Question

What does the following code do?

obj *x = new obj[100];
delete x; // Note the omission of []

Does it delete only the first element in the array?

Answer 1

Even though it is undefined behavior, you should probably take note of the most likely result (for the sake of debugging such issues).

When you create an array via new Object[100], the memory is first allocated. The default behavior (provided there were no overrides to the default allocator) is to simply call malloc(100 * sizeof(Object)). After that, the constructor for Object needs to be called on each Object-sized region. This is an important detail: the memory is allocated once, but the constructor is called in 100 locations.

When a block is allocated via malloc, it cannot be freed in pieces. Only a call to free(block) will release that memory. The C++ keyword delete internally calls free if the keyword new calls malloc. So, the proper way to delete an array is to call delete [] array. So, what happens if you call delete array? The likely answer is that the memory will be freed (all of it, not just the first element), but only one destructor will be called: the first element’s destructor.

Obviously, there are lots of facts to consider. new and delete are not necessarily bound to malloc and free. They may use system calls unique to a specific architecture or operating system. (Windows, in particular, has a whole set of heap management functions outside of malloc and free in its C API.) I simply demonstrated the example with malloc and free because that is what I have seen the most often when stepping through code. Visual Studio, for example, lets you step into new calls and actually see the new function code. (That’s another important detail. new and delete are simply function calls, which you can even override in many cases.)

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You can demonstrate this concept with this little program. Simply create an Object class that outputs something during the constructor and outputs something else during the destructor.

int main(int argc, char** argv)
{
    Object* o = new Object[4];
    delete o;
    return 0;
}

I ran it, and sure enough: the constructor was called 4 times, and the destructor was called once.