Hello,

Try to create Upp::String basing on std::string like this:

String(fields[0]);

IMO, you should use Core library to deal with text files. The easiest way is to use LoadFile(path):

Upp::String fileContent = Upp::LoadFile("PathToTextFile.txt");

// now you could use some powerfull functions from Core like Split;
Upp::Vector<String> fileContentLines = Upp::Split(fileContent, "\n");
for (const String& line : fileContentLines) {
    // Loop over the line...
}

I will start from rewriting you code from standard library into Upp approch. This could save you problems with file encoding conversion.