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An often heard complaint is that operator tt(new[]) calls the default
constructor
hi(new[]: and non-default constructors) of a class to initialize the
allocated objects. For example, to allocate an array of 10 strings we can do
verb(
new string[10];
)
but it is not possible to use another constructor. Assuming that we'd want
to initialize the strings with the text tt(hello world), we can't write
something like:
verb(
new string("hello world")[10];
)
The initialization of a dynamically allocated object usually consists of a
two-step process: first the array is allocated (implicitly calling the default
constructor); second the array's elements are initialized, as in the following
little example:
verb(
string *sp = new string[10];
fill(sp, sp + 10, string("hello world"));
)
These approaches all suffer from `i(double initialization)s', comparable
to not using member initializers in constructors.
One way to avoid double initialization is to use inheritance.
Inheritance can profitably be used to call non-default constructors in
combination with operator tt(new[]). The approach capitalizes on the
following:
itemization(
it() A base class pointer may point to a derived class object;
it() A derived class without (non-static)
hi(derived class vs. base class size)
hi(sizeof derived vs base classes)
data members has the same size as its base class.
)
The above also suggests a possible approach:
itemization(
it() Derive a simple, member-less class from the class we're interested
in;
it() Use the appropriate base class initializer in its default
constructor;
it() Allocate the required number of derived class objects, and assign
tt(new[])'s return expression to a pointer to base class objects.
)
Here is a simple example, producing 10 lines containing
the text tt(hello world):
verbinclude(-a examples/derivenew.cc)
Of course, the above example is fairly unsophisticated, but it's easy to
polish the example: the class tt(Xstr) can be defined in
an anonymous namespace, accessible only to a function tt(getString()) which
may be given a tt(size_t nObjects) parameter, allowing users to specify the
number of tt(hello world)-initialized strings they would like to allocate.
Instead of hard-coding the base class arguments it's also possible to use
variables or functions providing the appropriate values for the base class
constructor's arguments. In the next example a
emi(local class)hi(class: local) tt(Xstr) is defined inside a function
tt(nStrings(size_t nObjects, char const *fname)), expecting the number of
tt(string) objects to allocate and the name of a file whose subsequent lines
are used to initialize the objects. The local class
is invisible outside of the function tt(nStrings), so no special namespace
safeguards are required.
As discussed in section ref(LOCAL), members of local classes cannot access
local variables from their surrounding function. However, they can access
global and static data defined by the surrounding function.
Using a local class neatly allows us to hide the implementation details
within the function tt(nStrings), which simply opens the file, allocates the
objects, and closes the file again. Since the local class is derived from
tt(string), it can use any tt(string) constructor for its base class
initializer. In this particular case it calls the tt(string(char const *))
constructor, providing it with subsequent lines of the just opened stream via
its static member function tt(nextLine()). This latter function is, as it is a
static member function, available to tt(Xstr) default constructor's member
initializers even though no tt(Xstr) object is available by that time.
verbinclude(-a examples/nstrings.cc)
When this program is run, it displays the first 10 lines of the file
tt(nstrings.cc).
Note that the above implementation can't safely be used in a multithreaded
environment. In that case a emi(mutex) should be used to protect the three
statements just before the function's return statement.
A completely different way to avoid the double initialization (not using
inheritance) is to use placement new (cf. section ref(PLACEMENT)): simply
allocate the required amount of memory followed by the proper in-place
allocation of the objects, using the appropriate constructors. The following
example can also be used in multithreaded environments. The approach uses a
pair of static tt(construct/destroy) members to perform the required
initialization.
In the program shown below tt(construct) expects a tt(istream) that
provides the initialization strings for objects of a class tt(String) simply
containing a tt(std::string) object. tt(Construct) first allocates enough
memory for the tt(n) tt(String) objects plus room for an initial tt(size_t)
value. This initial tt(size_t) value is then initialized with tt(n). Next, in
a tt(for) statement, lines are read from the provided stream and the lines are
passed to the constructors, using placement new calls. Finally the address of
the first tt(String) object is returned.
The member tt(destroy) handles the destruction of the objects. It
retrieves the number of objects to destroy from the tt(size_t) it finds just
before the location of the address of the first object to destroy. The objects
are then destroyed by explicitly calling their destructors. Finally the raw
memory, originally allocated by tt(construct) is returned.
verbinclude(-a examples/placement.cc)
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