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Possible Duplicate:
How could pairing new[] with delete possibly lead to memory leak only?
delete vs delete[]

I just started learning C/C++ and I was told to use delete to delete a single object and to use delete [] for an array.

Then I found this website which asks this question

Anything wrong with this code?

T *p = new T[10];
delete p;

Note: Incorrect replies: “No, everything is correct”, “Only the first element of the array will be deleted”, “The entire array will be deleted, but only the first element destructor will be called”.

Which raises the question, what does happen in that block of code? I would have logically thought that it was "Only the first element of the array will be deleted” but it seems not. Can anyone shed some light on this?

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Caesar
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    C/C++ are two completely different languages. Removing the C tag, since `new` is C++ only. – David Rodríguez - dribeas Jul 03 '12 at 03:48
  • Possible duplicates: http://stackoverflow.com/questions/1913343/how-could-pairing-new-with-delete-possibly-lead-to-memory-leak-only, http://stackoverflow.com/questions/6171814/why-is-it-undefined-behavior-to-delete-an-array-of-derived-objects-via-a-base – jogojapan Jul 03 '12 at 03:53
  • Not mentioned in the linked questions directly, but to know how many objects have been constructed, the count must be stored in the allocated block, usually the compiler will inject code that allocates memory, stores the integer and returns a pointer `sizeof(size_t)` characters into the buffer, where the first element is constructed. That is not the case for `new` where the count need not be stored. `delete` (paired with `new`) would just call the destructor and release the memory in the pointer, `delete []` must call the destructors, and pass a pointer offsetted by `sizeof(size_t)` to release – David Rodríguez - dribeas Jul 03 '12 at 04:03
  • ... Because the pointer being released is not the pointer that was allocated, in many cases (including `malloc`/`free`) that is a breach of the contract. – David Rodríguez - dribeas Jul 03 '12 at 04:05

1 Answers1

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  • delete: This frees the memory currently allocated by the pointer the delete is performed upon. It only deletes the memory pointed to by the first variable.

  • delete []: This frees the memory allocated for the whole array. An array consists of several variables - delete frees memory only allocated for the first variable, while delete [] does the whole thing.

A good way to think of it is considering delete as an instruction while delete [] as a loop; where the array is looped through and delete is called individually on each variable in the array. This is NOT how it works in reality (the real workings are a bit more complicated), but is a good way to understand the diff.

The destructor is called on all objects, because in some cases such as in the case of an array of objects that contain pointers, calling the destructor on only the first element doesn't free all memory.

fdh
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