Apache C++ Standard Library Reference Guide

## vector

Library: Containers

    Does not inherit

- Local Index
- Summary
- Synopsis
- Description
- Special Case
- Interface
- Constructors
- Iterators
- Assignment Operator
- Allocator
- Reference Operators
- Member Functions
- Nonmember Operators
- Specialized Algorithms
- Example
- Warnings
- See Also
- Standards Conformance

### Local Index

#### Members

assign() at() back() begin() capacity() clear() empty() | end() erase() flip() front() get_allocator() insert() max_size() | operator=() operator[]() pop_back() push_back() rbegin() rend() reserve() | resize() size() swap() vector()

#### Non-Members

operator!=() operator>() | operator>=() operator<() | operator<=() operator==() |

### Summary

A sequence that supports random access iterators

### Synopsis

    #include <vector>

    namespace std {
      template <class T, class Allocator = allocator<T> >
      class vector;
    }

### Description

vector is a sequence that supports random access iterators. It also supports amortized constant time insert and erase operations at the end (Insert and erase in the middle take linear time.). Storage management is handled automatically. In vector, `iterator` is a random access iterator referring to `T`. `const_iterator` is a constant random access iterator that returns a `const T&` when dereferenced. A constructor for `iterator` and `const_iterator` is guaranteed. `size_type` is an unsigned integral type. `difference_type` is a signed integral type.

Any type used for the template parameter `T` must provide the following (where `T` is the `type`, `t` is a `value` of `T` and `u` is a `const` `value` of `T`):

      Copy constructors     T(t) and T(u)
      Destructor            t.~T()
      Address of            &t and &u yielding T* and const T*
                            respectively
      Assignment            t = a where a is a
                            (possibly const) value of T

### Special Case

Vectors of bit values, that is boolean 1/0 values, are handled as a special case by the standard library, so that they can be efficiently packed several elements to a word. The operations for a boolean vector, vector<bool>, are a superset of those for an ordinary vector, only the implementation is more efficient.

Two member functions are available to the boolean vector data type. One is `flip()`, which inverts all the bits of the vector. Boolean vectors also return as reference an internal value that also supports the `flip()` member function. The other member function specific to vector<bool> is a second form of the `swap()` function.

### Interface

    namespace std {

      template <class T, class Allocator = allocator<T> >
      class vector {

      public:

      // Types
      typedef T value_type;
      typedef Allocator allocator_type;
      typedef typename
              Allocator::reference reference;
      typedef typename
              Allocator::const_reference const_reference;
      class iterator;
      class const_iterator;
      typedef typename
              Allocator::size_type size_type;
      typedef typename
              Allocator::difference_type difference_type;
      typedef typename Allocator::pointer pointer;
      typedef typename Allocator::const_pointer const_pointer;
      typedef typename std::reverse_iterator<iterator>
                       reverse_iterator;
      typedef typename std::reverse_iterator<const iterator>
                       const_reverse_iterator;

      // Construct/Copy/Destroy
      explicit vector(const Allocator& = Allocator());
      explicit vector(size_type, const Allocator& = Allocator ());
      vector(size_type, const T&, const Allocator& = Allocator());
      vector(const vector<T, Allocator>&);
      template <class InputIterator>
      vector(InputIterator, InputIterator,
             const Allocator& = Allocator());
      vector<T,Allocator>& operator=(const vector<T, Allocator>&);
      template <class InputIterator>
      void assign(InputIterator start, InputIterator finish);
      void assign(size_type, const);
      allocator_type get_allocator () const;

      // Iterators
      iterator begin();
      const_iterator begin() const;
      iterator end();
      const_iterator end() const;
      reverse_iterator rbegin();
      const_reverse_iterator rbegin() const;
      reverse_iterator rend();
      const_reverse_iterator rend() const;

      // Capacity
      size_type size() const;
      size_type max_size() const;
      void resize(size_type);
      void resize(size_type, T);
      size_type capacity() const;
      bool empty() const;
      void reserve(size_type);

      // Element Access
      reference operator[](size_type);
      const_reference operator[](size_type) const;
      reference at(size_type);
      const_reference at(size_type) const;
      reference front();
      const_reference front() const;
      reference back();
      const_reference back() const;

      // Modifiers
      void push_back(const T&);
      void pop_back();
      iterator insert(iterator, const T&);
      void insert(iterator, size_type, const T&);
      template <class InputIterator>
      void insert(iterator, InputIterator, InputIterator);
      iterator erase(iterator);
      iterator erase(iterator, iterator);
      void swap(vector<T, Allocator>&);
      void clear()
      };

      // Nonmember Operators
      template <class T>
      bool operator==(const vector<T,Allocator>&,
      const vector <T,Allocator>&);
      template <class T>
      bool operator!=(const vector<T,Allocator>&,
      const vector <T,Allocator>&);
      template <class T>
      bool operator<(const vector<T,Allocator>&,
      const vector<T,Allocator>&);
      template <class T>
      bool operator>(const vector<T,Allocator>&,
      const vector<T,Allocator>&);
      template <class T>
      bool operator<=(const vector<T,Allocator>&,
      const vector<T,Allocator>&);
      template <class T>
      bool operator>=(const vector<T,Allocator>&,
      const vector<T,Allocator>&);

      // Specialized Algorithms
      template <class T, class Allocator>
      void swap (const vector<T,Allocator>&,
                 const vector<T,Allocator>&);
    }

### Constructors

    explicit vector(const Allocator& alloc = Allocator());

The default constructor. Creates a vector of length zero. The vector will use the allocator `alloc` for all storage management.

    explicit vector(size_type n);

Creates a vector of length `n`, containing `n` copies of the default value for type `T`. Requires that `T` have a default constructor. The vector will use the allocator `Allocator()` for all storage management.

    vector(size_type n, const T& value,
    const Allocator& alloc = Allocator());

Creates a vector of length `n`, containing `n` copies of value. The vector will use the allocator `alloc` for all storage management.

    vector(const vector<T, Allocator>& x);

Creates a copy of `x`.

    template <class InputIterator>
    vector(InputIterator start, InputIterator finish,
    const Allocator& alloc = Allocator());

Creates a vector of length `finish - start`, filled with all values obtained by dereferencing the `InputIterators` on the range `[start, finish)`. The vector will use the allocator `alloc` for all storage management.

### Iterators

    iterator
    begin();

Returns a random access `iterator` that points to the first element.

    const_iterator
    begin() const;

Returns a random access `const_iterator` that points to the first element.

    iterator
    end();

Returns a random access `iterator` that points to the past-the-end value.

    const_iterator
    end() const;

Returns a random access `const_iterator` that points to the past-the-end value.

    reverse_iterator
    rbegin();

Returns a random access `reverse_iterator` that points to the past-the-end value.

    const_reverse_iterator
    rbegin() const;

Returns a random access `const_reverse_iterator` that points to the past-the-end value.

    reverse_iterator
    rend();

Returns a random access `reverse_iterator` that points to the first element.

    const_reverse_iterator
    rend() const;

Returns a random access `const_reverse_iterator` that points to the first element.

### Assignment Operator

    vector<T, Allocator>&
    operator=(const vector<T, Allocator>& x);

Erases all elements in self then inserts into self a copy of each element in `x`. Returns a reference to self.

### Allocator

    allocator_type
    get_allocator() const;

Returns a copy of the allocator used by self for storage management.

### Reference Operators

    reference
    operator[](size_type n);

Returns a reference to element `n` of self. The result can be used as an lvalue. The index `n` must be between 0 and the `size` less one.

    const_reference
    operator[](size_type n) const;

Returns a constant reference to element `n` of self. The index `n` must be between 0 and the `size` less one.

### Member Functions

    template <class InputIterator>
    void
    assign(InputIterator start, InputIterator finish);

If InputIterator is an integral type, the function calls `assign(size_type(start), value_type (finish))`. Otherwise, the function replaces elements in `*this` with copies of those in the range `[start, finish)`. The function invalidates all iterators and references to elements in `*this`.

    void
    assign(size_type n, const T& t);

Replaces elements in `*this` with `n` copies of `t`. The function invalidates all iterators and references to elements in `*this`.

    reference
    at(size_type n);

Returns a reference to element `n` of self. The result can be used as an lvalue. The index `n` must be between 0 and the `size` less one.

    const_reference
    at(size_type) const;

Returns a constant reference to element `n` of self. The index `n` must be between 0 and the `size` less one.

    reference
    back();

Returns a reference to the last element.

    const_reference
    back() const;

Returns a constant reference to the last element.

    size_type
    capacity() const;

Returns the size of the allocated storage, as the number of elements that can be stored.

    void
    clear() ;

Deletes all elements from the vector.

    bool
    empty() const;

Returns `true` if the `size` is zero.

    iterator
    erase(iterator position);

Deletes the vector element pointed to by the iterator `position`. Returns an `iterator` pointing to the element following the deleted element, or `end()` if the deleted element was the last one in this vector.

    iterator
    erase(iterator start, iterator finish);

Deletes the vector elements in the range `[start, finish)`. Returns an `iterator` pointing to the element following the last deleted element, or `end()` if there were no elements in the deleted range.

    void
    flip();

Flips all the bits in the vector. This member function is only defined for vector<bool>.

    reference
    front();

Returns a reference to the first element.

    const_reference
    front() const;

Returns a constant reference to the first element.

    iterator
    insert(iterator position, const T& x);

Inserts `x` before `position`. The return value points to the inserted `x`.

    void
    insert(iterator position, size_type n, const  T& x);

Inserts `n` copies of `x` before `position`.

    template <class InputIterator>
    void
    insert(iterator position, InputIterator start,
            InputIterator finish);

Inserts copies of the elements in the range `[start, finish)` before `position`.

    size_type
    max_size() const;

Returns `size()` of the largest possible vector.

    void
    pop_back();

Removes the last element of self.

    void
    push_back(const T& x);

Inserts a copy of `x` to the end of self.

    void
    reserve(size_type n);

Increases the capacity of self in anticipation of adding new elements. `reserve` itself does not add any new elements. After a call to `reserve,` `capacity()` is greater than or equal to `n` and subsequent insertions will not cause a reallocation until the size of the vector exceeds `n`. Reallocation does not occur if `n` is less than `capacity()`. If reallocation does occur, then all iterators and references pointing to elements in the vector are invalidated. `reserve` takes at most linear time in the size of self. `reserve` throws a length_error exception if `n` is greater than `max_size()`.

    void
    resize(size_type sz);

Alters the size of self. If the new size (`sz`) is greater than the current size, then `sz-size()` instances of the default value of type`T` are inserted at the end of the vector. If the new size is smaller than the current `capacity`, then the vector is truncated by erasing `size()-sz` elements off the end. If `sz` is equal to `capacity` then no action is taken.

    void
    resize(size_type sz, T c);

Alters the size of self. If the new size (`sz`) is greater than the current size, then `sz-size()` `c`'s are inserted at the end of the vector. If the new size is smaller than the current `capacity`, then the vector is truncated by erasing `size()-sz` elements off the end. If `sz` is equal to `capacity` then no action is taken.

    size_type
    size() const;

Returns the number of elements.

    void
    swap(vector<T, Allocator>& x);

Exchanges self with `x`, by swapping all elements.

    void
    swap(reference x, reference y);

Swaps the values of `x` and `y`. This is a member function of vector<bool> only.

### Nonmember Operators

    template <class T, class Allocator>
    bool operator==(const vector<T, Allocator>& x,
                    const vector<T, Allocator>& y);

Returns `true` if `x` is the same as `y`.

    template <class T, class Allocator>
    bool operator!=(const vector<T, Allocator>& x,
                    const vector<T, Allocator>& y);

Returns `!(x==y)`.

    template <class T>
    bool operator<(const vector<T, Allocator>& x,
                   const vector<T, Allocator>& y);

Returns `true` if the elements contained in `x` are lexicographically less than the elements contained in `y`.

    template <class T>
    bool operator>(const vector<T, Allocator>& x,
                   const vector<T, Allocator>& y);

Returns `y < x`.

    template <class T>
    bool operator<=(const vector<T, Allocator>& x,
                    const vector<T, Allocator>& y);

Returns `!(y < x)`.

    template <class T>
    bool operator>=(const vector<T, Allocator>& x,
                    const vector<T, Allocator>& y);

Returns `!(x < y)`.

### Specialized Algorithms

    template <class T, class Allocator>void
    swap(vector <T, Allocator>& a, vector <T, Allocator>& b);

Efficiently swaps the contents of `a` and `b`.

### Example

    //
    //  vector.cpp
    //

    #include <vector>
    #include <iostream>

    typedef std::ostream_iterator<int, char,
                                  std::char_traits<char> >
            os_iter_t;
    typedef std::vector<int,std::allocator<int> > vector_t;

    std::ostream& operator<< (std::ostream& out,
                              const vector_t& v) {
        std::copy(v.begin(), v.end(), os_iter_t(out, " "));
        return out;
    }

    int main () {
        // Create a vector of doubles, and one of integers.
        vector_t             vi;
        vector_t::size_type   i;

        // Insert values before the beginning.
        for (i = 0; i < 10; ++i)
            vi.insert(vi.begin(), i);

        // Print out the vector.
        std::cout << vi << std::endl;

        // Now let's erase half of the elements.
        vector_t::size_type half = vi.size() / 2;
        for (i = 0; i < half; ++i)
            vi.erase(vi.begin());

        // Print it out again.
        std::cout << vi << std::endl;

        return 0;
    }

    Program Output:

    9 8 7 6 5 4 3 2 1 0
    4 3 2 1 0

### Warnings

Member function templates are used in all containers provided by the Standard Template Library. For example, the constructor for vector takes two templatized iterators:

    template <class InputIterator>
    vector (InputIterator, InputIterator,
            const Allocator = Allocator());

vector also has an insert function of this type. These functions, when not restricted by compiler limitations, allow you to use any type of input iterator as arguments. For compilers that do not support this feature we provide substitute functions that allow you to use an iterator obtained from the same type of container as the one you are constructing (or calling a member function on), or you can use a pointer to the type of element you have in the container.

For example, if your compiler does not support member function templates you can construct a vector in the following two ways:

    int intarray[10];
    vector<int> first_vector(intarray, intarray + 10);
    vector<int> second_vector(first_vector.begin(),
                              first_vector.end());

but not this way:

    vector<long>
    long_vector(first_vector.begin(),first_vector.end());

since the `long_vector` and `first_vector` are not the same type.

If your compiler does not support default template parameters, you must supply the `Allocator` template argument. For instance, you must write :

    vector<int, allocator<int> >

instead of :

    vector<int>

### See Also

allocator, Containers, Iterators, `lexicographical_compare()`

### Standards Conformance

ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 23.2.4