C++ – use a pointer rather than the object itself

c++c++-faqc++11pointers

I'm coming from a Java background and have started working with objects in C++. But one thing that occurred to me is that people often use pointers to objects rather than the objects themselves, for example this declaration:

Object *myObject = new Object;

rather than:

Object myObject;

Or instead of using a function, let's say testFunc(), like this:

myObject.testFunc();

we have to write:

myObject->testFunc();

But I can't figure out why should we do it this way. I would assume it has to do with efficiency and speed since we get direct access to the memory address. Am I right?

Best Solution

It's very unfortunate that you see dynamic allocation so often. That just shows how many bad C++ programmers there are.

In a sense, you have two questions bundled up into one. The first is when should we use dynamic allocation (using new)? The second is when should we use pointers?

The important take-home message is that you should always use the appropriate tool for the job. In almost all situations, there is something more appropriate and safer than performing manual dynamic allocation and/or using raw pointers.

Dynamic allocation

In your question, you've demonstrated two ways of creating an object. The main difference is the storage duration of the object. When doing Object myObject; within a block, the object is created with automatic storage duration, which means it will be destroyed automatically when it goes out of scope. When you do new Object(), the object has dynamic storage duration, which means it stays alive until you explicitly delete it. You should only use dynamic storage duration when you need it. That is, you should always prefer creating objects with automatic storage duration when you can.

The main two situations in which you might require dynamic allocation:

You need the object to outlive the current scope - that specific object at that specific memory location, not a copy of it. If you're okay with copying/moving the object (most of the time you should be), you should prefer an automatic object.
You need to allocate a lot of memory, which may easily fill up the stack. It would be nice if we didn't have to concern ourselves with this (most of the time you shouldn't have to), as it's really outside the purview of C++, but unfortunately, we have to deal with the reality of the systems we're developing for.

When you do absolutely require dynamic allocation, you should encapsulate it in a smart pointer or some other type that performs RAII (like the standard containers). Smart pointers provide ownership semantics of dynamically allocated objects. Take a look at std::unique_ptr and std::shared_ptr, for example. If you use them appropriately, you can almost entirely avoid performing your own memory management (see the Rule of Zero).

Pointers

However, there are other more general uses for raw pointers beyond dynamic allocation, but most have alternatives that you should prefer. As before, always prefer the alternatives unless you really need pointers.

You need reference semantics. Sometimes you want to pass an object using a pointer (regardless of how it was allocated) because you want the function to which you're passing it to have access that that specific object (not a copy of it). However, in most situations, you should prefer reference types to pointers, because this is specifically what they're designed for. Note this is not necessarily about extending the lifetime of the object beyond the current scope, as in situation 1 above. As before, if you're okay with passing a copy of the object, you don't need reference semantics.
You need polymorphism. You can only call functions polymorphically (that is, according to the dynamic type of an object) through a pointer or reference to the object. If that's the behavior you need, then you need to use pointers or references. Again, references should be preferred.
You want to represent that an object is optional by allowing a nullptr to be passed when the object is being omitted. If it's an argument, you should prefer to use default arguments or function overloads. Otherwise, you should preferably use a type that encapsulates this behavior, such as std::optional (introduced in C++17 - with earlier C++ standards, use boost::optional).
You want to decouple compilation units to improve compilation time. The useful property of a pointer is that you only require a forward declaration of the pointed-to type (to actually use the object, you'll need a definition). This allows you to decouple parts of your compilation process, which may significantly improve compilation time. See the Pimpl idiom.
You need to interface with a C library or a C-style library. At this point, you're forced to use raw pointers. The best thing you can do is make sure you only let your raw pointers loose at the last possible moment. You can get a raw pointer from a smart pointer, for example, by using its get member function. If a library performs some allocation for you which it expects you to deallocate via a handle, you can often wrap the handle up in a smart pointer with a custom deleter that will deallocate the object appropriately.

Related Solutions

C++ – What are the differences between a pointer variable and a reference variable in C++

A pointer can be re-assigned:

int x = 5;
int y = 6;
int *p;
p = &x;
p = &y;
*p = 10;
assert(x == 5);
assert(y == 10);

A reference cannot be re-bound, and must be bound at initialization:

int x = 5;
int y = 6;
int &q; // error
int &r = x;

A pointer variable has its own identity: a distinct, visible memory address that can be taken with the unary & operator and a certain amount of space that can be measured with the sizeof operator. Using those operators on a reference returns a value corresponding to whatever the reference is bound to; the reference’s own address and size are invisible. Since the reference assumes the identity of the original variable in this way, it is convenient to think of a reference as another name for the same variable.
```
int x = 0;
int &r = x;
int *p = &x;
int *p2 = &r;

assert(p == p2); // &x == &r
assert(&p != &p2);
```

You can have arbitrarily nested pointers to pointers offering extra levels of indirection. References only offer one level of indirection.

int x = 0;
int y = 0;
int *p = &x;
int *q = &y;
int **pp = &p;

**pp = 2;
pp = &q; // *pp is now q
**pp = 4;

assert(y == 4);
assert(x == 2);

A pointer can be assigned nullptr, whereas a reference must be bound to an existing object. If you try hard enough, you can bind a reference to nullptr, but this is undefined and will not behave consistently.

/* the code below is undefined; your compiler may optimise it
 * differently, emit warnings, or outright refuse to compile it */

int &r = *static_cast<int *>(nullptr);

// prints "null" under GCC 10
std::cout
    << (&r != nullptr
        ? "not null" : "null")
    << std::endl;

bool f(int &r) { return &r != nullptr; }

// prints "not null" under GCC 10
std::cout
    << (f(*static_cast<int *>(nullptr))
        ? "not null" : "null")
    << std::endl;

You can, however, have a reference to a pointer whose value is nullptr.

Pointers can iterate over an array; you can use ++ to go to the next item that a pointer is pointing to, and + 4 to go to the 5th element. This is no matter what size the object is that the pointer points to.
A pointer needs to be dereferenced with * to access the memory location it points to, whereas a reference can be used directly. A pointer to a class/struct uses -> to access its members whereas a reference uses a ..
References cannot be put into an array, whereas pointers can be (Mentioned by user @litb)
Const references can be bound to temporaries. Pointers cannot (not without some indirection):
```
const int &x = int(12); // legal C++
int *y = &int(12); // illegal to take the address of a temporary.
```
This makes const & more convenient to use in argument lists and so forth.

C++ – a smart pointer and when should I use one

UPDATE

This answer is rather old, and so describes what was 'good' at the time, which was smart pointers provided by the Boost library. Since C++11, the standard library has provided sufficient smart pointers types, and so you should favour the use of std::unique_ptr, std::shared_ptr and std::weak_ptr.

There was also std::auto_ptr. It was very much like a scoped pointer, except that it also had the "special" dangerous ability to be copied — which also unexpectedly transfers ownership.
It was deprecated in C++11 and removed in C++17, so you shouldn't use it.

std::auto_ptr<MyObject> p1 (new MyObject());
std::auto_ptr<MyObject> p2 = p1; // Copy and transfer ownership. 
                                 // p1 gets set to empty!
p2->DoSomething(); // Works.
p1->DoSomething(); // Oh oh. Hopefully raises some NULL pointer exception.

OLD ANSWER

A smart pointer is a class that wraps a 'raw' (or 'bare') C++ pointer, to manage the lifetime of the object being pointed to. There is no single smart pointer type, but all of them try to abstract a raw pointer in a practical way.

Smart pointers should be preferred over raw pointers. If you feel you need to use pointers (first consider if you really do), you would normally want to use a smart pointer as this can alleviate many of the problems with raw pointers, mainly forgetting to delete the object and leaking memory.

With raw pointers, the programmer has to explicitly destroy the object when it is no longer useful.

// Need to create the object to achieve some goal
MyObject* ptr = new MyObject(); 
ptr->DoSomething(); // Use the object in some way
delete ptr; // Destroy the object. Done with it.
// Wait, what if DoSomething() raises an exception...?

A smart pointer by comparison defines a policy as to when the object is destroyed. You still have to create the object, but you no longer have to worry about destroying it.

SomeSmartPtr<MyObject> ptr(new MyObject());
ptr->DoSomething(); // Use the object in some way.

// Destruction of the object happens, depending 
// on the policy the smart pointer class uses.

// Destruction would happen even if DoSomething() 
// raises an exception

The simplest policy in use involves the scope of the smart pointer wrapper object, such as implemented by boost::scoped_ptr or std::unique_ptr.

void f()
{
    {
       std::unique_ptr<MyObject> ptr(new MyObject());
       ptr->DoSomethingUseful();
    } // ptr goes out of scope -- 
      // the MyObject is automatically destroyed.

    // ptr->Oops(); // Compile error: "ptr" not defined
                    // since it is no longer in scope.
}

Note that std::unique_ptr instances cannot be copied. This prevents the pointer from being deleted multiple times (incorrectly). You can, however, pass references to it around to other functions you call.

std::unique_ptrs are useful when you want to tie the lifetime of the object to a particular block of code, or if you embedded it as member data inside another object, the lifetime of that other object. The object exists until the containing block of code is exited, or until the containing object is itself destroyed.

A more complex smart pointer policy involves reference counting the pointer. This does allow the pointer to be copied. When the last "reference" to the object is destroyed, the object is deleted. This policy is implemented by boost::shared_ptr and std::shared_ptr.

void f()
{
    typedef std::shared_ptr<MyObject> MyObjectPtr; // nice short alias
    MyObjectPtr p1; // Empty

    {
        MyObjectPtr p2(new MyObject());
        // There is now one "reference" to the created object
        p1 = p2; // Copy the pointer.
        // There are now two references to the object.
    } // p2 is destroyed, leaving one reference to the object.
} // p1 is destroyed, leaving a reference count of zero. 
  // The object is deleted.

Reference counted pointers are very useful when the lifetime of your object is much more complicated, and is not tied directly to a particular section of code or to another object.

There is one drawback to reference counted pointers — the possibility of creating a dangling reference:

// Create the smart pointer on the heap
MyObjectPtr* pp = new MyObjectPtr(new MyObject())
// Hmm, we forgot to destroy the smart pointer,
// because of that, the object is never destroyed!

Another possibility is creating circular references:

struct Owner {
   std::shared_ptr<Owner> other;
};

std::shared_ptr<Owner> p1 (new Owner());
std::shared_ptr<Owner> p2 (new Owner());
p1->other = p2; // p1 references p2
p2->other = p1; // p2 references p1

// Oops, the reference count of of p1 and p2 never goes to zero!
// The objects are never destroyed!

To work around this problem, both Boost and C++11 have defined a weak_ptr to define a weak (uncounted) reference to a shared_ptr.