template argument defaults to list c++

Question

I have a class that is using a list for its implementation :

template<class T>
class Ring {
 list<T> lst; 
..

I want the underliyng implementation (data structure) to be chosen using the template arguments and it should defaults to list. I thought it should be done like that:

template<class T,class Y = list<T>>
class Ring {
    Y lst;
..

where T is the generic type of the data structure and Y the type of container. But it doesn't compile, where is the problem ?

Edited: Here is the full code. Now I see that the problem is in using the iterator and not in template.

#include <iostream>
#include <list>
#include <string>
using namespace std;
template<class T,class Y = list<T>>
class Ring{
    Y lst;
public:
    // Declaration necessary so the following
    // 'friend' statement sees this 'iterator'
    // instead of std::iterator:
    class iterator;
    friend class iterator;
    class iterator : public std::iterator<
        std::bidirectional_iterator_tag, T, ptrdiff_t> {
        Y::iterator it;
        Y* r;
    public:
        // "typename" necessary to resolve nesting:
        iterator(Y& lst,
            const typename Y::iterator& i)
            : r(&lst), it(i) {}
        bool operator==(const iterator& x) const {
            return it == x.it;
        }
        bool operator!=(const iterator& x) const {
            return !(*this == x);
        }
        Y::reference operator*() const {
            return *it;
        }
        iterator& operator++() {
            ++it;
            if (it == r->end())
                it = r->begin();
            return *this;
        }
        iterator operator++(int) {
            iterator tmp = *this;
            ++*this;
            return tmp;
        }
        iterator& operator--() {
            if (it == r->begin())
                it = r->end();
            --it;
            return *this;
        }
        iterator operator--(int) {
            iterator tmp = *this;
            --*this;
            return tmp;
        }
        iterator insert(const T& x) {
            return iterator(*r, r->insert(it, x));
        }
        iterator erase() {
            return iterator(*r, r->erase(it));
        }
    };
    void push_back(const T& x) {
        lst.push_back(x);
    }
    iterator begin() {
        return iterator(lst, lst.begin());
    }
    int size() { return lst.size(); }
};
int main() {
    Ring<string> rs;
    rs.push_back("one");
    rs.push_back("two");
    rs.push_back("three");
    rs.push_back("four");
    rs.push_back("five");

    Ring<string>::iterator it = rs.begin();
    it++; it++;
    it.insert("six");
    it = rs.begin();
    // Twice around the ring:
    for (int i = 0; i < rs.size() * 2; i++)
        cout << *it++ << endl;
} ///:~

Errors :

Severity    Code    Description Project File    Line    Suppression State
Error   C2061   syntax error: identifier 'iterator' STLCont d:\c++ projects\volume 2\stlcont\stlcont\ring.cpp   16  
Error   C2238   unexpected token(s) preceding ';'   STLCont d:\c++ projects\volume 2\stlcont\stlcont\ring.cpp   16  
Error   C2061   syntax error: identifier 'reference'    STLCont d:\c++ projects\volume 2\stlcont\stlcont\ring.cpp   29  
Error   C2079   'it' uses undefined class 'Ring<std::string,std::list<T,std::allocator<_Ty>>>::iterator'    STLCont d:\c++ projects\volume 2\stlcont\stlcont\ring.cpp   77  
Error   C2334   unexpected token(s) preceding '{'; skipping apparent function body  STLCont d:\c++ projects\volume 2\stlcont\stlcont\ring.cpp   29  
Error   C2027   use of undefined type 'Ring<std::string,std::list<T,std::allocator<_Ty>>>::iterator'    STLCont d:\c++ projects\volume 2\stlcont\stlcont\ring.cpp   77  
Error   C2228   left of '.insert' must have class/struct/union  STLCont d:\c++ projects\volume 2\stlcont\stlcont\ring.cpp   79  
Error   C2027   use of undefined type 'Ring<std::string,std::list<T,std::allocator<_Ty>>>::iterator'    STLCont d:\c++ projects\volume 2\stlcont\stlcont\ring.cpp   80  
Error   C2100   illegal indirection STLCont d:\c++ projects\volume 2\stlcont\stlcont\ring.cpp   83  
Error   C2088   '<<': illegal for class STLCont d:\c++ projects\volume 2\stlcont\stlcont\ring.cpp   83

If you assume that `list` implies `std::list` it compiles just fine. — Johan, Oct 26 '17 at 11:15
"where is the problem?" -- what error messages are you getting? — Tristan Brindle, Oct 26 '17 at 11:15
you need to show real code and error messages otherwise we can only guess. The code you show here does not compile because `...` and missing `};` are syntax errors — 463035818_is_not_an_ai, Oct 26 '17 at 11:17
we dont need to see the full code. We need a [mcve] and the error messages. What is wrong with that code? What is "the problem in using the iterator"? — 463035818_is_not_an_ai, Oct 26 '17 at 11:23

score 0 · Accepted Answer · answered Oct 26 '17 at 11:32

The problem is nothing to do with the default template argument -- that bit is correct.

The reason this code won't compile is that Y::iterator is a dependent type, that is, it depends on the template parameter Y. This means that you need to say

typename Y::iterator it;

in your iterator inner class's member list, and

typename Y::reference operator*() const { ... }

In your iterator::operator*() declaration.

For a detailed explanation of why this is required, see this question.

template argument defaults to list c++

1 Answers1