GotW #54

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This is the original GotW problem and solution substantially as posted to Usenet. See the book More Exceptional C++ (Addison-Wesley, 2002) for the most current solution to this GotW issue. The solutions in the book have been revised and expanded since their initial appearance in GotW. The book versions also incorporate corrections, new material, and conformance to the final ANSI/ISO C++ standard.

Using Vector and Deque 
Difficulty: 8 / 10

What is the difference between vector and deque? When should you use each one? And how can you properly shrink such containers when you no longer need their full capacity? These answers and more, as we consider news updates from the standards front.


JG Question

1. In the standard library, vector and deque provide similar services. Which should you typically use? Why? Under what circumstances would you use the other?

Guru Questions

2. What does the following code do?

    vector<C> c( 10000 );
    c.erase( c.begin()+10, c.end() );
    c.reserve( 10 );

3. A vector or deque typically reserves extra internal storage as a hedge against future growth, to prevent too-frequent reallocation as new elements are added. Is it possible to completely clear a vector or deque (that is, remove all contained elements AND free all internally reserved storage)? Demonstrate why or why not.

Warning: Answers 2 and 3 may be subtle. Each has a facile answer, but don't stop at the surface; try to be as detailed as possible.


In Most Cases, Prefer Using deque (Controversial)

1. In the standard library, vector and deque provide similar services. Which should you typically use? Why? Under what circumstances would you use the other?

The C++ Standard, in section 23.1.1, offers some advice on which containers to prefer. It says:

vector is the type of sequence that should be used by default... deque is the data structure of choice when most insertions and deletions take place at the beginning or at the end of the sequence.

I'd like to present an amiably dissenting point of view: I recommend that you consider preferring deque by default instead of vector, especially when the contained type is a class or struct and not a builtin type, unless you really need the container's memory to be contiguous.

vector and deque offer nearly identical interfaces and are generally interchangeable. deque also offers push_front() and pop_front(), which vector does not. (True, vector offers capacity() and reserve(), which deque does not, but that's no loss -- those functions are actually a weakness of vector, as I'll demonstrate in a moment.)

The main structural difference between vector and deque lies in the way the two containers organize their internal storage: Under the covers, a deque allocates its storage in pages, or "chunks," with a fixed number of contained elements in each page; this why a deque is often compared to (and pronounced as) a "deck" of cards, although its name originally came from "double-ended queue" because of is ability to insert elements efficiently at either end of the sequence. On the other hand, a vector allocates a contiguous block of memory, and can only insert elements efficiently at the end of the sequence.

The paged organization of a deque offers significant benefits:

1. A deque offers constant-time insert() and erase() operations at the front of the container, whereas a vector does not -- hence the note in the Standard about using a deque if you need to insert or erase at both ends of the sequence.

2. A deque uses memory in a more operating system-friendly way, particularly on systems without virtual memory. For example, a 10-megabyte vector uses a single 10-megabyte block of memory, which is usually less efficient in practice than a 10-megabyte deque that can fit in a series of smaller blocks of memory.

3. A deque is easier to use, and inherently more efficient for growth, than a vector. The only operations supplied by vector that deque doesn't have are capacity() and reserve() -- and that's because deque doesn't need them! For vector, calling reserve() before a large number of push_back()s can eliminate reallocating ever-larger versions of the same buffer every time it finds out that the current one isn't big enough after all. A deque has no such problem, and having a deque::reserve() before a large number of push_back()s would not eliminate any allocations (or any other work) because none of the allocations are redundant; the deque has to allocate the same number of extra pages whether it does it all at once or as elements are actually appended.

Interestingly, the Standard stack adapter, which can only grow in one direction and so never needs insertion in the middle or at the other end, has its default implementation as a deque:

    template <class T, class Container = deque<T> >
    class stack {
      // ...

Aside: For Those Concerned About the Above

Some readers are going to be concerned about my advice to prefer deque, saying perhaps: "But deque is a more complex container than vector, and so deque must be very inefficient compared to vector, right?" As always, beware premature optimization before you actually measure: I have found the "deque must be inefficient" assumption to be generally untrue on popular implementations. Using MSVC 5.0 SP3 with the current patches for its bundled Dinkumware C++ Standard Library implementation (probably by far the most widely-used compiler and library configuration), I tested the performance of the following operations, in order:

  Operation   Description
  ----------- -------------------------------------------
  grow        first, perform 1,000,000 push_back()s
  traverse    then traverse, simply incrementing
                iterators, from begin() to end()
  at          then access each element in turn using at()
  shuffle     then random_shuffle() the entire container
  sort        then sort() the entire container (for list,
                uses list::sort())

I tested each operation against several standard containers including deque. I expected vector to outperform deque in "traverse" and "at", and deque to win in "grow" for reason #3 above. In fact, note that deque did outperform vector on the "grow" test even though in fairness I gave vector special treatment by calling vector::reserve(1000000) first to avoid any reallocations. All timings varied less than 5% across multiple runs, and all runs executed fully in-memory without paging or other disk accesses.

First, consider what happened for containers of a simple builtin type, int:

  Times in ms     grow traverse      at  shuffle    sort
  ------------ ------- -------- ------- -------- -------
  vector<int>     1015       55     219      621    3624
  deque<int>       761      265     605     1370    7820
  list<int>       1595      420     n/a      n/a   16070

Here list was always the slowest, and the difference between vector and deque wasn't as great as several people had led me to expect. Of course, the performance differences between the containers will fade when you use a contained type that is more expensive to manipulate. What's interesting is that most of the differences go away even when the contained type is as simple (and common) as a struct E { int i; char buf[100]; };:

  Times in ms     grow traverse      at  shuffle    sort
  ------------ ------- -------- ------- -------- -------
  vector<E>       3253       60     519     3825   17546
  deque<E>        3471      274     876     4950   21766
  list<E>         3740      497     n/a      n/a   15134

Now deque's performance disadvantage for even an intensive operation like sort is less than 25%.

Finally, note that the popular library implementation that I tested has since been revised and now includes a streamlined version of deque with simplified iterators. I do not yet have a copy of that library, but it will be interesting to see how much of the deque disadvantage in even the raw iterator "traverse" and element-accessing "at" tests will remain compared to vector.

So, for the three reasons cited earlier: Consider preferring deque by default in your programs, especially when the contained type is a class or struct and not a builtin type, unless the actual performance difference in your situation is known to be important or you specifically require the contained elements to be contiguous in memory (which typically means that you intend to pass the contents to a function that expects an array).

NOTE: If you think that relying on vector storage to be contiguous is a Bad Thing, in light of recent newsgroup discussions, see my upcoming article in the July/August 1999 C++ Report... I give reasons why doing so is arguably okay, safe, and even portable.

The Incredible Shrinking vector

One of std::vector's most endearing features, at least compared to C-style arrays, is its encapsulated storage management. As we push elements onto a vector it just grows its storage automatically, and we can even give the vector hints about how much capacity to keep ready under the covers as it grows (by first calling reserve()) and only incur at most a single reallocation hit. This allows optimally efficient growth.

But what if we're doing the opposite? What if we're using a vector that's pretty big, and then we remove elements that we no longer need and want the vector to shrink to fit; that is, we want it to get rid of the now-unneeded extra capacity? You might think that the following would work:

2. What does the following code do?

    vector<C> c( 10000 );

Line 1 creates a vector<C> object named c that initially contains 10,000 default-constructed C objects. At this point, we know that c.capacity() >= 10,000.

    c.erase( c.begin()+10, c.end() );

Line 2 erases all but the first 10 elements in c. At this point, c's capacity is probably unchanged.

    c.reserve( 10 );

Alas, line 3 does NOT shrink c's internal buffer to fit! Now c.capacity() is still >= 10000 as before.

This example doesn't do what you might expect because calling reserve() will never shrink the vector's capacity; it can only increase the capacity, or do nothing if the capacity is already sufficient.

The Right Way To "Shrink-To-Fit" a vector or deque

So, can we write code that does shrink a vector "to fit" so that its capacity is just enough to hold the contained elements? Obviously reserve() can't do the job, but fortunately there is indeed a way:

    vector<Customer>( c ).swap( c );
    // c.capacity() == c.size(), or
    // perhaps a little more than c.size()

Do you see how the shrink-to-fit statement works? It's a little subtle:

1. First, we create a temporary (unnamed) vector<Customer> and initialize it to hold the same contents as c. The salient difference between the temporary vector and c is that, while c still carries around a lot of extra capacity in its oversize internal buffer, the temporary vector has just enough capacity to hold its copy of c's contents. (Some implementations may choose to round up the capacity slightly to their next larger internal "chunk size," with the result that the capacity actually ends up being slightly larger than the size.)

2. Next, we call swap() to exchange the internals of c with the temporary vector. Now the temporary vector owns the oversize buffer with the extra capacity that we're trying to get rid of, and c owns the "rightsized" buffer with the just-enough capacity.

3. Finally, the temporary vector goes out of scope, carrying away the old oversize buffer with it; the old buffer is deleted when the temporary vector is destroyed. Now all we're left with is c itself, but now c has a "rightsized" capacity.

Note that this procedure is not needlessly inefficient. Even if vector had a special-purpose shrink_to_fit() member function, it would have to do pretty much all of the same work just described above.

The Right Way To Completely Clear a vector or deque

3. A vector or deque typically reserves extra internal storage as a hedge against future growth, to prevent too-frequent reallocation as new elements are added. Is it possible to completely clear a vector or deque (that is, remove all contained elements AND free all internally reserved storage)? Demonstrate why or why not.

Again, the answer is Yes, it is possible. If you want to completely clear a vector, so that it has no contents and no extra capacity at all, the code is nearly identical to the shrink-to-fit code... you just initialize the temporary vector to be empty instead of making it a copy of c:

    vector<Customer>().swap( c );
    // c.capacity() == 0, unless the
    // implementation happens to enforce a
    // minimum size even for empty vectors

Again, note that the vector implementation you're using may choose to make even empty vectors have some slight capacity, but now you're guaranteed that c's capacity will be the smallest possible allowed by your implementation: it will be the same as the capacity of an empty vector.

These techniques work for deque, too, but you don't need to do this kind of thing for list, set, map, multiset, or multimap because they allocate storage on an "exactly-as-needed" basis and so should never have excess capacity lying around.

Copyright 2009 Herb Sutter