miterator: Iterate over groups of containers in range based for loops

The following code is supposed to iterate over two vectors and sum over the product of their elements if each element is larger than zero. Can you spot the two bugs?

vector<int>  v1{ 1, -1, 2, -2, 3, -3, 4, -4 };
vector<int>  v2{ 5,  5, 6,  6 };
int result= 0;
auto it= v2.begin();
for( auto i1 : v1 )
{
  auto i2= *it;
  if( i1 <= 0 || i2 <= 0 ) continue;
  result += i1 * i2;
  ++it;
}

1. Iterator it is not increased in case the if statement triggers.
2. Vector v2 has less elements than v1. Therefore it will eventually point past the end of v2.

miterator

miterator is a header only library built for easing iteration over an arbitrary number of containers in range based for loops. With miterator we solve the task above as follows.

#include "miterator.hpp"
#include <vector>

using namespace miterator;

std::vector<int>  v1{ 1, -1, 2, -2, 3, -3, 4, -4 };
std::vector<int>  v2{ 5,  5, 6,  6 };

int result= 0;
for( auto const &p : make_miterator( v1, v2 ) )
{
  auto i1= *get<0>( p );
  auto i2= *get<1>( p );
  if( i1 <= 0 || i2 <= 0 ) continue;
  result += i1 * i2;
}

This can further be simplified using miterator::apply

int result= 0;
for( auto const &p : make_miterator( v1, v2 ) )
{
  apply( p, [&result]( int i1, int i2 )
            { if( i1 > 0 && i2 > 0 ) result += i1 * i2; } );
}

make_miterator returns an object of a proxy type which wraps the iterators of the containers. Each of these iterators is incremented automatically in each step of the for loop. Condition it != end is true only if for each of the underlying iterators it != end is true. This ensures that iteration ends as soon as one iterator points behind the last element.

get returns the wrapped iterator indexed by the template parameter.

apply takes the bunch of iterators, dereferences each of them and submits the resulting values to the function, i.e. the second argument of apply. apply returns the result of the function.

See the code in the test directory for further examples.

Stream iterators

Stream iterators are supported via surrogate containers defined in stream_wrapper.hpp. The following example shows how they can be used.

#include "miterator.hpp"
#include "stream_wrapper.hpp"
#include <iostream>
#include <sstream>
using namespace miterator;
std::istringstream  str( "0.5 0.6 0.7 0.8" );
auto  sc1= make_ostream_cont<double>( std::cout, ", " );
auto  sc2= make_istream_cont<double>( str );
for( auto &p : make_miterator( sc1, sc2 ) )
     *get<0>( p )= *get<1>( p ); // 0.5, 0.6, 0.7, 0.8,

ostream iterators are supported for symmetry reasons only. Since increment of an ostream iterator is a noop, there is actually no need for wrapping them.

Const correctness

miterator is written such that const correctness is ensured. Therefore none of the following assignments compiles.

std::vector<int>   v1{ 1, 2, 3 };
int const          v2[]= { -2, -3, -4, -5 };
for( auto const &p : make_miterator( v1, v2 ) )
{
  *get<0>( p )= *get<1>( p ); // Must not compile since p is const
}
for( auto &p : make_miterator( v1, v2 ) )
{
  *get<1>( p )= *get<0>( p ); // Must not compile since v2 is const
}

Caveats

• miterator requires a C++14 compliant compiler.
• Tested with gcc 4.9 and 5.2.
• A miterator does not fulfill the requirements of an iterator. It might not be used anywhere else than in range based for loops.