Construction a vector from the concatenation of 2 vectors

Question

Is there a way to construct a vector as the concatenation of 2 vectors (Other than creating a helper function?)

For example:

Answer 1

I came across this question looking for the same thing, and hoping there was an easier way than the one I came up with... seems like there isn't.

So, some iterator trickery should do it if you don't mind a helper template class:

#include 
#include 

template
class concat
{
public:
    using value_type = typename std::vector::const_iterator::value_type;
    using difference_type = typename std::vector::const_iterator::difference_type;
    using reference = typename std::vector::const_iterator::reference;
    using pointer = typename std::vector::const_iterator::pointer;
    using iterator_category = std::forward_iterator_tag;
    concat(
        const std::vector& first,
        const std::vector& last,
        const typename std::vector::const_iterator& iterator) : 
            mFirst{first},
            mLast{last},
            mIterator{iterator}{}
    bool operator!= ( const concat& i ) const
    {
        return mIterator != i.mIterator;
    }
    concat& operator++()
    {
        ++mIterator;
        if(mIterator==mFirst.end())
        {
            mIterator = mLast.begin();
        }
        return *this;
    }
    reference operator*() const
    {
        return *mIterator;
    }
private:
    const std::vector& mFirst;
    const std::vector& mLast;
    typename std::vector::const_iterator mIterator;
};

int main()
{
    const std::vector first{0,1,2,3,4};
    const std::vector last{5,6,7,8,9};
    const std::vector concatenated(
        concat(first,last,first.begin()),
        concat(first,last,last.end()));
    for(auto i: concatenated)
    {
        std::cout << i << std::endl;
    }
    return 0;
}

You may have to implement operator++(int) or operator== depending on how your STL implements the InputIterator constructor, this is the minimal iterator code example I could come up with for MingW GCC.

Have Fun! :)