N4058: Atomic Smart Pointers -- Herb Sutter
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Document number: N4058
Date: 2014-06-12
Atomic Smart Pointers
by Herb Sutter
Excerpt:
We encourage that modern C++ code should avoid all uses of owning raw pointers and explicit
delete. Instead, programmers should useunique_ptrandshared_ptr(withweak_ptr), as this is known to lead to simpler and leak-free memory-safe code. This is especially important when lifetimes are unstructured or nondeterministic, which arises especially in concurrent code, and it has long been well-known that the smart pointers would be useful there; for an example, see [1].Unfortunately, lock-free code is still mostly forced to use owning raw pointers. Our
unique_ptr,shared_ptr, andweak_ptrwould directly benefit lock-free code just as they do regular code (see next section), but they are not usable easily or at all in lock-free code because we do not supportatomic<unique_ptr<T>>,atomic<shared_ptr<T>>, andatomic<weak_ptr<T>>....
If we had
atomic<unique_ptr<T>>we could (and should) write the following equivalent code that is safer, no slower, and less error-prone because we can directly express the unique ownership semantics including ownership transfer:
atomic<unique_ptr<X>> p_root;
void producer() {
auto temp = make_unique<X>();
load_from_disk_and_store_in( *temp ); // build data structure
p_root = move(temp); // atomically publish it
}
This righteous code should be supported...