Three possibilities:
- if you're interested in ASCII only, make sure that the value is lesser than 128 and cast the value to a char
- if your locale is Latin-1 (or compatible), you can safely cast to char with values up to 255
- otherwise, if you want a real conversion from UTF-32 to your current 8-bits locale, you have to use the functions in sf::Unicode
Most people don't care about non-ASCII or non-Latin-1 characters, and just do a cast.