Method volume

Exercises/NoModules/Chapter 13/Soln13_04/Box.h:15–15

  Box(double l, double w, double h) : m_length{ l }, m_width{ w }, m_height{ h } {}

  double volume() const { return m_length * m_width * m_height; }

  // Accessors
  double getLength() const { return m_length; }