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Before using the tt(unique_ptr) class presented in this section the
tthi(memory) header file must be included.
When pointers are used to access dynamically allocated memory strict
bookkeeping is required to prevent memory leaks. When a pointer variable
referring to dynamically allocated memory goes out of scope, the dynamically
allocated memory becomes inaccessible and the program suffers from a
i(memory leak).
Consequently, the programmer has to make sure that the dynamically
allocated memory is returned to the common pool just before the pointer
variable goes out of scope.
When a pointer variable points to a dynamically allocated single value or
object, bookkeeping requirements are greatly simplified when the pointer
variable is defined as a tt(std::unique_ptr)hi(unique_ptr) object.
Unique_ptrs are em(objects) masquerading as pointers. Since they are objects,
their destructors are called when they go out of scope. Their destructors
automatically delete the dynamically allocated memory to which they
point. Unique_ptrs (and their cousins shared_ptrs (cf. section ref(SHAREDPTR))
are also called
hi(smart pointer)em(smart pointers)).
tt(Unique_ptrs) have several special characteristics:
itemization(
it() when assigning a tt(unique_ptr) to another em(move semantics) is
used. If move semantics is not available compilation fails. On the other
hand, if compilation succeeds then the used containers or generic algorithms
support the use of tt(unique_ptr)s. Here is an example:
verb(std::unique_ptr<int> up1(new int);
std::unique_ptr<int> up2(up1); // compilation error)
The second definition fails to compile as tt(unique_ptr)'s copy
constructor is private (the same holds true for the assignment operator). But
the tt(unique_ptr) class em(does) offer facilities to initialize and assign
from em(rvalue references):
verb(class unique_ptr // interface partially shown
{
public:
unique_ptr(unique_ptr &&tmp); // rvalues bind here
private:
unique_ptr(const unique_ptr &other);
};)
In the next example move semantics is used and so it compiles correctly:
verb(unique_ptr<int> cp(unique_ptr<int>(new int));)
it() a tt(unique_ptr) object should only point to memory that was made
available dynamically, as only dynamically allocated memory can be deleted.
it() multiple tt(unique_ptr) objects should not be allowed to point to the
same block of dynamically allocated memory. The tt(unique_ptr)'s interface was
designed to prevent this from happening. Once a tt(unique_ptr) object goes out
of scope, it deletes the memory it points to, immediately changing any other
object also pointing to the allocated memory into a hi(pointer: wild) wild
pointer.
it() When a class tt(Derived) is derived from tt(Base), then a newly
allocated tt(Derived) class object can be assigned to a tt(unique_ptr<Base>),
without having to define a virtual destructor for tt(Base). The tt(Base *)
pointer that is returned by the tt(unique_ptr) object can simply be cast
statically to tt(Derived), and tt(Derived's) destructor is automatically
called as well, if the tt(unique_ptr) definition is provided with a
em(deleter) function address. This is illustrated in the next example:
verb( class Base
{ ... };
class Derived: public Base
{
...
public:
// assume Derived has a member void process()
static void deleter(Base *bp);
};
void Derived::deleter(Base *bp)
{
delete static_cast<Derived *>(bp);
}
int main()
{
unique_ptr<Base, void (*)(Base *)> bp(new Derived, &Derived::deleter);
static_cast<Derived *>(bp.get())->process(); // OK!
} // here ~Derived is called: no polymorphism required.)
)
The class tt(unique_ptr) offers several member functions to access the
pointer itself or to have a tt(unique_ptr) point to another block of
memory. These member functions (and tt(unique_ptr) constructors) are
introduced in the next few sections.
tt(Unique_ptr) can also be used with containers and (generic) algorithms. They
can properly destruct any type of object, as their constructors accept
customizable deleters. In addition, arrays can be handled by tt(unique_ptrs).
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