A derived class inherits functionality from its base class (or base classes,
as bf(C++) supports multiple inheritance, cf. section ref(MULTIPLE)). When a
derived class object is constructed it is built on top of its base class
object. As a consequence the base class must have been constructed before the
actual derived class elements can be initialized. This results in some
requirements that must be observed when defining
 hi(constructor: of derived classes) derived class constructors.

A constructor exists to initialize the object's data members. A derived class
constructor is also responsible for the proper initialization of its base
class. Looking at the definition of the class tt(Land) introduced earlier
(section ref(VehicleSystem)), its constructor could simply be defined as
follows:
        verb(    Land::Land(size_t mass, size_t speed)
    {
        setMass(mass);
        setSpeed(speed);
    })

However, this implementation has several disadvantages.
    itemization(
    it() When constructing a derived class object a base class constructor is
em(always) called before any action is performed on the derived class object
itself. By default the base class's i(default constructor) is going to be
called.
    it() Using the base class constructor only to reassign new values to its
data members in the derived class constructor's body usually is inefficient,
but sometimes sheer impossible as in situations where base class reference or
tt(const) data members must be initialized. In those cases a specialized base
class constructor must be used instead of the base class default constructor.
    )
    A derived class's base class may be initialized using a dedicated base
class constructor by calling the base class constructor in the derived class
constructor's initializer clause. Calling a base class constructor in a
constructor's initializer clause is called a
 emi(base class initializer). The base class initializer must be called before
initializing any of the derived class's data members and when using the base
class initializer none of the derived class data members may be used. When
constructing a derived class object the base class is constructed first and
only after that construction has successfully completed
the derived class data members are available for initialization. tt(Land)'s
constructor may therefore be improved:
        verb(    Land::Land(size_t mass, size_t speed)
    :
        Vehicle(mass),
        d_speed(speed)
    {})

Derived class constructors always by default call their base class's
default constructor. This is of course not correct for a derived class's
 i(copy constructor). Assuming that the class tt(Land) must be provided with a
copy constructor its tt(Land const &other) parameter also represents the other
object's base class:
        verb(    Land::Land(Land const &other)   // assume a copy constructor is needed
    :
        Vehicle(other),             // copy-construct the base class part.
        d_speed(other.d_speed)      // copy-construct Land's data members
    {})

