fmt and scn are made for formatted output and input of strings, which is not the same as what I’m proposing here.
They are better than iostream, and libc equivalents, at parsing and extracting single value such as integers and floats. There are other libs like this, like fastfloat, C++17 added similar function std::to/from_chars, but they can not represent a data source.
We can use both, the stream which just serves at getting the input data (using istream::read), and scn, or any other library such as expat for XML, for parsing it. Like this, we won’t have significant performance issues caused by iostreams, giving us the best of each world!