@KirinCupr: just a quick confirmation - you're completely right, the creator of the Kata has defined "input_str" as a constant here, so while we can read its value, we must not change/manipulate it! ... The C++ documentation says about the use of the keyword "const": "an object whose type is 'const-qualified', or a non-mutable subobject of a const object. Such object cannot be modified: attempt to do so directly is a compile-time error ..."

Hello. I'm think we create new variable because "input_str" is is const (const std::string& input_str). So we can't change it. If I'm wrong, please, correct me.