Concatenating two std::vectors

Question

How do I concatenate two std::vectors?

Answer 1

I've implemented this function which concatenates any number of containers, moving from rvalue-references and copying otherwise

namespace internal {

// Implementation detail of Concatenate, appends to a pre-reserved vector, copying or moving if
// appropriate
template
void AppendNoReserve(Target* target, Head&& head, Tail&&... tail) {
    // Currently, require each homogenous inputs. If there is demand, we could probably implement a
    // version that outputs a vector whose value_type is the common_type of all the containers
    // passed to it, and call it ConvertingConcatenate.
    static_assert(
            std::is_same_v<
                    typename std::decay_t::value_type,
                    typename std::decay_t::value_type>,
            "Concatenate requires each container passed to it to have the same value_type");
    if constexpr (std::is_lvalue_reference_v) {
        std::copy(head.begin(), head.end(), std::back_inserter(*target));
    } else {
        std::move(head.begin(), head.end(), std::back_inserter(*target));
    }
    if constexpr (sizeof...(Tail) > 0) {
        AppendNoReserve(target, std::forward(tail)...);
    }
}

template
size_t TotalSize(const Head& head, const Tail&... tail) {
    if constexpr (sizeof...(Tail) > 0) {
        return head.size() + TotalSize(tail...);
    } else {
        return head.size();
    }
}

}  // namespace internal

/// Concatenate the provided containers into a single vector. Moves from rvalue references, copies
/// otherwise.
template
auto Concatenate(Head&& head, Tail&&... tail) {
    size_t totalSize = internal::TotalSize(head, tail...);
    std::vector::value_type> result;
    result.reserve(totalSize);
    internal::AppendNoReserve(&result, std::forward(head), std::forward(tail)...);
    return result;
}