Is it valid to use std::transform with std::back_inserter?

Question

Cppreference has this example code for std::transform:

std::vector ordinals;
std::transform(s.begin(), s.end(), std::back_inserter(ordinals),
         


        

Answer 1

From n4385:

§25.6.4 Transform:

template
constexpr OutputIterator
transform(InputIterator first1, InputIterator last1, OutputIterator result, UnaryOperation op);

template
ForwardIterator2
transform(ExecutionPolicy&& exec, ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 result, UnaryOperation op);

template
constexpr OutputIterator
transform(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, OutputIterator result, BinaryOperation binary_op);

template
ForwardIterator
transform(ExecutionPolicy&& exec, ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator result, BinaryOperation binary_op);

§23.5.2.1.2 back_inserter

template
constexpr back_insert_iterator back_inserter(Container& x);

Returns: back_insert_iterator(x).

§23.5.2.1 Class template back_insert_iterator

using iterator_category = output_iterator_tag;

So std::back_inserter can't be used with parallel versions of std::transform. The versions that support output iterators read from their source with input iterators. Since input iterators can only be pre- and post-incremented (§23.3.5.2 Input iterators) and there is only sequential (i.e. non-parallel) execution, order must be preserved between them and the output iterator.