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Consider the class tt(Strings), introduced in section ref(DESTRUCTOR),
once again. As it contains several primitive type data members as well as a
pointer to dynamically allocated memory it needs a constructor, a destructor,
and an overloaded assignment operator. In fact the class offers two
constructors: in addition to the default constructor it offers a constructor
expecting a tt(char const *const *) and a tt(size_t).
Now consider the following code fragment. The statement references are
discussed following the example:
verb( int main(int argc, char **argv)
{
Strings s1(argv, argc); // (1)
Strings s2; // (2)
Strings s3(s1); // (3)
s2 = s1; // (4)
})
itemization(
it() At 1 we see an i(initialization). The object
tt(s1) is initialized using tt(main)'s parameters: tt(Strings)'s second
constructor is used.
it() At 2 tt(Strings)'s emi(default constructor) is used, initializing
an empty tt(Strings) object.
it() At 3 yet another tt(Strings) object is created, using a
constructor accepting an existing tt(Strings) object. This form of
initializations has not yet been discussed. It is called a
hi(copy constructor) em(copy construction) and the constructor performing the
initialization is called the em(copy constructor). Copy constructions are also
encountered in the following form:
verb( Strings s3 = s1;)
This is a em(construction) and therefore an em(initialization). It is not
an em(assignment) as an assignment needs a left-hand operand that has already
been defined. bf(C++) allows the assignment syntax to be used for constructors
having only one parameter. It is somewhat deprecated, though.
it() At 4 we see a plain assignment.
)
In the above example three objects were defined, each using a different
constructor. The actually used constructor was deduced from the constructor's
argument list.
The copy constructor encountered here is new. It does not result in a
compilation error even though it hasn't been declared in the class
interface. This takes us to the following rule:
quote(
A copy constructor is (almost) always available, even if it isn't declared
in the class's interface.
)
The reason for the `(almost)' is given in section ref(MOVECONS).
The copy constructor made available by the compiler is also called the
i(trivial copy constructor). Its use can easily be suppressed (using the tt(=
delete) idiom). The trivial copy constructor performs a byte-wise copy
operation of the existing object's primitive data to the newly created object,
calls copy constructors to intialize the object's class data members from
their counterparts in the existing object and, when inheritance is used, calls
the copy constructors of the base class(es) to initialize the new object's
base classes.
Consequently, in the above example the trivial copy constructor is
used. As it performs a byte-by-byte copy operation of the object's
primitive type data members that is exactly what happens at statement 3.
By the time tt(s3) ceases to exist its destructor deletes its array of
strings. Unfortunately tt(d_string) is of a primitive data type and so it also
deletes tt(s1)'s data. Once again we encounter wild pointers as a result of an
object going out of scope.
The remedy is easy: instead of using the trivial copy constructor a copy
constructor must explicitly be added to the class's interface and its
definition must prevent the wild pointers, comparably to the way this was
realized in the overloaded assignment operator. An object's dynamically
allocated memory is em(duplicated), so that it contains its own allocated
data. But note that if a class also reserves extra (raw) memory, i.e., if it
supports extra memory capacity, then that unused extra capacity is not made
available in the copy-constructed object.
hi(capacity: reduce) Copy construction can therefore be used to shed
excess capacity. The copy constructor is simpler than the overloaded
assignment operator in that it doesn't have to delete previously allocated
memory. Since the object is going to be created no memory has already been
allocated.
tt(Strings)'s copy constructor can be implemented as follows:
verb( Strings::Strings(Strings const &other)
:
d_string(new string[other.d_size]),
d_size(other.d_size)
{
for (size_t idx = 0; idx != d_size; ++idx)
d_string[idx] = other.d_string[idx];
})
The copy constructor is always called when an object is initialized using
another object of its class. Apart from the plain copy construction that we
encountered thus far, here are other situations where the copy constructor is
used:
itemization(
it() it is used when a function defines a hi(object: parameter) class type
value parameter rather than a pointer or a reference. The function's argument
initializes the function's parameter using the copy constructor. Example:
verb(void process(Strings store) // no pointer, no reference
{
store.at(3) = "modified"; // doesn't modify `outer'
}
int main(int argc, char **argv)
{
Strings outer(argv, argc);
process(outer);
})
it() it is used when a function defines a class type value return type.
Example:
verb(Strings copy(Strings const &store)
{
return store;
})
)
Here tt(store) is used to initialize tt(copy)'s return value. The returned
tt(Strings) object is a temporary, anonymous object that may be
immediately used by code calling tt(copy) but no assumptions can be made about
its lifetime thereafter.
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