std::list and iterator

[ghost1111]

how do you return a value using std::list and iterator

here an example

include "example.h"

public
const example* getExample(unsigned int i) const
{
       return exampleOne[i];//i know i cant do this because of the []
} 

private:
std::list<example> exampleOne;

this is just an example it not really code is there something i call form the std::list to return a value?

[coder777]

Use advance(). See:

http://www.cplusplus.com/reference/iterator/advance/?kw=advance

[ghost1111]

Where does the 5 come form and how would that work with "return"

[Peter87]

5 is the index. In your code you probably want to use i instead.

[ghost1111]

What would you return instead of "return exampleOne [i]

[JLBorges]

#include <iostream>
#include <list>
#include <iterator>
#include <stdexcept>

namespace detail
{
    template < typename SEQUENCE_CONTAINER > // random access iterator
    typename SEQUENCE_CONTAINER::const_reference
    at( const SEQUENCE_CONTAINER& seq, typename SEQUENCE_CONTAINER::size_type pos, std::random_access_iterator_tag )
    { return seq.at(pos) ; }

    template < typename SEQUENCE_CONTAINER > // bidirectional iterator
    typename SEQUENCE_CONTAINER::const_reference
    at( const SEQUENCE_CONTAINER& seq, typename SEQUENCE_CONTAINER::size_type pos, std::bidirectional_iterator_tag )
    {
        const auto sz = seq.size() ;

        // check for valid position
        if( pos >= sz ) throw std::out_of_range( "invalid position" ) ;

        // if the position is in the first half of the sequence iterate forward from begin
        // http://en.cppreference.com/w/cpp/iterator/next
        if( pos <= sz/2 ) return *std::next( seq.begin(), pos ) ;

        // if the position is in the second half of the sequence iterate backwards from end
        else return *std::next( seq.rbegin(), sz-pos-1 ) ;
    }

    template < typename SEQUENCE_CONTAINER > // forward iterator
    typename SEQUENCE_CONTAINER::const_reference
    at( const SEQUENCE_CONTAINER& seq, typename SEQUENCE_CONTAINER::size_type pos, std::forward_iterator_tag )
    {
        const auto sz = seq.size() ;

        // check for valid position
        if( pos >= sz ) throw std::out_of_range( "invalid position" ) ;

        return *std::next( seq.begin(), pos ) ;
    }
}

template < typename SEQUENCE_CONTAINER >
typename SEQUENCE_CONTAINER::const_reference
at( const SEQUENCE_CONTAINER& seq, typename SEQUENCE_CONTAINER::size_type pos )
{ return detail::at( seq, pos, typename std::iterator_traits< decltype(seq.begin()) >::iterator_category{} ) ; }

template < typename SEQUENCE_CONTAINER >
typename SEQUENCE_CONTAINER::reference
at( SEQUENCE_CONTAINER& seq, typename SEQUENCE_CONTAINER::size_type pos )
{
    const auto& const_ref = at( const_cast< const SEQUENCE_CONTAINER& >(seq), pos ) ;
    return const_cast< typename SEQUENCE_CONTAINER::reference >(const_ref) ;
}

int main()
{
    struct example { int value ; /* ...... */ };

    std::list<example> my_list { {0}, {1}, {2}, {3}, {4}, {5} } ;

    try
    {
        for( std::size_t pos : { 1, 4, 9 } )
        {
            std::cout << "my_list[" << pos << "] == " ;
            example& ex = at( my_list, pos ) ;
            std::cout << "example{" << ex.value << "}\n" ;
        }
    }

    catch( const std::out_of_range& error )
    {
        std::cerr << " ***error: " << error.what() << '\n' ;
    }
}

http://coliru.stacked-crooked.com/a/2c419ec0af30ad4d
http://rextester.com/PKAK37352

[coder777]

What would you return instead of "return exampleOne [i]

include "example.h"

public
const example* getExample(unsigned int i) const
{
    std::list<int>::const_iterator it=exampleOne.begin();
    std::advance(it, i);
    return &*it;
} 

private:
std::list<example> exampleOne;