Which data type perimits sharing memory among different types of data?

Answer 1

Types cannot share memory; only instances of a type (objects or variables) can share memory. This is achieved through the use of shared resource handles or "smart pointers". We can also use C-style pointer variables to share memory, however they are problematic because as soon as the shared memory is released, all pointers to it become invalid but there's no way to tell if a pointer is valid or not. Even without using shared memory, there is no notion of "ownership" as far as pointers are concerned.

In the absence of resource handles, the best strategy is to allocate memory in an outer scope and then create shared pointers within an inner scope. The shared pointers MUST NOT release the memory they refer to. When we return to the outer scope, the shared references will have fallen from scope, thus we can safely release the memory using the same pointer variable we used to allocate the memory. In this way we get some notion of ownership.

With resource handles we don't have this problem because the shared memory cannot be released accidently until the very last reference falls from scope. The last reference need not be the one we originally used to allocate the memory.

Sharing memory between threads is best kept to an absolute minimum. Although we can use locks to prevent data races, we achieve better performance with lock-free code and there's less risk of deadlock (where two threads are waiting on each other to release the memory they've locked). Shared memory is often unavoidable, but don't use it just because you can, only use it when it is appropriate to do so; there are always alternatives.