When std::seed_seq is constructed from random access iterators we can detect the internal vector size in O(1). Reserving memory for elements in such cases may avoid multiple memory allocations.
libstdc++-v3/ChangeLog: * include/bits/random.tcc: Optimize seed_seq construction. -- Best regards, Antony Polukhin
diff --git a/libstdc++-v3/include/bits/random.tcc b/libstdc++-v3/include/bits/random.tcc index bf43970..816bfc1 100644 --- a/libstdc++-v3/include/bits/random.tcc +++ b/libstdc++-v3/include/bits/random.tcc @@ -3234,14 +3234,31 @@ namespace __detail template<typename _IntType> seed_seq::seed_seq(std::initializer_list<_IntType> __il) { + _M_v.reserve(__il.size()); for (auto __iter = __il.begin(); __iter != __il.end(); ++__iter) _M_v.push_back(__detail::__mod<result_type, __detail::_Shift<result_type, 32>::__value>(*__iter)); } + template<typename _Vector, typename _InputIterator> + void __reserve_if_distance_cheap(_Vector& __vec, _InputIterator __begin, + _InputIterator __end, random_access_iterator_tag) + { + __vec.reserve(__end - __begin); + } + + template<typename _Vector, typename _InputIterator, typename _Tag> + void __reserve_if_distance_cheap(_Vector&, _InputIterator, + _InputIterator, _Tag) + { + // computing the distance between __begin and __end is not O(1) + } + template<typename _InputIterator> seed_seq::seed_seq(_InputIterator __begin, _InputIterator __end) { + std::__reserve_if_distance_cheap(_M_v, __begin, __end, + typename iterator_traits<_InputIterator>::iterator_category()); for (_InputIterator __iter = __begin; __iter != __end; ++__iter) _M_v.push_back(__detail::__mod<result_type, __detail::_Shift<result_type, 32>::__value>(*__iter));