Why should I use a pointer rather than the object

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 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?

Answer 1

Technically it is a memory allocation issue, however here are two more practical aspects of this. It has to do with two things: 1) Scope, when you define an object without a pointer you will no longer be able to access it after the code block it is defined in, whereas if you define a pointer with "new" then you can access it from anywhere you have a pointer to this memory until you call "delete" on the same pointer. 2) If you want to pass arguments to a function you want to pass a pointer or a reference in order to be more efficient. When you pass an Object then the object is copied, if this is an object that uses a lot of memory this might be CPU consuming (e.g. you copy a vector full of data). When you pass a pointer all you pass is one int (depending of implementation but most of them are one int).

Other than that you need to understand that "new" allocates memory on the heap that needs to be freed at some point. When you don't have to use "new" I suggest you use a regular object definition "on the stack".

Answer 2

"Necessity is the mother of invention." The most of important difference that I would like to point out is the outcome of my own experience of coding. Sometimes you need to pass objects to functions. In that case, if your object is of a very big class then passing it as an object will copy its state (which you might not want ..AND CAN BE BIG OVERHEAD) thus resulting in an overhead of copying object .while pointer is fixed 4-byte size (assuming 32 bit). Other reasons are already mentioned above...

Answer 3

Another good reason to use pointers would be for forward declarations. In a large enough project they can really speed up compile time.

Answer 4

Object *myObject = new Object;

Doing this will create a reference to an Object (on the heap) which has to be deleted explicitly to avoid memory leak.

Object myObject;

Doing this will create an object(myObject) of the automatic type (on the stack) that will be automatically deleted when the object(myObject) goes out of scope.

Answer 5

In C++, objects allocated on the stack (using Object object; statement within a block) will only live within the scope they are declared in. When the block of code finishes execution, the object declared are destroyed. Whereas if you allocate memory on heap, using Object* obj = new Object(), they continue to live in heap until you call delete obj.

I would create an object on heap when I like to use the object not only in the block of code which declared/allocated it.

Answer 6

You shouldn't. People (many people, sadly) write it out of ignorance.

Sometimes dynamic allocation has its place but, in the examples you give, it is wrong.

If you want to think about efficiency, then this is worse, because it introduces indirection for no good reason. This sort of programming is slower and more error-prone.