Implementing views on a std::vector Goal:


Wrap a vector. The wrapper can create view on part of the vector.
Support const_view and view. 
view allows modification of elements while const_view does not.
The views are ranges (has begin() and end() methods). 


Problem:


Code duplication between the const_view and view.


The actual code implements a 2D sparse array that saves more memory than usual. So I want to learn how to solve this problem.

Simplified Code (implements a dense 2D array)

#include &lt;vector&gt;
class A_csub;
class A_sub;

typedef std::vector&lt;int&gt; V;
typedef typename V::iterator It;
typedef typename V::const_iterator Cit;

class A {
// 2D dense array with _r rows and _c columns.
public:
    A(const int r, const int c);
    A_csub cview(const int n) const;
    A_sub view(const int n); // view nth row
    It sub_begin(const int n); // iterator at begin of nth row
    Cit sub_cbegin(const int n) const;
    int _r;
    int _c;
private:
    V _v;
};

class A_csub {
public:
    A_csub(const A&amp; a, const int n);
    Cit begin() const;
    Cit end() const;
    const int size() const;
private:
    const A&amp; _a;
    const int _n;
};

class A_sub {
public:
    A_sub(A&amp; a, const int n);
    It begin();
    It end();
    const int size() const;
private:
    A&amp; _a;
    const int _n;
};

// -- A -- //
A::A(const int r, const int c) : _r(c), _c(c), _v(r*c) {}
A_csub A::cview(const int n) const { return A_csub(*this, n); }
A_sub A::view(const int n) { return A_sub(*this, n); }
It A::sub_begin(const int n) { return _v.begin() + n*_r; }
Cit A::sub_cbegin(const int n) const { return _v.cbegin() + n*_r; }

// -- A_csub -- //
A_csub::A_csub(const A&amp; a, const int n) : _a(a), _n(n) {}
Cit A_csub::begin() const { return _a.sub_cbegin(_n); }
Cit A_csub::end() const { return begin() + _a._r; }
const int A_csub::size() const { return _a._r; }

// -- A_sub -- //
A_sub::A_sub(A&amp; a, const int n) : _a(a), _n(n) {}
It A_sub::begin() { return _a.sub_begin(_n); }
It A_sub::end() { return begin() + _a._r; }
const int A_sub::size() const { return _a._r; }

int main() {
    A a(10,5);
    a.cview(4);
    A_sub b = a.view(4);
    for (auto &amp;&amp; e:b) {e=1;}
}
The variable names you chose are quite bad. I can't seem to figure out what r and c are as constructor input.

Creating such views only for vectors (of ints) is a bit wasteful. You are basically wrapping an iterator pair, so why not make the code generic so that it woks with iterators of any type of container?

Boost, for example, provides Boost.Range for doing just this.

When you implemented a specialized container, you create specialized iterators that know how to traverse it. The view/range stays the same, it's job is still to wrap a pair of iterators.