Is there a back_inserter variant that takes advant

问题:

In generic code I was trying to tell an output iterator (in practice a std::back_inserter_iterator to move a range of elements. To my surprise it looked as if elements were moved in a move-to-back_inserter operation.

#include<algorithm> // move
#include<iterator> // back_inserter
#include<vector>
int main(){
    std::vector<std::vector<double> > v(10, std::vector<double>(100));
    std::vector<std::vector<double> > w;

    assert( not v[0].empty() and w.size() == 0 );
    std::copy(v.begin(), v.end(), std::back_inserter(w));
    assert( not v[0].empty() and w.size() == 10 );

    std::move(v.begin(), v.end(), std::back_inserter(w));
    assert( v[0].empty() and w.size() == 20 ); // were v elements moved to w?
}

However I don't think it is possible that elements of v were really moved to w, because after all back_inserter will do a push_back which implies a copy to w.

It seems more likely that in the std::move case, v elements were moved to a temporary and only then copied into w.

Is that correct? Is there a std::back_inserter really moving somehow?

Is there already a variant of std::back_inserter that takes advantage of move/emplace? Something like an std::back_emplacer?

回答1:

If std::back_insert_iterator does nothing but calling push_back on the container it's being constructed with, the question can be answered looking at the std::vector::push_back overload set:

void push_back(const T& value);
void push_back(T&& value);

So, here we obviously have an overload for rvalue references. Wouldn't it be weird, if the corresponding std::back_insert_iterator copies its argument when invoked with an rvalue reference? And indeed, we again have a very similar overload set:

back_insert_iterator<Container>&
    operator=(typename Container::const_reference value);
back_insert_iterator<Container>&
    operator=(const typename Container::value_type& value);

with the additional comment that

1) Results in container->push_back(value)
2) Results in container->push_back(std::move(value))

Coming back to your original question

Is there a std::back_inserter really moving somehow?

Yes, the iterator this function creates does take advantage of rvalue reference arguments.

Is there already a variant of std::back_inserter that takes advantage of move/emplace?

No, but it can be implemented. See this answer.

回答2:

std::back_inserter() is a convenience function template for creating an std::back_insert_iterator object.

The class std::back_insert_iterator has already operator= overloaded for taking rvalue reference types, i.e.:

back_insert_iterator<Container>& operator=(typename Container::value_type&& value);

Therefore, std::back_insert_iterator is already prepared to take advantage of move semantics.