Smart pointers help solve memory management issues like leaks by automatically freeing memory when an object goes out of scope. They support the RAII idiom where resource acquisition is tied to object lifetime. Common smart pointers include std::shared_ptr, std::unique_ptr, std::weak_ptr, which help avoid leaks from exceptions or cycles in object graphs. make_shared is preferable to separate allocation as it can allocate the object and control block together efficiently in one allocation.
3. Introduction
In programming, the use of pointers are the main source of
errors (or bugs) when developing code.
The main problem found are the occurrences of memory
leaks, this is due to the way pointers interact with memory,
such as allocation and deallocation, which when
performed inefficiently can cause the pointer to hang (or
dangle, meaning that the pointer points to a previous
removed object).
The solution to this problem is the use of Smart Pointers.
4. RAII
This is a programming idiom, In RAII, holding a
resource is tied to object lifetime: resource
allocation (acquisition) is done during object
creation (specifically initialization), by the
constructor, while resource deallocation (release)
is done during object destruction, by the
destructor. If objects are destructed properly,
resource leaks do not occur.
5. Goal of RAII
The main goal of this idiom is->
1] To ensure that resource acquisition occurs at the same
time that the object is initialized, so that all resources for
the object are created and made ready in one line of
code.
2] The resource is automatically freed when the object
gets out of scope.
7. Smart pointers
Smart pointers are crucial to the RAII or Resource Acquisition Is
Initialialization programming idiom.
Smart pointers are class objects that behave like built-in pointers.
They also support pointer operations like dereferencing (operator *)
and indirection (operator ->).
To be smarter than regular pointers, smart pointers need to do things
that regular pointers don't. What could these things be? Probably the
most common bugs in C++ (and C) are related to pointers and
memory management: dangling pointers, memory leaks, allocation
failures and other joys. Having a smart pointer take care of these things
can save a lot of aspirin..
9. It is common to write code such
as..
void myFunction()
{
myClass *p (new myClass());
p->Dosomething();
delete p;
}
10. Example of smart pointers
This code may work. But what if somewhere in the function body an
exception gets thrown? Suddenly, the delete code never gets called!
In this case a memory leak waiting to happen.
However the use of a smart pointer will remove this threat due to the
automatic clean up of the pointer because the pointer will be cleaned
up whenever it gets out of scope, whether it was during the normal
path of execution or during an exception.
Auto_ptr: the simplest smart pointer to use. For situations when there
are no special requirements.
11. auto_ptr
auto_ptr is a class template available in the
C++ Standard Library (declared in the
<memory> header file) that provides some
basic RAII features for C++ raw pointers.
The auto_ptr template class describes an
object that stores a pointer to a single
allocated object of type Type* that ensures
that the object to which it points gets
destroyed automatically when control leaves a
scope.
12. auto_ptr
void myFunction()
{
auto_ptr<myClass> *p (new myClass());
p->DoSomething();
//delete obj; /*memory allocated will
automatically be freed*/
}
13. An Example of Smart Pointers
void foo()
{
MyClass* p(new
MyClass);
p->DoSomething();
delete p;
}
void foo()
{
auto_ptr<MyClass>
p(new MyClass);
p->DoSomething();
}
14. auto_ptr
The auto_ptr has semantics of strict ownership, meaning that the
auto_ptr instance is the sole entity responsible for the object's lifetime. If
an auto_ptr is copied, the source loses the reference. For example:
MyClass* p(new MyClass);
MyClass* q = p;
delete p;
p->DoSomething(); // Watch out! p is now dangling!
p = NULL; // p is no longer dangling
q->DoSomething(); // q is still dangling!
15. An Example of Smart Pointers
For auto_ptr, this is solved by setting its pointer to NULL when it is copied:
int main(int argc, char **argv)
{
int *i = new int;
auto_ptr<int> x(i);
auto_ptr<int> y;
y = x;
cout << x.get() << endl; // Print NULL
cout << y.get() << endl; // Print non-NULL address i
16. An Example of Smart Pointers
This code will print a NULL address for the first
auto_ptr object and some non-NULL address for
the second, showing that the source object lost
the reference during the assignment (=). The raw
pointer i in the example should not be deleted, as
it will be deleted by the auto_ptr that owns the
reference. In fact, new int could be passed
directly into x, eliminating the need for i.
17. #include <memory> // for std::auto_ptr
#include <stdlib.h> // for EXIT_SUCCESS
using namespace std;
typedef struct { int a, b; } IntPair;
int main(int argc, char **argv) {
auto_ptr<int> x(new int(5));
// Return a pointer to the pointed-to object.
int *ptr = x.get();
// Return a reference to the value of the pointed-to object.
int val = *x;
// Access a field or function of a pointed-to object.
auto_ptr<IntPair> ip(new IntPair);
ip->a = 100;
// Reset the auto_ptr with a new heap-allocated object.
x.reset(new int(1));
// Release responsibility for freeing the pointed-to object.
ptr = x.release();
delete ptr;
return EXIT_SUCCESS;
20. Problem with auto_ptr
The C++ Standard says that an STL element must be "copy-constructible" and
"assignable." In other words, an element must be able to be assigned or copied
and the two elements are logically independent. std::auto_ptr does not fulfill this
requirement. For example-void
foo() {
vector<auto_ptr< > > ivec;
ivec.push_back(auto_ptr< >(new (5)));
ivec.push_back(auto_ptr< >(new (6)));
auto_ptr< > z = ivec[0];
}
21. Problem with auto_ptr
To overcome this limitation, you should use
the std::unique_ptr, std::shared_ptr or
std::weak_ptr smart pointers or the boost
equivalents if you don't have C++11.
22. shared_ptr
Shared pointer is a smart pointer (a C++ object wih
overloaded operator*() and operator->())
It keeps a pointer to an object and a pointer to a shared
reference count.
Every time a copy of the smart pointer is made using the
copy constructor, the reference count is incremented.
When a shared pointer is destroyed, the reference count
for its object is decremented.
After counts goes to zero then managed object
automatically get deleted.
23. int main(int argc, char **argv) {
// x contains a pointer to an int and has reference count 1.
boost::shared_ptr<int> x(new int(10));
{
// x and y now share the same pointer to an int, and they
// share the reference count; the count is 2.
boost::shared_ptr<int> y = x;
std::cout << *y << std::endl;
}
// y fell out of scope and was destroyed. Therefore, the
// reference count, which was previously seen by both x and y,
// but now is seen only by x, is decremented to 1.
return EXIT_SUCCESS;
}
24. Finally, something that works!
it is safe to store shared_ptrs in containers, since
copy/assign maintain a shared reference count and
pointer-
26. How they work
The process starts when the managed object is dynamically allocated,
and the first shared_ptr (sp1) is created to point to it; the shared_ptr
constructor creates a manager object (dynamically allocated). The
manager object contains a pointer to the managed object; the
overloaded member functions like shared_ptr::operator-> access the
pointer in the manager object to get the actual pointer to the
managed object.1 The manager object also contains two reference
counts: The shared count counts the number of shared_ptrs pointing to
the manager object, and the weak count counts the number of
weak_ptrs pointing to the manager object. When sp1 and the
manager object are first created, the shared count will be 1, and the
weak count will be 0.
28. How they work….
If another shared_ptr (sp2) is created by copy or assignment from sp1,
then it also points to the same manager object, and the copy
constructor or assignment operator increments the shared count to
show that 2 shared_ptrs are now pointing to the managed object.
Likewise, when a weak pointer is created by copy or assignment from
a shared_ptr or another weak_ptr for this object, it points to the same
manager object, and the weak count is incremented. The diagram
shows the situation after three shared_ptrs and two weak_ptrs have
been created to point to the same object.
29. Problem with shared_ptr
If you used shared_ptr and have a cycle in the
sharing graph, the reference count will never hit
zero.
30. cycle of shared_ptr’s
#include <boost/shared_ptr.hpp>
boost::shared_ptr;
A {
shared_ptr<A> next;
shared_ptr<A> prev;
};
int main(int argc, char **argv) {
shared_ptr<A> head( A());
head->next = shared_ptr<A>( A());
head->next->prev = head;
}
2
0
2
1
next
prev
0
next
prev
head
31. breaking the cycle with weak_ptr
A {
shared_ptr<A> next;
weak_ptr<A> prev;
};
int main(int argc, char **argv) {
shared_ptr<A> head(new A());
head->next = shared_ptr<A>(new A());
head->next->prev = head;
}
1
0
1
1
next
prev
0
next
prev
head
33. use of make_shared to create an
object
If you need to create an object using a custom allocator, you can use
make_shared. So, why use make_shared ? There are two main reasons:
simplicity, and efficiency.
First, with make_shared the code is simpler. Write for clarity and
correctness first.
Second, using make_shared is more efficient.
The shared_ptr implementation has to maintain housekeeping
information in a control block shared by all shared_ptrs
and weak_ptrs referring to a given object. In particular, that
housekeeping information has to include not just one but two
reference counts:
34. use of make_shared to create an
object
A “strong reference” count to track the number
of shared_ptrs currently keeping the object alive.
A “weak reference” count to track the number
of weak_ptrs currently observing the object.
Example-sp1
= shared_ptr<widget>{ new widget{} };
sp2 = sp1
36. use of make_shared to create an
object
We’d like to avoid doing two separate allocations
here. If you use make_shared to allocate the
object and the shared_ptr all in one go, then the
implementation can fold them together in a single
allocation, as shown in Example-sp1
= make_shared<widget>();
sp2 = sp1;
38. use of make_shared to create an
object
It reduces allocation overhead, including
memory fragmentation.
It improves locality. The reference counts
are frequently used with the object, and
for small objects are likely to be on the
same cache line, which improves cache
performance.
