vector


class template
<vector>

Vector

Vectors are a kind of sequence containers. As such, their elements are ordered following a strict linear sequence.

Vector containers are implemented as dynamic arrays; Just as regular arrays, vector containers have their elements stored in contiguous storage locations, which means that their elements can be accessed not only using iterators but also using offsets on regular pointers to elements.

But unlike regular arrays, storage in vectors is handled automatically, allowing it to be expanded and contracted as needed.

Vectors are good at:
  • Accessing individual elements by their position index (constant time).
  • Iterating over the elements in any order (linear time).
  • Add and remove elements from its end (constant amortized time).


Compared to arrays, they provide almost the same performance for these tasks, plus they have the ability to be easily resized. Although, they usually consume more memory than arrays when their capacity is handled automatically (this is in order to accomodate for extra storage space for future growth).

Compared to the other base standard sequence containers (deques and lists), vectors are generally the most efficient in time for accessing elements and to add or remove elements from the end of the sequence. For operations that involve inserting or removing elements at positions other than the end, they perform worse than deques and lists, and have less consistent iterators and references than lists.

Internally, vectors -like all containers- have a size, which represents the amount of elements contained in the vector. But vectors, also have a capacity, which determines the amount of storage space they have allocated, and which can be either equal or greater than the actual size. The extra amount of storage allocated is not used, but is reserved for the vector to be used in the case it grows. This way, the vector does not have to reallocate storage on each occasion it grows, but only when this extra space is exhausted and a new element is inserted (which should only happen in logarithmic frequence in relation with its size).

Reallocations may be a costly operation in terms of performance, since they generally involve the entire storage space used by the vector to be copied to a new location. Therefore, whenever large increases in size are planned for a vector, it is recommended to explicitly indicate a capacity for the vector using member function vector::reserve.

In their implementation in the C++ Standard Template Library vectors take two template parameters:
 
template < class T, class Allocator = allocator<T> > class vector;

Where the template parameters have the following meanings:
  • T: Type of the elements.
  • Allocator: Type of the allocator object used to define the storage allocation model. By default, the allocator class template for type T is used, which defines the simplest memory allocation model and is value-independent.

In the reference for the vector member functions, these same names are assumed for the template parameters.

Member functions



Iterators:


Capacity:


Element access:


Modifiers:


Allocator:


Member types

of template <class T, class Allocator=allocator<T> > class vector;

member typedefinition
referenceAllocator::reference
const_referenceAllocator::const_reference
iteratorRandom access iterator
const_iteratorConstant random access iterator
size_typeUnsigned integral type (usually same as size_t)
difference_typeSigned integral type (usually same as ptrdiff_t)
value_typeT
allocator_typeAllocator
pointerAllocator::pointer
const_pointerAllocator::const_pointer
reverse_iteratorreverse_iterator<iterator>
const_reverse_iteratorreverse_iterator<const_iterator>


Vector specialization: vector<bool>

The vector class template has a special template specialization for the bool type.

This specialization is provided to optimize for space allocation: In this template specialization, each element occupies only one bit (which is eight times less than the smallest type in C++: char).

The references to elements of a bool vector returned by the vector members are not references to bool objects, but a special member type which is a reference to a single bit, defined inside the vector<bool> class specialization as:

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class vector<bool>::reference {
  friend class vector;
  reference();                                 // no public constructor
public:
  ~reference();
  operator bool () const;                      // convert to bool
  reference& operator= ( const bool x );       // assign from bool
  reference& operator= ( const reference& x );  // assign from bit
  void flip();                                 // flip bit value.
}


For a similar container class to contain bits, but with a fixed size, see bitset.