Question

I’ve been using C++ for a short while, and I’ve been wondering about the new keyword. Simply, should I be using it, or not?

1. With the new keyword…

    MyClass* myClass = new MyClass();     myClass->MyField = "Hello world!"; 

2. Without the new keyword…

    MyClass myClass;     myClass.MyField = "Hello world!"; 

From an implementation perspective, they don’t seem that different (but I’m sure they are)… However, my primary language is C#, and of course the 1st method is what I’m used to.

The difficulty seems to be that method 1 is harder to use with the std C++ classes.

Which method should I use?

Update 1:

I recently used the new keyword for heap memory (or free store) for a large array which was going out of scope (i.e. being returned from a function). Where before I was using the stack, which caused half of the elements to be corrupt outside of scope, switching to heap usage ensured that the elements were intact. Yay!

Update 2:

A friend of mine recently told me there’s a simple rule for using the new keyword; every time you type new, type delete.

    Foobar *foobar = new Foobar();     delete foobar; // TODO: Move this to the right place. 

This helps to prevent memory leaks, as you always have to put the delete somewhere (i.e. when you cut and paste it to either a destructor or otherwise).

Answer 1

Method 1 (using new)

• Allocates memory for the object on the free store (This is frequently the same thing as the heap)
• Requires you to explicitly delete your object later. (If you don’t delete it, you could create a memory leak)
• Memory stays allocated until you delete it. (i.e. you could return an object that you created using new)
• The example in the question will leak memory unless the pointer is deleted; and it should always be deleted, regardless of which control path is taken, or if exceptions are thrown.

Method 2 (not using new)

• Allocates memory for the object on the stack (where all local variables go) There is generally less memory available for the stack; if you allocate too many objects, you risk stack overflow.
• You won’t need to delete it later.
• Memory is no longer allocated when it goes out of scope. (i.e. you shouldn’t return a pointer to an object on the stack)

As far as which one to use; you choose the method that works best for you, given the above constraints.

Some easy cases:

• If you don’t want to worry about calling delete, (and the potential to cause memory leaks) you shouldn’t use new.
• If you’d like to return a pointer to your object from a function, you must use new