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Unless specified otherwise object data members of classes are initialized by
their default constructors. Using the default constructor might not always be
the optimal way to intialize an object and it might not even be possible: a
class might simply not define a default constructor.
Earlier we've encountered the following constructor of the tt(Person):
verb(
Person::Person(string const &name, string const &address,
string const &phone, size_t mass)
{
d_name = name;
d_address = address;
d_phone = phone;
d_mass = mass;
}
)
Think briefly about what is going on in this constructor. In the
constructor's body we encounter assignments to string objects. Since
assignments are used in the constructor's body their left-hand side objects
must exist. But when objects are coming into existence constructors em(must)
have been called. The initialization of those objects is thereupon immediately
undone by the body of tt(Person)'s constructor. That is
not only inefficient but sometimes downright impossible. Assume that the class
interface mentions a tt(string const) data member: a data member whose value
is not supposed to change at all (like a birthday, which usually doesn't
change very much and is therefore a good candidate for a tt(string const) data
member). Constructing a birthday object and providing it with an initial value
is OK, but changing the initial value isn't.
The body of a constructor allows assignments to data members. The
em(initialization) of data members happens before that. bf(C++) defines the
emi(member initializer) syntax allowing us to specify the way data members
are initialized at construction time. Member initializers are specified as a
list of constructor specifications between a colon following a constructor's
parameter list and the opening curly brace of a constructor's body, as
follows:
verb(
Person::Person(string const &name, string const &address,
string const &phone, size_t mass)
:
d_name(name),
d_address(address),
d_phone(phone),
d_mass(mass)
{}
)
Member initialization em(always) occurs when objects are composed in
classes: if em(no) constructors are mentioned in the member initializer list
the default constructors of the objects are called. Note that this only holds
true for em(objects). Data members of primitive data types are em(not)
initialized automatically.
Member initialization can, however, also be used for primitive data members,
like tt(int) and tt(double). The above example shows the initialization of the
data member tt(d_mass) from the parameter tt(mass). When member
initializers are used the data member could even have the same name as the
constructor's parameter (although this is deprecated) as there is no ambiguity
and the first (left) identifier used in a member initializer is always a data
member that is initialized whereas the identifier between parentheses is
interpreted as the parameter.
The em(order) in which class type data members are initialized is defined by
the order in which those members are defined in the composing class
interface. If the order of the initialization in the constructor differs from
the order in the class interface, the compiler complains, and reorders the
initialization so as to match the order of the class interface.
Member initializers should be used as often as possible. As shown it may be
required to use them (e.g., to initialize const data members, or to initialize
objects of classes lacking default constructors) but em(not) using member
initializers also results in inefficient code as the default constructor of a
data member is always automatically called unless an explicit member
initializer is specified. Reassignment in the constructor's body following
default construction is then clearly inefficient. Of course, sometimes it is
fine to use the default constructor, but in those cases the explicit member
initializer can be omitted.
As a i(rule of thumb): if a value is assigned to a data member in the
constructor's body then try to avoid that assignment in favor of using a
member initializer.
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