Question

I’m aware that questions about the stack vs. the heap have been asked several times, but I’m confused about one small aspect of choosing how to declare objects in C++.

I understand that the heap–accessed with the “new” operator–is used for dynamic memory allocation. According to an answer to another question on Stack Overflow, “the heap is for storage of data where the lifetime of the storage cannot be determined ahead of time”. The stack is faster than the heap, and seems to be used for variables of local scope, i.e., the variables are automatically deleted when the relevant section of code is completed. The stack also has a relatively limited amount of available space.

In my case, I know prior to runtime that I will need an array of pointers to exactly 500 objects of a particular class, and I know I will need to store the pointers and the objects throughout the duration of runtime. The heap doesn’t make sense because I know beforehand how long I will need the memory and I know exactly how man objects I will need. The stack also doesn’t make sense if it is limited in scope; plus, I don’t know if it can actually hold all of my objects/pointers.

What would be the best way to approach this situation and why? Thanks!

Answer 1

Objects allocated on the stack in main() have a lifetime of the entire run of the program, so that’s an option. An array of 500 pointers is either 2000 or 4000 bytes depending on whether your pointers are 32 or 64 bits wide — if you were programming in an environment whose stack limit was that small, you would know it (such environments do exist: for instance, kernel mode stacks are often 8192 bytes or smaller in total) so I wouldn’t hesitate to put the array there.

Depending on how big your objects are, it might also be reasonable to put them on the stack — the typical stack limit in user space nowadays is order of 8 megabytes, which is not so large that you can totally ignore it, but is not peanuts, either.

If they are too big for the stack, I would seriously consider making a global variable that was an array of the objects themselves. The major downside of this is you can’t control precisely when they are initialized. If the objects have nontrivial constructors this is very likely to be a problem. An alternative is to allocate storage for the objects as a global variable, initialize them at the appropriate point within main using placement new, and explicitly call their destructors on the way out. This requires care in the presence of exceptions; I’d write a one-off RAII class that encapsulated the job.