Why would you ever want to allocate memory on the heap rather than the stack? [duplicate]

Answer 1

Size limits are a huge dealbreaker in a lot of cases. The stack is usually measured in the low megabytes or even kilobytes (that's for everything on the stack), whereas all modern PCs allow you a few gigabytes of heap. So if you're going to be using a large amount of data, you absolutely need the heap.

Answer 2

There are a few reasons:

• The main one is that with heap allocation, you have the most flexible control over the object's lifetime (from malloc/calloc to free);
• Stack space is typically a more limited resource than heap space, at least in default configurations;
• A failure to allocate heap space can be handled gracefully, whereas running out of stack space is often unrecoverable.

Without the flexible object lifetime, useful data structures such as binary trees and linked lists would be virtually impossible to write.

Answer 3

The most obvious rationale for using the heap is when you call a function and need something of unknown length returned. Sometimes the caller may pass a memory block and size to the function, but at other times this is just impractical, especially if the returned stuff is complex (e.g. a collection of different objects with pointers flying around, etc.).