Wednesday, May 9, 2012

Abilities of Deques

Abilities of Deques

Deques have the following differences compared with the abilities of vectors:
  • Inserting and removing elements is fast both at the beginning and at the end (for vectors it is only fast at the end). These operations are done in amortized constant time.
  • The internal structure has one more indirection to access the elements, so element access and iterator movement of deques are usually a bit slower.
  • Iterators must be smart pointers of a special type rather than ordinary pointers because they must jump between different blocks.
  • In systems that have size limitations for blocks of memory (for example, some PC systems), a deque might contain more elements because it uses more than one block of memory. Thus, max_size() might be larger for deques.
  • Deques provide no support to control the capacity and the moment of reallocation. In particular, any insertion or deletion of elements other than at the beginning or end invalidates all pointers, references, and iterators that refer to elements of the deque. However, reallocation may perform better than for vectors, because according to their typical internal structure, deques don't have to copy all elements on reallocation.
  • Blocks of memory might get freed when they are no longer used, so the memory size of a deque might shrink (however, whether and how this happens is implementation specific).
The following features of vectors also apply to deques:
  • Inserting and deleting elements in the middle is relatively slow because all elements up to either of both ends may be moved to make room or to fill a gap.
  • Iterators are random access iterators.

In summary, you should prefer a deque if the following is true:
  • You insert and remove elements at both ends (this is the classic case for a queue).
  • You don't refer to elements of the container.
  • It is important that the container frees memory when it is no longer used (however, the standard does not guarantee that this happens).

The interface of vectors and deques is almost the same, so trying both is very easy when no special feature of a vector or a deque is necessary.

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