Why does the use of 'new' cause memory leaks?

Question

I learned C# first, and now I\'m starting with C++. As I understand, operator new in C++ is not similar to the one in C#.

Can you explain the reason of

Answer 1

B object2 = *(new B());

This line is the cause of the leak. Let's pick this apart a bit..

object2 is a variable of type B, stored at say address 1 (Yes, I'm picking arbitrary numbers here). On the right side, you've asked for a new B, or a pointer to an object of type B. The program gladly gives this to you and assigns your new B to address 2 and also creates a pointer in address 3. Now, the only way to access the data in address 2 is via the pointer in address 3. Next, you dereferenced the pointer using * to get the data that the pointer is pointing to (the data in address 2). This effectively creates a copy of that data and assigns it to object2, assigned in address 1. Remember, it's a COPY, not the original.

Now, here's the problem:

You never actually stored that pointer anywhere you can use it! Once this assignment is finished, the pointer (memory in address3, which you used to access address2) is out of scope and beyond your reach! You can no longer call delete on it and therefore cannot clean up the memory in address2. What you are left with is a copy of the data from address2 in address1. Two of the same things sitting in memory. One you can access, the other you can't (because you lost the path to it). That's why this is a memory leak.

I would suggest coming from your C# background that you read up a lot on how pointers in C++ work. They are an advanced topic and can take some time to grasp, but their use will be invaluable to you.