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Mark Nelson

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C++11: unique_ptr

June 26, 2012

There are a lot of great features in C++11, but unique_ptr stands out in the area of code hygiene. Simply put, this is a magic bullet for dynamically created objects. It won't solve every problem, but it is really, really good at what it does: managing dynamically created objects with simple ownership semantics.

The Basics

The class template unique_ptr<T> manages a pointer to an object of type T. You will usually construct an object of this type by calling new to create an object in the unique_ptr constructor:

std::unique_ptr<foo> p( new foo(42) );

After calling the constructor, you can use the object very much like a raw pointer. The * and -> operators work exactly like you would expect, and are very efficient — usually generating nearly the same assembly code as raw pointer access.

As you might expect from a class that wraps raw pointers, the first benefit you will get from using unique_ptr is automatic destruction of the contained object when the pointer goes out of scope. You don't have to track every possible exit point from a routine to make sure the object is freed properly — it is done automatically. And more importantly, it will be destroyed if your function exits via an exception.

Containers

So far this is nice, but hardly revolutionary. Writing a class that just does what I've described is fairly trivial, and you could have done it with the original C++ standard. In fact, the ill-fated (and now deprecated) auto_ptr was just that, a first stab at an RIAA pointer wrapper.

Unfortunately, the language hadn't evolved to the point where auto_ptr could be done properly. As a result, you couldn't use it for some pretty basic things. For example, you couldn't store auto_ptr objects in most containers. Kind of a big problem.

C++11 fixed these problems with the addition of rvalue references and move semantics. As a result, unique_ptr objects can be stored in containers, work properly when containers are resized or moved, and will still be destroyed when the container is destroyed. Just like you want.

Uniqueness and Move Semantics

So what exactly is the meaning of the word unique in this context? Mostly just what it says: When you create a unique_ptr object, you are declaring that you are going to have exactly one copy of this pointer. There is never any doubt about who owns it, because you can't inadvertently make copies of the pointer.

With a classic raw pointer, this kind of code is a bug lying in wait:

    foo *p = new foo("useful object");
    make_use( p );

Here I have allocated an object, and I have a pointer to it. When I call make_use, what happens to that pointer? Does make_use make a copy of it for later use? Does it take ownership of it and delete when done? Does it simply borrow it for a while and then return it to the caller for later destruction?

We can't really answer any of these questions with confidence, because C++ doesn't make it easy to have a contract regarding the use of a pointer. You end up relying on code inspection, memory, or documentation. All of these things break regularly.

With unique_ptr, you won't have these problems. If you want to pass the pointer to another routine, you won't make a duplicate copy of the pointer that has to be accounted for — the compiler prohibits it.

Who Owns the Pointer

Let's take a simple example — I create a pointer and want to store it in a container. As a new user of unique_ptr, I write some pretty straightforward code:

std::unique_ptr<foo> q( new foo(42) );
v.push_back( q );

This seems reasonable, but doing this gets me into a gray area: Who owns the pointer? Will the container destroy it at some point in its lifetime? Or is it still my job do so?

The rules of using unique_ptr prohibit this kind of code, and trying to compile it will lead to the classic cascade of template-based compiler errors (the ones that were going to be fixed with concepts, remember?), ending thus:

unique.cpp(26) : see reference to class template instantiation
 'std::vector<_Ty>' being compiled

Anyway, the problem here is that we are only allowed to have one copy of the pointer — unique ownership rules apply. If I want to give the object to another piece of code, I have to invoke move semantics:

v.push_back( std::move(q) );

After I move the object into the container, my original unique_ptr, q, has given up ownership of the pointer and it now rests with the container. Although object q still exists, any attempt to dereference it will generate a null pointer exception. In fact, after the move operation, the internal pointer owned by q has been set to null.

Move semantics will be used automatically any place you create an rvalue reference. For example, returning a unique_ptr from a function doesn't require any special code:

    return q;

Nor does passing a newly constructed object to a calling function:

process( std::unique_ptr<foo>( new foo(41) ) );

Legacy Code

We all have legacy code to deal with, and even when using unique_ptr, you will find that there are times you just have to pass some function a raw pointer. There are two ways to do this:

    do_something( q.get() );          //retain ownership
    do_something_else( q.release() ); //give up ownership

Calling get() returns a pointer to the underlying method. You really want to avoid calling this if you can, because as soon as you release that raw pointer into the wild, you have lost much of the advantage you achieved by switching to unique_ptr. With careful code inspection you can probably convince yourself that the pointer is indeed only being used ephemerally, and it will disappear once the called routine is done with it.

Extracting the pointer with release() is a more realistic way to do it. At this point you are saying "I'm done with the ownership of this pointer, it's yours now." Fair enough.

As your code base matures, you will need to say this less and less often.

One other place you will find that your code differs slightly from that using raw pointers is that you will now generally be passing unique_ptr as a reference argument:

void inc_baz( std::unique_ptr<foo> &p )
{
    p->baz++;
}

When passing by reference, we don't have to worry about values being inadvertently copied and muddying ownership.

The Cost

Proponents of unique_ptr argue that the cost of using this wrapper is minimal, and this seems like it is true. Below I show you a pair of routines that increment a member of the class foo. One is passed a raw pointer and increment routine, the other a unique_ptr.

Let's look at the disassembled code that was compiled in Release mode:

int inc_bazp( foo *p )
{
01331700  push        ebp  
01331701  mov         ebp,esp  
    return p->baz++;
01331703  mov         edx,dword ptr [p]  
01331706  mov         eax,dword ptr [edx+18h]  
01331709  lea         ecx,[eax+1]  
0133170C  mov         dword ptr [edx+18h],ecx  
}
0133170F  pop         ebp  
01331710  ret

int inc_baz( std::unique_ptr<foo> &p )
{
00AC16E0  push        ebp  
00AC16E1  mov         ebp,esp  
    return p->baz++;
00AC16E3  mov         eax,dword ptr [p]  
00AC16E6  mov         edx,dword ptr [eax]  
00AC16E8  mov         eax,dword ptr [edx+18h]  
00AC16EB  lea         ecx,[eax+1]  
00AC16EE  mov         dword ptr [edx+18h],ecx  
}
00AC16F1  pop         ebp  
00AC16F2  ret  

The unique_ptr version does indeed have one extra pointer dereference in comparison to the raw pointer version. While it is not so easy to count cycles these days, it seems likely that the difference between these two cases are going to be quite low, perhaps as low as 10%. A routine that performed multiple operations on the object would see the penalty reduced further.

One Final Note

I'm sold on unique_ptr, so you will be seeing it plenty in my code. However, you might be feeling a bit more cautious. And that's okay; with C++11, it is pretty easy for you to test the waters without too much extra work. Just make sure that the routines that use pointers use the auto type to hold all pointers — this means no changes in the consumer code if you change to/from raw pointers to unique_pointer. And routines that are passed these pointers can be defined as function templates, making it easy to adapt to whatever type of data is passed in.

In a perfect world, with no legacy code requiring raw pointer semantics, unique_ptr can literally guarantee that you won't leak data allocated from the free store. This was simply not possible in a reasonable way until C++11. Use it!

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