Well, I have been wondering about this for quite a while now, and I have tried to run some tests to better understand what’s going on under the hood. The standard answer is that after you call delete you should not expect anything good from accessing that memory spot. However, this did not seem enough to me. What is it really happening when calling delete(ptr)? Even though there no standard behavior, what could happen, anyway? Here’s what I’ve found. I’m using g++ on Ubuntu 16.04, so this may play a role in the results.
What I first expected when using the delete operator was that the freed memory would be handed back to the system for usage in other processes. Let me say this does not happen under any of the circumstances I have tried.
Memory released with delete still seem to be allocated to the program it first allocated it with new. I have tried, and there is no memory usage decrease after calling delete. I had a software which allocated around 30MB of lists through new calls, and then released them with subsequent delete calls. What happened is that, looking at the System monitor while the program was running, even a long sleep after the delete calls, memory consumption my the program was the same. No decrease! This means that delete does not release memory to the system.
In fact, it looks like memory allocated by a program is his forever! However, the point is that, if deallocated, memory can be used again by the same program without having to allocate any more. I tried to allocate 15MB, freeing them, and then allocating another 15MB of data after, and the program never used 30MB. System monitor always showed it around 15MB. What I did, in respect to the previous test, was just to change the order in which things happened: half allocation, half deallocation, other half of allocation.
So, apparently memory used by a program can increase, but never shrink. Continue reading