pub type JSErrorNotes_iterator_value_type = u64;
Expand description
UniquePtr is a smart pointer that wholly owns a resource. Ownership may be transferred out of a UniquePtr through explicit action, but otherwise the resource is destroyed when the UniquePtr is destroyed.
UniquePtr is similar to C++98’s std::auto_ptr, but it improves upon auto_ptr in one crucial way: it’s impossible to copy a UniquePtr. Copying an auto_ptr obviously can’t copy ownership of its singly-owned resource. So what happens if you try to copy one? Bizarrely, ownership is implicitly transferred, preserving single ownership but breaking code that assumes a copy of an object is identical to the original. (This is why auto_ptr is prohibited in STL containers.)
UniquePtr solves this problem by being movable rather than copyable. Instead of passing a |UniquePtr u| directly to the constructor or assignment operator, you pass |Move(u)|. In doing so you indicate that you’re moving ownership out of |u|, into the target of the construction/assignment. After the transfer completes, |u| contains |nullptr| and may be safely destroyed. This preserves single ownership but also allows UniquePtr to be moved by algorithms that have been made move-safe. (Note: if |u| is instead a temporary expression, don’t use |Move()|: just pass the expression, because it’s already move-ready. For more information see Move.h.)
UniquePtr is also better than std::auto_ptr in that the deletion operation is
customizable. An optional second template parameter specifies a class that
(through its operator()(T*)) implements the desired deletion policy. If no
policy is specified, mozilla::DefaultDelete
Use of UniquePtr proceeds like so:
UniquePtr
UniquePtr
struct S { int x; S(int x) : x(x) {} };
UniquePtr g3, g4(new S(5));
g3 = std::move(g4); // g3 owns the S, g4 cleared
S* p = g3.get(); // g3 still owns |p|
assert(g3->x == 5); // operator-> works (if .get() != nullptr)
assert((g3).x == 5); // also operator (again, if not cleared)
std::swap(g3, g4); // g4 now owns the S, g3 cleared
g3.swap(g4); // g3 now owns the S, g4 cleared
UniquePtr g5(std::move(g3)); // g5 owns the S, g3 cleared
g5.reset(); // deletes the S, g5 cleared
struct FreePolicy { void operator()(void* p) { free(p); } }; UniquePtr<int, FreePolicy> g6(static_cast<int*>(malloc(sizeof(int)))); int* ptr = g6.get(); g6 = nullptr; // calls free(ptr)
Now, carefully note a few things you can’t do:
UniquePtr
UniquePtr
(Note that changing a UniquePtr to store a direct |new| expression is permitted, but usually you should use MakeUnique, defined at the end of this header.)
A few miscellaneous notes:
UniquePtr, when not instantiated for an array type, can be move-constructed and move-assigned, not only from itself but from “derived” UniquePtr<U, E> instantiations where U converts to T and E converts to D. If you want to use this, you’re going to have to specify a deletion policy for both UniquePtr instantations, and T pretty much has to have a virtual destructor. In other words, this doesn’t work:
struct Base { virtual ~Base() {} }; struct Derived : Base {};
UniquePtr
UniquePtr
UniquePtr is specialized for array types. Specializing with an array type creates a smart-pointer version of that array – not a pointer to such an array.
UniquePtr<int[]> arr(new int[5]); arr[0] = 4;
What else is different? Deletion of course uses |delete[]|. An operator[] is provided. Functionality that doesn’t make sense for arrays is removed. The constructors and mutating methods only accept array pointers (not T*, U* that converts to T*, or UniquePtr<U[]> or UniquePtr) or |nullptr|.
It’s perfectly okay for a function to return a UniquePtr. This transfers the UniquePtr’s sole ownership of the data, to the fresh UniquePtr created in the calling function, that will then solely own that data. Such functions can return a local variable UniquePtr, |nullptr|, |UniquePtr(ptr)| where |ptr| is a |T*|, or a UniquePtr |Move()|’d from elsewhere.
UniquePtr will commonly be a member of a class, with lifetime equivalent to that of that class. If you want to expose the related resource, you could expose a raw pointer via |get()|, but ownership of a raw pointer is inherently unclear. So it’s better to expose a |const UniquePtr&| instead. This prohibits mutation but still allows use of |get()| when needed (but operator-> is preferred). Of course, you can only use this smart pointer as long as the enclosing class instance remains live – no different than if you exposed the |get()| raw pointer.
To pass a UniquePtr-managed resource as a pointer, use a |const UniquePtr&| argument. To specify an inout parameter (where the method may or may not take ownership of the resource, or reset it), or to specify an out parameter (where simply returning a |UniquePtr| isn’t possible), use a |UniquePtr&| argument. To unconditionally transfer ownership of a UniquePtr into a method, use a |UniquePtr| argument. To conditionally transfer ownership of a resource into a method, should the method want it, use a |UniquePtr&&| argument.