[string.substr]

constexpr basic_string substr(size_type pos = 0, size_type n = npos) const &;

Effects: Equivalent to: return basic_string(*this, pos, n);

constexpr basic_string substr(size_type pos = 0, size_type n = npos) &&;

Effects: Equivalent to: return basic_string(std::move(*this), pos, n);

constexpr basic_string_view<charT, traits> subview(size_type pos = 0, size_type n = npos) const;

Effects: Equivalent to: return basic_string_view<charT, traits>(*this).subview(pos, n);

27 Strings library [strings]