custom iterator, C++ (vc++)

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//Microsoft (R) C/C++ Optimizing Compiler Version 19.00.23506 for x64

#include <iostream>
#include <vector>
#include <iterator>

template < typename T > struct collection
{
    collection( std::size_t nrows, std::size_t ncols, const T& v = {} )
        : nrows(nrows), ncols(ncols), items( nrows*ncols, v ) {}

struct iterator
    {
        using value_type = T ;
        using reference = T& ;
        using pointer = T* ;
        using difference_type = std::ptrdiff_t ;
        using iterator_category	= std::forward_iterator_tag ;

reference operator* () { return *curr ; }
        pointer operator& () { return &**this ; }

iterator& operator++ () { ++curr ; return *this ; }
        iterator operator++ (int) { const auto temp(*this) ; ++*this ; return temp ; }

bool operator== ( const iterator& that ) const { return curr == that.curr ; }
        bool operator!= ( const iterator& that ) const { return !(*this==that) ; }

iterator& next_row()
        {
            if( std::distance( curr, end ) >= difference_type(ncols) ) std::advance( curr, ncols ) ;
            else curr = end ;
            return *this ;
        }

typename std::vector<T>::iterator curr ;
        typename std::vector<T>::iterator end ;
        std::size_t ncols ;

};

iterator begin() { return { std::begin(items), std::end(items), ncols } ; }
    iterator end() { return { std::end(items), std::end(items), ncols } ; }

std::size_t nrows ;
    std::size_t ncols ;
    std::vector<T> items ;
};

int main()
{
    collection<int> coll( 10, 12 ) ;

int n = 0 ;
    for( int& v : coll ) v = n++ ;

// print column #3
    for( auto iter = ++++++std::begin(coll) ; iter != std::end(coll) ; iter.next_row() )
        std::cout << *iter << ' ' ;
    std::cout << '\n' ;

// print column #5
    for( auto iter = std::next( std::begin(coll), 5 ) ; iter != std::end(coll) ; iter.next_row() )
        std::cout << *iter << ' ' ;
    std::cout << '\n' ;

// print column #11 (the last column)
    for( auto iter = std::next( std::begin(coll), 11 ) ; iter != std::end(coll) ; iter.next_row() )
        std::cout << *iter << ' ' ;
    std::cout << '\n' ;
}

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Compilation time: 7,98 sec, absolute running time: 0,22 sec, absolute service time: 8,21 sec