D Dynamic Arrays - RAII

Question

I admit I have no deep understanding of D at this point, my knowledge relies purely on what documentation I have read and the few examples I have tried.

In C++ you could rely on the RAII idiom to call the destructor of objects on exiting their local scope.

Can you in D?

I understand D is a garbage collected language, and that it also supports RAII. Why does the following code not cleanup the memory as it leaves a scope then?

import std.stdio;

void main() {
      {
            const int len = 1000 * 1000 * 256; // ~1GiB

            int[] arr;
            arr.length = len;
            arr[] = 99;
      }

      while (true) {}
}

The infinite loop is there so as to keep the program open to make residual memory allocations easy visible.

A comparison of a equivalent same program in C++ is shown below.

It can be seen that C++ immediately cleaned up the memory after allocation (the refresh rate makes it appear as if less memory was allocated), whereas D kept it even though it had left scope.

Therefore, when does the GC cleanup?

Answer 1

No, you cannot assume that the garbage collector will collect your object at any point in time.

There is, however, a delete keyword (as well as a scope keyword) that can delete an object deterministically.

scope is used like:

{
    scope auto obj = new int[5];
    //....
} //obj cleaned up here

and delete is used like in C++ (there's no [] notation for delete).

There are some gotcha's, though:

• It doesn't always work properly (I hear it doesn't work well with arrays)

• The developers of D (e.g. Andrei) are intending to remove them in later versions, because it can obviously mess up things if used incorrectly. (I personally hate this, given that it's so easy to screw things up anyway, but they're sticking with removing it, and I don't think people can convince them otherwise although I'd love it if that was the case.)

• In its place, there is already a clear method that you can use, like arr.clear(); however, I'm not quite sure what it exactly does yet myself, but you could look at the source code in object.d in the D runtime if you're interested.

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As to your amazement: I'm glad you're amazed, but it shouldn't be really surprising considering that they're both native code. :-)