C++ new int[0] — will it allocate memory?

问题:

A simple test app:

cout << new int[0] << endl;

outputs:

0x876c0b8

So it looks like it works. What does the standard say about this? Is it always legal to "allocate" empty block of memory?

回答1:

From 5.3.4/7

When the value of the expression in a direct-new-declarator is zero, the allocation function is called to allocate an array with no elements.

From 3.7.3.1/2

The effect of dereferencing a pointer returned as a request for zero size is undefined.

Also

Even if the size of the space requested [by new] is zero, the request can fail.

That means you can do it, but you can not legally (in a well defined manner across all platforms) dereference the memory that you get - you can only pass it to array delete - and you should delete it.

Here is an interesting foot-note (i.e not a normative part of the standard, but included for expository purposes) attached to the sentence from 3.7.3.1/2

[32. The intent is to have operator new() implementable by calling malloc() or calloc(), so the rules are substantially the same. C++ differs from C in requiring a zero request to return a non-null pointer.]

回答2:

Yes, it is legal to allocate a zero-sized array like this. But you must also delete it.

回答3:

What does the standard say about this? Is it always legal to "allocate" empty block of memory?

Every object has a unique identity, i.e. a unique address, which implies a non-zero length (the actual amount of memory will be silently increased, if you ask for zero bytes).

If you allocated more than one of these objects then you'd find they have different addresses.

回答4:

Yes it is completely legal to allocate a 0 sized block with new. You simply can't do anything useful with it since there is no valid data for you to access. int[0] = 5; is illegal.

However, I believe that the standard allows for things like malloc(0) to return NULL.

You will still need to delete [] whatever pointer you get back from the allocation as well.

回答5:

Curiously, C++ requires that operator new return a legitimate pointer even when zero bytes are requested. (Requiring this odd-sounding behavior simplifies things elsewhere in the language.)

I found Effective C++ Third Edition said like this in "Item 51: Adhere to convention when writing new and delete".

回答6:

I guarantee you that new int[0] costs you extra space since I have tested it.

For example, the memory usage of

int **arr = new int*[1000000000];

is significantly smaller than

int **arr = new int*[1000000000];
for(int i =0; i < 1000000000; i++) {
    arr[i]=new int[0];
}

The memory usage of the second code snippet minus that of the first code snippet is the memory used for the numerous new int[0].