Why [] is used in delete ( delete [] ) to free dynamically allocated array ?

Question

I know that when delete [] will cause destruction for all array elements and then releases the memory.

I initially thought that compiler wants it just to call destructor for all elements in the array, but I have also a counter - argument for that which is:

Heap memory allocator must know the size of bytes allocated and using sizeof(Type) its possible to find no of elements and to call appropriate no of destructors for an array to prevent resource leaks.

So my assumption is correct or not and please clear my doubt on it.

So I am not getting the usage of [] in delete [] ?

Answer 1

Scott Meyers says in his Effective C++ book: Item 5: Use the same form in corresponding uses of new and delete.

The big question for delete is this: how many objects reside in the memory being deleted? The answer to that determines how many destructors must be called.

Does the pointer being deleted point to a single object or to an array of objects? The only way for delete to know is for you to tell it. If you don't use brackets in your use of delete, delete assumes a single object is pointed to.

Also, the memory allocator might allocate more space that required to store your objects and in this case dividing the size of the memory block returned by the size of each object won't work.

Depending on the platform, the _msize (windows), malloc_usable_size (linux) or malloc_size (osx) functions will tell you the real length of the block that was allocated. This information can be exploited when designing growing containers.

Another reason why it won't work is that Foo* foo = new Foo[10] calls operator new[] to allocate the memory. Then delete [] foo; calls operator delete[] to deallocate the memory. As those operators can be overloaded, you have to adhere to the convention otherwise delete foo; calls operator delete which may have an incompatible implementation with operator delete []. It's a matter of semantics, not just keeping track of the number of allocated object to later issue the right number of destructor calls.

See also:

[16.14] After p = new Fred[n], how does the compiler know there are n objects to be destructed during delete[] p?

Short answer: Magic.

Long answer: The run-time system stores the number of objects, n, somewhere where it can be retrieved if you only know the pointer, p. There are two popular techniques that do this. Both these techniques are in use by commercial-grade compilers, both have tradeoffs, and neither is perfect. These techniques are:

• Over-allocate the array and put n just to the left of the first Fred object.
• Use an associative array with p as the key and n as the value.

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EDIT: after having read @AndreyT comments, I dug into my copy of Stroustrup's "The Design and Evolution of C++" and excerpted the following:

How do we ensure that an array is correctly deleted? In particular, how do we ensure that the destructor is called for all elements of an array?

...

Plain delete isn't required to handle both individual objects an arrays. This avoids complicating the common case of allocating and deallocating individual objects. It also avoids encumbering individual objects with information necessary for array deallocation.

An intermediate version of delete[] required the programmer to specify the number of elements of the array.

...

That proved too error prone, so the burden of keeping track of the number of elements was placed on the implementation instead.

As @Marcus mentioned, the rational may have been "you don't pay for what you don't use".

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EDIT2:

In "The C++ Programming Language, 3rd edition", §10.4.7, Bjarne Stroustrup writes:

Exactly how arrays and individual objects are allocated is implementation-dependent. Therefore, different implementations will react differently to incorrect uses of the delete and delete[] operators. In simple and uninteresting cases like the previous one, a compiler can detect the problem, but generally something nasty will happen at run time.

The special destruction operator for arrays, delete[], isn’t logically necessary. However, suppose the implementation of the free store had been required to hold sufficient information for every object to tell if it was an individual or an array. The user could have been relieved of a burden, but that obligation would have imposed significant time and space overheads on some C++ implementations.

Answer 2

The main reason why it was decided to keep separate delete and delete[] is that these two entities are not as similar as it might seem at the first sight. For a naive observer they might seem to be almost the same: just destruct and deallocate, with the only difference in the potential number of objects to process. In reality, the difference is much more significant.

The most important difference between the two is that delete might perform polymorphic deletion of objects, i.e. the static type of the object in question might be different from its dynamic type. delete[] on the other hand must deal with strictly non-polymorphic deletion of arrays. So, internally these two entities implement logic that is significantly different and non-intersecting between the two. Because of the possibility of polymorphic deletion, the functionality of delete is not even remotely the same as the functionality of delete[] on an array of 1 element, as a naive observer might incorrectly assume initially.

Contrary to the strange claims made in some other answers, it is, of course, perfectly possible to replace delete and delete[] with just a single construct that would branch at the very early stage, i.e. it would determine the type of the memory block (array or not) using the household information that would be stored by new/new[], and then jump to the appropriate functionality, equivalent to either delete or delete[]. However, this would be a rather poor design decision, since, once again, the functionality of the two is too different. Forcing both into a single construct would be akin to creating a Swiss Army Knife of a deallocation function. Also, in order to be able to tell an array from a non-array we'd have to introduce an additional piece of household information even into a single-object memory allocations done with plain new. This might easily result in notable memory overhead in single object allocations.

But, once again, the main reason here is the functional difference between delete and delete[]. These language entities possess only apparent skin-deep similarity that exists only at the level of naive specification ("destruct and free memory"), but once one gets to understand in detail what these entities really have to do one realizes that they are too different to be merged into one.

P.S. This is BTW one of the problems with the suggestion about sizeof(type) you made in the question. Because of the potentially polymorphic nature of delete, you don't know the type in delete, which is why you can't obtain any sizeof(type). There are more problems with this idea, but that one is already enough to explain why it won't fly.

Answer 3

The heap itself knows the size of allocated block - you only need the address. Look like free() works - you only pass the address and it frees memory.

The difference between delete (delete[]) and free() is that the former two first call the destructors, then free memory (possibly using free()). The problem is that delete[] also has only one argument - the address and having only that address it need to know the number of objects to run destructors on. So new[] uses som implementation-defined way of writing somewhere the number of elements - usually it prepends the array with the number of elements. Now delete[] will rely on that implementation-specific data to run destructors and then free memory (again, only using the block address).

Answer 4

delete[] just calls a different implementation (function);

There's no reason an allocator couldn't track it (in fact, it would be easy enough to write your own).

I don't know the reason they did not manage it, or the history of the implementation, if I were to guess: Many of these 'well, why wasn't this slightly simpler?' questions (in C++) came down to one or more of:

1. compatibility with C
2. performance

In this case, performance. Using delete vs delete[] is easy enough, I believe it could all be abstracted from the programmer and be reasonably fast (for general use). delete[] only requires only a few additional function calls and operations (omitting destructor calls), but that is per call to delete, and unnecessary because the programmer generally knows the type he/she is dealing with (if not, there's likely a bigger problem at hand). So it just avoids calling through the allocator. Additionally, these single allocations may not need to be tracked by the allocator in as much detail; Treating every allocation as an array would require additional entries for count for trivial allocations, so it is multiple levels of simple allocator implementation simplifications which are actually important for many people, considering it is a very low level domain.