How to resolve a series of errors about iterator constructors involving expected l-values, missing conversions, and viability

Question

I have a program I wrote for a class back in 2000 that I thought I would try to run just for the fun of it. It used to compile back in 2000, but I'm getting a bunch of errors now. I pretty much haven't looked at C++ code since that class, so I'm a bit bewildered. I resolved a bunch of complaints about iostream.h & list.h (putting "std::" in front of a bunch of things like std::cerr, etc). I don't know if that caused the current errors, or whether there's something inherently wrong with my iterators, but the only thing left to resolve involves a bunch of iterator constructors. First, here is a sample of the errors:

SolutionList.cpp:10:22: error: no matching constructor for initialization of 'PieceList::iterator'
        PieceList::iterator i=rhs.first();
                            ^ ~~~~~~~~~~~
./PieceList.h:27:5: note: candidate constructor not viable: expects an l-value for 1st argument
                                iterator(iterator& rhs){data = rhs.data;}
                                ^
./PieceList.h:28:5: note: candidate constructor not viable: no known conversion from 'PieceList::iterator' to 'std::list<Piece>::iterator &'
      (aka '__list_iterator<value_type, __void_pointer> &') for 1st argument
                                iterator(std::list<Piece>::iterator& rhs){data = rhs;}
                                ^
./PieceList.h:26:5: note: candidate constructor not viable: requires 0 arguments, but 1 was provided
                                iterator(){}
                                ^

And here's what I believe is the relevant portion of code:

class PieceList
{
    public:
        PieceList(){}
        PieceList(PieceList& rhs){pieces = rhs.pieces;}
        PieceList(std::list<Piece>& rhs){pieces = rhs;}

        friend std::ifstream& operator>>(std::ifstream&,PieceList&);
        friend std::istream& operator>>(std::istream&,PieceList&);

        class iterator
        {
            public:
                friend class PieceList;

                iterator(){}
                iterator(iterator& rhs){data = rhs.data;}
                iterator(std::list<Piece>::iterator& rhs){data = rhs;}

                Piece operator*(){return *data;}
                iterator& operator=(iterator& rhs){data=rhs.data;return *this;}
                iterator& operator=(std::list<Piece>::iterator& rhs){data = rhs;return *this;}
                bool operator==(iterator& rhs){return data==rhs.data;}
                bool operator==(const iterator& rhs) const{return data==rhs.data;}
                bool operator!=(iterator rhs){return data != rhs.data;}
                iterator& operator++(){++data;return *this;}
                iterator& operator--(){--data;return *this;}
            private:
                std::list<Piece>::iterator data;
        };

        iterator first(){iterator i;i.data=pieces.begin();return i;}
        iterator last(){iterator i;i.data=pieces.end();return i;}

    private:
        std::list<Piece> pieces;
};

So I know that it doesn't like the second constructor, but I'm not sure how to fix it. I couldn't make sense of any of my google results from googling the errors. Can anyone point me in the right direction?

Incidentally, the program solves soma cubes where the number of dimensions is a command-line parameter. I was telling someone about it last week and was pretty proud I'd solved 90 degree rotations in any number of dimensions. I kinda want put it on github or something or maybe rewrite it all together, but it would be nice if I could just get it to run. — hepcat72, Jul 18 '15 at 20:17

sehe · Accepted Answer · 2015-07-18T20:31:32.630

You cannot bind temporaries to non-const references.

    iterator(iterator const& rhs) : data(rhs.data) {}
    iterator(std::list<Piece>::iterator rhs) : data(rhs) {}

Note that iterators are conventially passed by value

Live On Coliru

#include <iterator>
#include <list>
#include <string>
#include <iostream>

struct Piece {
    std::string name;
};

class PieceList
{
    public:
        PieceList(){}
        PieceList(PieceList const& rhs){pieces = rhs.pieces;}
        PieceList(std::list<Piece> const& rhs){pieces = rhs;}

        friend std::ifstream& operator>>(std::ifstream&,PieceList&);
        friend std::istream& operator>>(std::istream&,PieceList&);

        class iterator
        {
            public:
                friend class PieceList;

                iterator(){}
                iterator(iterator const& rhs) : data(rhs.data) {}
                iterator(std::list<Piece>::iterator rhs) : data(rhs) {}

                Piece operator*(){return *data;}
                iterator& operator=(iterator& rhs){data=rhs.data;return *this;}
                iterator& operator=(std::list<Piece>::iterator& rhs){data = rhs;return *this;}
                bool operator==(iterator& rhs){return data==rhs.data;}
                bool operator==(const iterator& rhs) const{return data==rhs.data;}
                bool operator!=(iterator rhs){return data != rhs.data;}
                iterator& operator++(){++data;return *this;}
                iterator& operator--(){--data;return *this;}
            private:
                std::list<Piece>::iterator data;
        };

        iterator first(){iterator i;i.data=pieces.begin();return i;}
        iterator last(){iterator i;i.data=pieces.end();return i;}

    private:
        std::list<Piece> pieces;
};

int main() {
    PieceList pl { {
        { "aap" },
        { "noot" },
        { "mies" },
    } };
    for(auto it=pl.first(); it!=pl.last(); ++it) {
        std::cout << (*it).name << "\n";
    }
}

Prints

aap
noot
mies

You had a similar issue with the constructors of PieceList. Your iterator class is missing operator-> too. I'll leave that as an ExerciseForTheReader™

OK, I pasted the entirety of the rest of the class, barring the # lines. Does that address the blind? — hepcat72, Jul 18 '15 at 20:20
What does the colon do in your fix? I.e. What does " : data(rhs.data)" do? — hepcat72, Jul 18 '15 at 20:31
It's an initializer list: search for `[c++-faq] colon constructor`: http://stackoverflow.com/questions/1711990/what-is-this-weird-colon-member-syntax-in-the-constructor?s=1|2.8949 (only 106 upvotes... some 200+ in the answers) — sehe, Jul 18 '15 at 20:32
Awesome. That resolved the errors! Yay! Thanks so much! So fast too. I just have to propagate this fix to the other iterators and then see if anything new pops up in the errors. — hepcat72, Jul 18 '15 at 20:39
Alright. One of my iterator classes is a bit different than the others and is still generating errors about the != and the = operators. Should I edit my question above, post a brand new question, or is there some way to post followup questions that spawn from a current question? — hepcat72, Jul 18 '15 at 21:12
Post a brand new question, unless you think it's of no value to others, in which case consider just paste-binning and linking it here — sehe, Jul 18 '15 at 21:13
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/83635/discussion-between-sehe-and-hepcat72). — sehe, Jul 18 '15 at 21:13

How to resolve a series of errors about iterator constructors involving expected l-values, missing conversions, and viability

1 Answers1