std::basic_string<ch>&

This is a template class in C++ that represents a sequence of characters. The "ch" can be any character type, such as char, wchar_t, or char16_t. The basic_string class provides various operations for manipulating strings, such as concatenation, substring extraction, finding a character or substring, replacing a character or substring, and so on. It also provides iterators to traverse the elements of the string. The basic_string class is part of the standard C++ library and is typically used to represent text or character-based data in C++ programs.