17.5.1 Stream Output <<

Output stream class (chptr. 12) provides ability to write a single character
Stream library provides insertion operator for all built-in types:
ostream& operator<<(ostream& out, double value);

Note: the stream itself is returned

E.g.: Overload output for Fraction

ostream& operator<<( ostream &out, const Fraction &value ) { out << value.numerator() << "/" << value.denominator(); return out; }