CODEWITHSUNDEEP
c++
VolkerK
VolkerK

Reputation: 96159

How does delete[] "know" the size of the operand array?

Foo* set = new Foo[100];
// ...
delete [] set;

You don't pass the array's boundaries to delete[]. But where is that information stored? Is it standardised?

Upvotes: 313

Views: 53056

Answers (9)

Avt
Avt

Reputation: 17043

ONE OF THE approaches for compilers is to allocate a little more memory and to store a count of elements in a head element.

Example how it could be done:

Here

int* i = new int[4];

compiler will allocate sizeof(int)*5 bytes.

int *temp = malloc(sizeof(int)*5)

Will store "4" in the first sizeof(int) bytes

*temp = 4;

and set i

i = temp + 1;

So i will points to an array of 4 elements, not 5.

And deletion

delete[] i;

will be processed in the following way:

int *temp = i - 1;
int numbers_of_element = *temp; // = 4
... call destructor for numbers_of_element elements
... that are stored in temp + 1, temp + 2, ... temp + 4 if needed
free (temp)

Upvotes: 36

Chris Jefferson
Chris Jefferson

Reputation: 7157

This is a more interesting problem than you might think at first. This reply is about one possible implementation.

Firstly, while at some level your system has to know how to 'free' the memory block, the underlying malloc/free (which new/delete/new[]/delete[] generally call) don't always remember exactly how much memory you ask for, it can get rounded up (for example, once you are above 4K it is often rounded up to the next 4K-sized block).

Therefore, even if could get the size of the memory block, that doesn't tell us how many values are in the new[]ed memory, as it can be smaller. Therefore, we do have to store an extra integer telling us how many values there are.

EXCEPT, if the type being constructed doesn't have a destructor, then delete[] doesn't have to do anything except free the memory block, and therefore doesn't have to store anything!

Upvotes: 1

MSN
MSN

Reputation: 54604

It's defined in the C++ standard to be compiler specific. Which means compiler magic. It can break with non-trivial alignment restrictions on at least one major platform.

You can think about possible implementations by realizing that delete[] is only defined for pointers returned by new[], which may not be the same pointer as returned by operator new[]. One implementation in the wild is to store the array count in the first int returned by operator new[], and have new[] return a pointer offset past that. (This is why non-trivial alignments can break new[].)

Keep in mind that operator new[]/operator delete[]!=new[]/delete[].

Plus, this is orthogonal to how C knows the size of memory allocated by malloc.

Upvotes: 9

Peter Kühne
Peter Kühne

Reputation: 3274

When you allocate memory on the heap, your allocator will keep track of how much memory you have allocated. This is usually stored in a "head" segment just before the memory that you get allocated. That way when it's time to free the memory, the de-allocator knows exactly how much memory to free.

Upvotes: 234

Francisco Soto
Francisco Soto

Reputation: 10392

Basically its arranged in memory as:

[info][mem you asked for...]

Where info is the structure used by your compiler to store the amount of memory allocated, and what not.

This is implementation dependent though.

Upvotes: 5

Dominik Grabiec
Dominik Grabiec

Reputation: 10655

The information is not standardised. However in the platforms that I have worked on this information is stored in memory just before the first element. Therefore you could theoretically access it and inspect it, however it's not worth it.

Also this is why you must use delete [] when you allocated memory with new [], as the array version of delete knows that (and where) it needs to look to free the right amount of memory - and call the appropriate number of destructors for the objects.

Upvotes: 14

Andre
Andre

Reputation: 1607

It is not standardized. In Microsoft's runtime the new operator uses malloc() and the delete operator uses free(). So, in this setting your question is equivalent to the following: How does free() know the size of the block?

There is some bookkeeping going on behind the scenes, i.e. in the C runtime.

Upvotes: -1

Joel Coehoorn
Joel Coehoorn

Reputation: 415765

Because the array to be 'deleted' should have been created with a single use of the 'new' operator. The 'new' operation should have put that information on the heap. Otherwise, how would additional uses of new know where the heap ends?

Upvotes: 2

jeffm
jeffm

Reputation: 3151

This isn't something that's in the spec -- it's implementation dependent.

Upvotes: 4

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