Let’s Get Familiar with Smart Pointers in C++

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Many readers have probably heard about smart pointers and their benefits. One of the features of C++11, smart pointers are quite useful and have an inbuilt mechanism to ensure a program is free from memory leaks.

Before beginning, let’s have a look at why people prefer smart pointers over normal pointers. When dealing with normal pointers, the onus is on programmers to allocate and release the resources or memory, in order to avoid memory leaks and exceptions. Programmers also need to prevent access to the resources that are already freed up, or it will lead to dangling references. That is why many beginners dislike normal or raw pointers.

Smart pointers and their types
Smart pointers are not just pointers; they are objects that are smarter. Unlike normal pointers, memory is released once the pointer goes out of scope.

unique_ptr: An object can be held by a pointer at a given time — multiple pointers cannot point to the same object. For example, consider a pointer obj1 pointing to an object; another pointer named obj2 cannot point to the object pointed by obj1. unique_ptr doesn’t allow copying of pointers, but it allows transfer of ownership from one pointer to another.

  • Allows exactly single ownership of the object.
  • No two different pointers can hold the same object.
  • Object cannot be shared or copied to another pointer but can be moved.
  • It can manage a single object and dynamically allocate an array of objects.
#include <iostream>
#include <memory>

using namespace std;
#define UNIQUE 1

class smartclass {
string name;

public:
smartclass(string str)
{
name = str;
}

int display()
{
cout<<name<<endl;
}
};

int main()
{

unique_ptr<smartclass> P1(new smartclass(“smartclass”));
P1->display();
#ifdef UNIQUE
unique_ptr<smartclass> P2;
P2 = move(P1);
#else
unique_ptr<smartclass> P2(P1);
#endif
P2->display();

return 0;
}

The output is:

  smartclass
   smartclass

Comment #define UNIQUE 1 in the above code and compile; then the compiler throws an error. unique_ptr doesn’t have a copy constructor.

shared_ptr: Of all the smart pointers, this one is really smart. Multiple pointers can point to a single object. This is based on the shared ownership model. For example, pointers obj1 and obj2 can point to an object.

  • It is based on the shared ownership model.
  • It has a reference counting mechanism; when each pointer gets released or goes out of scope the reference count decreases by 1.
  • When the reference count reaches 0, the resource is freed up.

Reference count can be retrieved using the use_count() method.
shared_ptr manages two blocks:

  • The control block (contains meta data)
  • The object being managed
#include <iostream>
#include <memory>

using namespace std;
class smartclass {
string name;
public:
smartclass(string str)
{
name = str;
}

int display()
{
cout << name<<endl;
}
};

int main()
{

shared_ptr<smartclass> P1(new smartclass(“smartclass”));
cout << “P1:”;
P1->display();

shared_ptr<smartclass> P2;
P2 = P1;

cout << “P2:”;
P2->display();
cout << “reference count :”<<P1.use_count() << endl;

shared_ptr<smartclass> P3(new smartclass(“myclass”));
cout << “P3 :”;
P3->display();
cout << “is p1 unique ? “ << P1.unique()<<endl;
cout << “is p3 unique ? “ << P3.unique()<<endl;

P3.swap(P2); // swapping of pointers
cout << “P3 after swapping :”;
P3->display();

return 0;
}

The output is:

P1 :smartclass
P2 :smartclass
reference count :2
P3 :myclass
is p1 unique ? 0
is p3 unique ? 1
P3 after swapping :smartclass

In the above program, P1 and P2 point to the same resource; hence, the reference count yields 2.

Then we check whether the pointer is unique or not, to find out whether the ownership is shared. For P1, ownership is shared, so P1.unique() returns 0. For P3, P3.unique() returns 1, which means its ownership is not shared with any other pointer. Finally, we swap P3 with P2 just to show that shared_ptr supports pointer swapping.

weak_ptr: Using weak_ptr, it is possible to access the object but not own it. To use weak_ptr, the object must be already owned by some other pointer (shared_ptr). For example, if a shared pointer obj1 owns a resource, the same resource can be accessed by another weak_ptr without owning it.

  • Used to avoid cyclic reference between shared_ptr references.
    It must be converted to shared_ptr in order to access the referenced object.
#include <iostream>
#include <memory>

using namespace std;
int main()
{

weak_ptr<int> wptr;
shared_ptr<int> sp = std::make_shared<int>(10);
wptr = sp;

cout << “use_count = :”<<wptr.use_count();
shared_ptr<int> sptr = wptr.lock();
cout <<”\nvalue :”<<*sptr<<endl;
}

The output is:

use_count = :1
value :10

In the above program, change the weak_ptr to shared_ptr and comment the last two lines of code; then compile and run, and the use_count value yields 2. On the contrary, if we use exactly the same code as above, the use_count won’t increase as the weak_ptr doesn’t take ownership.

  • weak_ptr::lock(): Creates a shared_ptr to manage the object.
  • weak_ptr::use_count(): Returns the number of shared_ptr objects.
  • Std::make_shared: Makes a single heap-allocation for both the control block and the managed object.

The aim of this article is to make each and every reader understand smart pointers. In today’s world, everything is driven by software, directly or indirectly. But to make the software reliable and stable, it is imperative to understand concepts like smart pointers; otherwise, one has to spend many man-hours debugging and fixing issues. During my career, I have fixed many pointer related issues and noticed that many programmers don’t have the patience to write code for releasing the resource. Smart pointers are very helpful in minimising bugs. Finally, coding is easy if we understand the purpose and usage of different methods.

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