
        }catch(const char * e){
            cout<<*itv<<e<<endl;
        }
    }
    //works
    RingQueue<int,10>::iterator ite = ring.end();
    for(RingQueue<int,10>::iterator it = ring.begin(); it!=ite; ++it){
        cout<<*it<<endl;
    }
    // doesn't work 
    for(RingQueue<int,10>::iterator it = ring.begin(); it!=ring.end(); ++it){
        cout<<*it<<endl;
    }
    return 0;
}

When I compile the doesn’t work part, g++ dumps the following error

ringqueue.cpp: In function ‘int main()’:
ringqueue.cpp:112: error: no match for ‘operator!=’ in ‘it != ring.RingQueue<T, N>::end [with T = int, int N = 10]()’
ringqueue.cpp:48: note: candidates are: bool RingQueue<T, N>::iterator::operator!=(RingQueue<T, N>::iterator&) [with T = int, int N = 10]

the works part compiles seamlessly, when compiled without doesn’t work part. Can somebody explain me what’s wrong.

Answer 1

I think the issue is in these lines:

    bool operator==(iterator &rhs){
        return (this->_container==rhs._container && this->_idx == rhs._idx);
    }
    bool operator!=(iterator &rhs){
        return !(*this==rhs);
    }  

The problem here is that these functions take in lvalue references to their arguments. This means that if you try passing an rvalue (for example, a temporary object returned from a function) into these operators, you will get a compile-time error because references cannot bind to temporaries. To fix this, either change the arguments to be const references:

    bool operator==(const iterator &rhs){
        return (this->_container==rhs._container && this->_idx == rhs._idx);
    }
    bool operator!=(const iterator &rhs){
        return !(*this==rhs);
    }  

Since const references can bind to temporaries, or have them take their arguments by value:

    bool operator==(iterator rhs){
        return (this->_container==rhs._container && this->_idx == rhs._idx);
    }
    bool operator!=(iterator rhs){
        return !(*this==rhs);
    }  

Whichever choice you make, you should probably mark these functions const because they don’t mutate the receiver object.

Hope this helps!