operator+ and move semantics

问题

How is the proper way to implement move semantics with operator+? Similarly to how it works for std::string?

I have attempted the following, however I was hoping there was some more elegant and possibly more correct way to do it:

class path
{
    std::vector<std::string> path_;
public:

    path& path::operator+=(const path& other)
    {
        path_.insert(std::begin(path_), std::begin(other.path_), std::end(other.path_));
        return *this;
    }

    path& path::operator+=(path&& other)
    {
        path_.insert(std::begin(path_), std::make_move_iterator(std::begin(other.path_)), std::make_move_iterator(std::end(other.path_)));
        return *this;
    }
};

template<typename L, typename R>
typename std::enable_if<std::is_convertible<path, L>::value, path>::type operator+(const L& lhs, const R& rhs)
{
    auto tmp = std::forward<L>(lhs);
    tmp     += std::forward<R>(rhs);
    return tmp;
}

回答1:

Way too complicated. :) Just abide by the rule you already should've followed:

Take the lhs of operator+ by value
implement operator+ in terms of operator+= on the lhs

This was already true in C++03, because of copy elision and RVO. The rule of thumb: If you make a copy anyways, make it in the parameters.

With that in mind:

#include <iterator>
#include <utility>

class path
{
    std::vector<std::string> path_;
public:

    path& operator+=(path other)
    {
        auto op_begin = std::make_move_iterator(std::begin(other.path_));
        auto op_end = std::make_move_iterator(std::end(other.path_));
        path_.reserve(path_.size() + other.path_.size());
        path_.insert(std::end(path_), op_begin, op_end);
        return *this;
    }
};

path operator+(path lhs, path rhs)
{
  return std::move(lhs += std::move(rhs));
}

This should be the most optimal form. Note that I also changed your operator+= to actually append the path, and not prepend (I hope that's what you had in mind. If not, feel free to change it to std::begin(path_) again).

I also made the rhs of operator+ and operator+= values and then just moved them around. std::make_move_iterator is also a nice utility. As the name implies, instead of copying, it moves the pointed-at elements. This should really be as fast as it's going to get.

Another version might be to use the iterator version of std::move in operator+=:

path& operator+=(path other)
{
    path_.reserve(path_.size() + other.path_.size());
    std::move(other.begin(), other.end(), std::back_inserter(path_));
    return *this;
}

来源：https://stackoverflow.com/questions/8687701/operator-and-move-semantics

标签

c++

move

addition