Concatenating two std::vectors

Question

How do I concatenate two std::vectors?

Answer 1

Here's a general purpose solution using C++11 move semantics:

template 
std::vector concat(const std::vector& lhs, const std::vector& rhs)
{
    if (lhs.empty()) return rhs;
    if (rhs.empty()) return lhs;
    std::vector result {};
    result.reserve(lhs.size() + rhs.size());
    result.insert(result.cend(), lhs.cbegin(), lhs.cend());
    result.insert(result.cend(), rhs.cbegin(), rhs.cend());
    return result;
}

template 
std::vector concat(std::vector&& lhs, const std::vector& rhs)
{
    lhs.insert(lhs.cend(), rhs.cbegin(), rhs.cend());
    return std::move(lhs);
}

template 
std::vector concat(const std::vector& lhs, std::vector&& rhs)
{
    rhs.insert(rhs.cbegin(), lhs.cbegin(), lhs.cend());
    return std::move(rhs);
}

template 
std::vector concat(std::vector&& lhs, std::vector&& rhs)
{
    if (lhs.empty()) return std::move(rhs);
    lhs.insert(lhs.cend(), std::make_move_iterator(rhs.begin()), std::make_move_iterator(rhs.end()));
    return std::move(lhs);
}

Note how this differs from appending to a vector.