Question

I have this small piece of code in the middle of a larger code:

int *p = new int[100];
p += 50;
delete []p;

Will the compiler delete only the memory from the 51^st location? I think it does. However, in the case of array pointers, the compiler holds an additional item which tells the number of objects allocated. So, in that case, shouldn’t it go ahead and delete memory beyond the allocated size?
Or does it delete the 51^st–100^th elements and keep the 1^st–50^th in the memory, in which case a memory leak can happen.

Answer 1

It’s undefined behavior. The C++ standard says:

3.7.4.2 Deallocation functions

3 … Otherwise, the behavior is undefined
if the value supplied to operator delete(void*) in the standard library is not one of the values returned
by a previous invocation of either operator new(std::size_t) or operator new(std::size_t, const
std::nothrow_t&) in the standard library, and the behavior is undefined if the value supplied to operator
delete[](void*) in the standard library is not one of the values returned by a previous invocation of
either operator new[](std::size_t) or operator new[](std::size_t, const std::nothrow_t&) in the
standard library.

4 … The effect of using an invalid pointer value
(including passing it to a deallocation function) is undefined. (On some implementations, it causes a system-generated runtime fault.)