Why should I use a pointer rather than the object itself?

Question

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 t

Answer 1

Preface

Java is nothing like C++, contrary to hype. The Java hype machine would like you to believe that because Java has C++ like syntax, that the languages are similar. Nothing can be further from the truth. This misinformation is part of the reason why Java programmers go to C++ and use Java-like syntax without understanding the implications of their code.

Onwards we go

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?

To the contrary, actually. The heap is much slower than the stack, because the stack is very simple compared to the heap. Automatic storage variables (aka stack variables) have their destructors called once they go out of scope. For example:

{
    std::string s;
}
// s is destroyed here

On the other hand, if you use a pointer dynamically allocated, its destructor must be called manually. delete calls this destructor for you.

{
    std::string* s = new std::string;
}
delete s; // destructor called

This has nothing to do with the new syntax prevalent in C# and Java. They are used for completely different purposes.

Benefits of dynamic allocation

1. You don't have to know the size of the array in advance

One of the first problems many C++ programmers run into is that when they are accepting arbitrary input from users, you can only allocate a fixed size for a stack variable. You cannot change the size of arrays either. For example:

char buffer[100];
std::cin >> buffer;
// bad input = buffer overflow

Of course, if you used an std::string instead, std::string internally resizes itself so that shouldn't be a problem. But essentially the solution to this problem is dynamic allocation. You can allocate dynamic memory based on the input of the user, for example:

int * pointer;
std::cout << "How many items do you need?";
std::cin >> n;
pointer = new int[n];

Side note: One mistake many beginners make is the usage of variable length arrays. This is a GNU extension and also one in Clang because they mirror many of GCC's extensions. So the following int arr[n] should not be relied on.

Because the heap is much bigger than the stack, one can arbitrarily allocate/reallocate as much memory as he/she needs, whereas the stack has a limitation.

2. Arrays are not pointers

How is this a benefit you ask? The answer will become clear once you understand the confusion/myth behind arrays and pointers. It is commonly assumed that they are the same, but they are not. This myth comes from the fact that pointers can be subscripted just like arrays and because of arrays decay to pointers at the top level in a function declaration. However, once an array decays to a pointer, the pointer loses its sizeof information. So sizeof(pointer) will give the size of the pointer in bytes, which is usually 8 bytes on a 64-bit system.

You cannot assign to arrays, only initialize them. For example:

int arr[5] = {1, 2, 3, 4, 5}; // initialization 
int arr[] = {1, 2, 3, 4, 5}; // The standard dictates that the size of the array
                             // be given by the amount of members in the initializer  
arr = { 1, 2, 3, 4, 5 }; // ERROR

On the other hand, you can do whatever you want with pointers. Unfortunately, because the distinction between pointers and arrays are hand-waved in Java and C#, beginners don't understand the difference.

3. Polymorphism

Java and C# have facilities that allow you to treat objects as another, for example using the as keyword. So if somebody wanted to treat an Entity object as a Player object, one could do Player player = Entity as Player; This is very useful if you intend to call functions on a homogeneous container that should only apply to a specific type. The functionality can be achieved in a similar fashion below:

std::vector vector;
vector.push_back(&square);
vector.push_back(&triangle);
for (auto& e : vector)
{
     auto test = dynamic_cast(e); // I only care about triangles
     if (!test) // not a triangle
        e.GenericFunction();
     else
        e.TriangleOnlyMagic();
}

So say if only Triangles had a Rotate function, it would be a compiler error if you tried to call it on all objects of the class. Using dynamic_cast, you can simulate the as keyword. To be clear, if a cast fails, it returns an invalid pointer. So !test is essentially a shorthand for checking if test is NULL or an invalid pointer, which means the cast failed.

Benefits of automatic variables

After seeing all the great things dynamic allocation can do, you're probably wondering why wouldn't anyone NOT use dynamic allocation all the time? I already told you one reason, the heap is slow. And if you don't need all that memory, you shouldn't abuse it. So here are some disadvantages in no particular order:

• It is error-prone. Manual memory allocation is dangerous and you are prone to leaks. If you are not proficient at using the debugger or valgrind (a memory leak tool), you may pull your hair out of your head. Luckily RAII idioms and smart pointers alleviate this a bit, but you must be familiar with practices such as The Rule Of Three and The Rule Of Five. It is a lot of information to take in, and beginners who either don't know or don't care will fall into this trap.

• It is not necessary. Unlike Java and C# where it is idiomatic to use the new keyword everywhere, in C++, you should only use it if you need to. The common phrase goes, everything looks like a nail if you have a hammer. Whereas beginners who start with C++ are scared of pointers and learn to use stack variables by habit, Java and C# programmers start by using pointers without understanding it! That is literally stepping off on the wrong foot. You must abandon everything you know because the syntax is one thing, learning the language is another.

1. (N)RVO - Aka, (Named) Return Value Optimization

One optimization many compilers make are things called elision and return value optimization. These things can obviate unnecessary copys which is useful for objects that are very large, such as a vector containing many elements. Normally the common practice is to use pointers to transfer ownership rather than copying the large objects to move them around. This has lead to the inception of move semantics and smart pointers.

If you are using pointers, (N)RVO does NOT occur. It is more beneficial and less error-prone to take advantage of (N)RVO rather than returning or passing pointers if you are worried about optimization. Error leaks can happen if the caller of a function is responsible for deleteing a dynamically allocated object and such. It can be difficult to track the ownership of an object if pointers are being passed around like a hot potato. Just use stack variables because it is simpler and better.