Is there any use for unique_ptr with array?

Solution 1:

Some people do not have the luxury of using std::vector, even with allocators. Some people need a dynamically sized array, so std::array is out. And some people get their arrays from other code that is known to return an array; and that code isn't going to be rewritten to return a vector or something.

By allowing unique_ptr<T[]>, you service those needs.

In short, you use unique_ptr<T[]> when you need to. When the alternatives simply aren't going to work for you. It's a tool of last resort.

Solution 2:

There are tradeoffs, and you pick the solution which matches what you want. Off the top of my head:

Initial size

• vector and unique_ptr<T[]> allow the size to be specified at run-time
• array only allows the size to be specified at compile time

Resizing

• array and unique_ptr<T[]> do not allow resizing
• vector does

Storage

• vector and unique_ptr<T[]> store the data outside the object (typically on the heap)
• array stores the data directly in the object

Copying

• array and vector allow copying
• unique_ptr<T[]> does not allow copying

Swap/move

• vector and unique_ptr<T[]> have O(1) time swap and move operations
• array has O(n) time swap and move operations, where n is the number of elements in the array

Pointer/reference/iterator invalidation

• array ensures pointers, references and iterators will never be invalidated while the object is live, even on swap()
• unique_ptr<T[]> has no iterators; pointers and references are only invalidated by swap() while the object is live. (After swapping, pointers point into to the array that you swapped with, so they're still "valid" in that sense.)
• vector may invalidate pointers, references and iterators on any reallocation (and provides some guarantees that reallocation can only happen on certain operations).

Compatibility with concepts and algorithms

• array and vector are both Containers
• unique_ptr<T[]> is not a Container

I do have to admit, this looks like an opportunity for some refactoring with policy-based design.

Solution 3:

One reason you might use a unique_ptr is if you don't want to pay the runtime cost of value-initializing the array.

std::vector<char> vec(1000000); // allocates AND value-initializes 1000000 chars

std::unique_ptr<char[]> p(new char[1000000]); // allocates storage for 1000000 chars

The std::vector constructor and std::vector::resize() will value-initialize T - but new will not do that if T is a POD.

See Value-Initialized Objects in C++11 and std::vector constructor

Note that vector::reserve is not an alternative here: Is accessing the raw pointer after std::vector::reserve safe?

It's the same reason a C programmer might choose malloc over calloc.

Solution 4:

An std::vector can be copied around, while unique_ptr<int[]> allows expressing unique ownership of the array. std::array, on the other hand, requires the size to be determined at compile-time, which may be impossible in some situations.