Heap Memory

From Stanford CS Ed Library: Pointers and Memory

"Heap" memory, also known as "dynamic" memory, is an alternative to local stack memory. Local memory is quite automatic — it is allocated automatically on function call and it is deallocated automatically when a function exits. Heap memory is different in every way. The programmer explicitly requests the allocation of a memory "block" of a particular size, and the block continues to be allocated until the programmer
explicitly requests that it be deallocated. Nothing happens automatically. So the programmer has much greater control of memory, but with greater responsibility since the memory must now be actively managed.

The advantages of heap memory are...

1. Lifetime. Because the programmer now controls exactly when memory is allocated and deallocated, it is possible to build a data structure in memory, and return that data structure to the caller. This was never possible with local memory which was automatically deallocated when the function exited.
2. Size. The size of allocated memory can be controlled with more detail. For example, a string buffer can be allocated at run-time which is exactly the right size to hold a particular string. With local memory, the code is more likely to declare a buffer size 1000 and hope for the best.

The disadvantages of heap memory are...

1. More Work. Heap allocation needs to arranged explicitly in the code which is just more work.
2. More Bugs. Because it's now done explicitly in the code, realistically on occasion the allocation will be done incorrectly leading to memory bugs. Local memory is constrained, but at least it's never wrong.

Nonetheless, there are many problems that can only be solved with heap memory, so that's that way it has to be. In languages with garbage collectors such as Perl, LISP, or Java, the above disadvantages are mostly eliminated. The garbage collector takes over most of the responsibility for heap management at the cost of a little extra time taken at run-time.