Heap is an area of dynamically-allocated memory that is managed automatically by the operating system or the memory manager library. You can allocate a block at any time and free it at any time. Heap allocation requires maintaining a full record of what memory is allocated and what isn’t, as well as some overhead maintenance to reduce fragmentation, find contiguous memory segments big enough to fit the requested size, and so on. Memory can be deallocated at any time leaving free space. As the heap grows new blocks are often allocated from lower addresses towards higher addresses. Thus you can think of the heap as a heap of memory blocks that grows in size as memory is allocated. If the heap is too small for an allocation the size can often be increased by acquiring more memory from the underlying operating system. Memory allocated from the heap will remain allocated until one of the following occurs:
- The memory is freed
- The program terminates
Stack
- Stored in computer RAM just like the heap.
- Variables created on the stack will go out of scope and are automatically deallocated.
- Much faster to allocate in comparison to variables on the heap.
- Stores local data, return addresses, used for parameter passing.
- Can have a stack overflow when too much of the stack is used (mostly from infinite or too deep recursion, very large allocations).
- You would use the stack if you know exactly how much data you need to allocate before compile time and it is not too big.
- Usually has a maximum size already determined when your program starts.
Heap
- Stored in computer RAM just like the stack.
- In C++, variables on the heap must be destroyed manually and never fall out of scope. The data is freed with delete, delete[], or free.
- Slower to allocate in comparison to variables on the stack.
- Used on demand to allocate a block of data for use by the program.
- Can have fragmentation when there are a lot of allocations and deallocations.
- In C++ or C, data created on the heap will be pointed to by pointers and allocated with new or malloc respectively.
- Can have allocation failures if too big of a buffer is requested to be allocated.
- You would use the heap if you don’t know exactly how much data you will need at run time or if you need to allocate a lot of data.
- Responsible for memory leaks.