Question

What are the instances where you need to explicitly call a destructor?

Answer 1

When you use placement-new is a common reason (the only reason?):

struct foo {};

void* memoryLocation = ::operator new(sizeof(foo));
foo* f = new (memoryLocation) foo(); // note: not safe, doesn't handle exceptions

// ...

f->~foo();
::operator delete(memoryLocation);

This is mostly present in allocators (used by containers), in the construct and destroy functions, respectively.

Otherwise, don’t. Stack-allocations will be done automatically, as it will when you delete pointers. (Use smart pointers!)

Well, I suppose that makes one more reason: When you want undefined behavior. Then feel free to call it as many times as you want… 🙂