https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110945
--- Comment #15 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jonathan Wakely <r...@gcc.gnu.org>: https://gcc.gnu.org/g:cc3d7baf2741777e99567d4301802c99f5775619 commit r14-3306-gcc3d7baf2741777e99567d4301802c99f5775619 Author: Jonathan Wakely <jwak...@redhat.com> Date: Tue Aug 8 16:31:42 2023 +0100 libstdc++: Optimize std::string::assign(Iter, Iter) [PR110945] Calling string::assign(Iter, Iter) with "foreign" iterators (not the string's own iterator or pointer types) currently constructs a temporary string and then calls replace to copy the characters from it. That means we copy from the iterators twice, and if the replace operation has to grow the string then we also allocate twice. By using *this = basic_string(first, last, get_allocator()) we only perform a single allocation+copy and then do a cheap move assignment instead of a second copy (and possible allocation). But that alternative has to be done conditionally, so that we don't pessimize the native iterator case (the string's own iterator and pointer types) which currently select efficient overloads of replace which will not allocate at all if the string already has sufficient capacity. For C++20 we can extend that efficient case to work for any contiguous iterator with the right value type, not just for the string's native iterators. So the change is to inline the code that decides whether to work in place or to allocate+copy (instead of deciding that via overload resolution for replace), and for the allocate+copy case do a move assignment instead of another call to replace. For C++98 there is no change, as we can't do an efficient move assignment anyway, so keep the current code. We can also simplify assign(initializer_list<CharT>) because the backing array for an initializer_list is always disjunct with *this, so most of the code in _M_replace is not needed. libstdc++-v3/ChangeLog: PR libstdc++/110945 * include/bits/basic_string.h (basic_string::assign(Iter, Iter)): Dispatch to _M_replace or move assignment from a temporary, based on the iterator type.
