[string.substr]
27 Strings library [strings]
27.4 String classes [string.classes]
27.4.3 Class template basic_string [basic.string]
27.4.3.8 String operations [string.ops]
27.4.3.8.3 basic_string::substr [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);