Is it safe to use parallel execution policies with std::back_inserter?

Question

I would like to know whether using execution policies std::execution::par and std::execution::par_unseq is safe when the output is std::back_insert_iterator . For example:

std::vector<std::string> src(100, "test"), dst;
std::move(std::execution::par_unseq, src.begin(), src.end(), std::back_inserter(dst));

Is it guaranteed that no concurrent insertions will be made to the destination vector?

Answer 1

Calling push_back on a std::vector does not match the requirements of par_unseq or par .

So no, it isn't safe.

By saying par you are promising that access to the iterators used for input and output contains no race conditions. That fails here.

par_unseq requires more promises from you; basically that you are both data-race free, and your algorithm is suitable for vectorization (doing things in batches).

push_back does not qualify, and back_inserter is defined in terms of push_back .

On the other hand:

std::vector<std::string> src(100, "test"), dst(100, "");
std::move(std::execution::par_unseq, src.begin(), src.end(), dst.begin());

this is valid, because you are allowed to parallel and concurrently access individual elements of a vector (except vector<bool> )