The definition of a constructor may contain an initialization section in the function heading. For example, suppose you have the following class definition:

class Rational
{
public:
   Rational( );
   Rational(int whole_number);
   Rational(int numerator, int denominator);
   ...
private:
   int top;
   int bottom;
};

//Initializes top to 0 and bottom to 1
Rational::Rational( ) : top(0), bottom(1)
{
   //Empty body
}

//Initializes top to whole_number and bottom to 1
Rational::Rational(int whole_number) : top(whole_number), bottom(1)
{
   //Empty body
}

//Initializes top to numerator and bottom to denominator
Rational::Rational(int numerator, int denominator)
                : top(numerator), bottom(denominator)
{
   //Empty body
}

The constructor can then be called either with no parameter, one parameter or two parameters.