Overview

Simplified

template< typename T, /* ... */ >
class vector;

Detailed

Regular

template<
    class T,
    class Allocator = std::allocator<T>
> class vector;

Polymorphic (since C++17)

namespace pmr {
    template< class T >
    using vector = std::vector<T, std::pmr::polymorphic_allocator<T>>;
}

std::vector is a container that encapsulates dynamic size arrays.

Memory

The elements are stored contiguously, one after another. This means that a pointer to an element of a vector may be passed to any function that expects a pointer to an element of an array.

Storage size

The storage of the vector is handled automatically, being expanded and contracted as needed. Vectors usually occupy more space than static arrays, because more memory is allocated to handle future growth. This way a vector does not need to reallocate each time an element is inserted, but only when the additional memory is exhausted. The total amount of allocated memory can be queried using capacity() function. Extra memory can be returned to the system via a call to shrink_to_fit() (od C++11)

Reallocations are usually costly operations in terms of performance. The reserve() function can be used to eliminate reallocations if the number of elements is known beforehand.

Technical details

Complexity

The complexity (efficiency) of common operations on vectors is as follows:

• random access - constant 𝓞(1)
• insertion or removal of elements at the end - amortized constant 𝓞(1)
• insertion or removal of elements - linear in the distance to the end of the vector 𝓞(n)

Named requirements

std::vector (for T other than bool) meets the requirements of:

• Container
• AllocatorAwareContainer
• SequenceContainer
• ContiguousContainer (od C++17)
• ReversibleContainer

Template type requirements

T

od C++17

The requirements that are imposed on the elements depend on the actual operations performed on the container. Generally, it is required that element type meets the requirements of Erasable, but many member functions impose stricter requirements. This container (but not its members) can be instantiated with an incomplete element type if the allocator satisfies the allocator completeness requirements.

do C++17

The requirements that are imposed on the elements depend on the actual operations performed on the container. Generally, it is required that element type is a complete type and meets the requirements of Erasable, but many member functions impose stricter requirements.

do C++11

T must meet the requirements of CopyAssignable and CopyConstructible.

Allocator

The type must meet the requirements of Allocator.

od C++20

The program is ill-formed if Allocator::value_type is not the same as T.

do C++20

The behavior is undefined if Allocator::value_type is not the same as T.

std::vector

Defined in

vector

Template parameters

See technical details for more detailed information.

pub	T	Type of the elements.
pub	Allocator	Allocator that is used to acquire/release memory and to construct/destroy the elements in that memory.

Type names

pub	value_type	T
pub	allocator_type	Allocator
pub	size_type	Unsigned integer type (usually std::size_t)
pub	difference_type	Signed integer type (usually std::ptrdiff_t)
pub	reference	value_type&
pub	const_reference	value_type const&
pub	pointer	Allocator::pointer (do C++11) std::allocator_traits<Allocator>::pointer (od C++11)
pub	const_pointer	Allocator::const_pointer (do C++11) std::allocator_traits<Allocator>::const_pointer (od C++11)
pub	iterator	LegacyRandomAccessIterator and  LegacyContiguousIterator to value_type   (do C++20) LegacyRandomAccessIterator and  LegacyConstexprIterator and  contiguous_iterator to value_type   (od C++20)
pub	const_iterator	LegacyRandomAccessIterator and  LegacyContiguousIterator to value_type   (do C++20) LegacyRandomAccessIterator and  LegacyConstexprIterator and  contiguous_iterator to const value_type   (od C++20)
pub	reverse_iterator	std::reverse_iterator<iterator>
pub	const_reverse_iterator	std::reverse_iterator<const_iterator>

Member functions

pub	(constructors)	Constructs a vector.
pub	(destructor)	Destroys the vector, deallocating internal storage if used.
pub	operator=	Assigns values to the container.
pub	assign	Assigns values to the container.
pub	get_allocator	Returns the associated allocator.

Element access

pub	at	Accesses a specified element with bounds checking.
pub	operator[]	Accesses a specified element.
pub	front	Returns the first element.
pub	back	Returns the last element.
pub	data	Returns a pointer to the first element of the underlying array.

Iterators

pub	begin cbegin  (od C++11)	Returns an iterator/const_iterator to the beginning.
pub	end cend  (od C++11)	Returns an iterator/const_iterator to the end.
pub	rbegin crbegin  (od C++11)	Returns a reverse iterator/const_iterator to the beginning.
pub	rend crend  (od C++11)	Returns a reverse iterator/const_iterator to the end.

Capacity

pub	empty	Returns true if the vector is empty, otherwise false.
pub	size	Returns the number of elements.
pub	max_size	Returns the maximum possible number of elements.
pub	reserve	Reserves storage.
pub	capacity	Returns the number of elements that can be held in currently allocated storage.
pub	shrink_to_fit  (od C++11)	Reduces memory usage by freeing unused memory.

Operations

pub	clear	Clears the contents.
pub	insert	Inserts elements.
pub	emplace  (od C++11)	Constructs elements in-place.
pub	erase	Removes elements.
pub	push_back	Appends an element to the end.
pub	emplace_back  (od C++11)	Constructs elements in-place at the end.
pub	pop_back	Removes the last element.
pub	resize	Changes the number of elements stored.
pub	swap	Swaps the contents.

Non-member functions

pub	operator== operator!=  (removed in C++20) operator<  (removed in C++20) operator>  (removed in C++20) operator<=  (removed in C++20) operator>=  (removed in C++20) operator<=>	Lexicographically compares the values in the vector.
pub	std::swap (std::vector)	An overload for a std::swap algorithm.
pub	erase (std::vector) erase_if (std::vector)	Overloads for std::erase/std::erase_if algorithms.

Deduction guides (since C++17)

Click to expand

Deduction guides

// (1)
template< class InputIt,
          class Alloc = std::allocator<typename std::iterator_traits<InputIt>::value_type> >
vector( InputIt, InputIt, Alloc = Alloc() )
  -> vector<typename std::iterator_traits<InputIt>::value_type, Alloc>;

(1) allows deduction from an iterator range.

Overload resolution

In order for any of the deduction guides to participate in overload resolution, the folllowing requirements must be met:

• InputIt satisfies the requirements of LegacyInputIterator
• Alloc satisfies the requirements of Allocator

zanotuj

The extent to which the library determines that a type does not satisfy LegacyInputIterator is unspecified, except that as a minimum:

• Integral types do not qualify as input iterators.

Likewise, the extent to which it determines that a type does not satisfy Allocator is unspecified, except that as a minimum:

• The member type Alloc::value_type must exist.
• The expression std::declval<Alloc&>().allocate(std::size_t{}) must be well-formed when treated as an unevaluated operand.

Examples

Basic usage

Calculate arithmetic mean

#include <iostream>
#include <vector>

auto main() -> int
{
  auto const numbers = std::vector<double>{
    5, 2.4, 3.2, 1.8, 4.6,
    3.1, 2.9, 4.7, 3.8, 2.2,
  };

  auto sum = 0.0;
  for (auto number : numbers) {
    sum += number;
  }
  
  auto const mean = sum / numbers.size();
  std::cout << "Mean: " << mean << '\n';
}

Result

Mean: 3.37

Find the maximum element

#include <iostream>
#include <string>
#include <algorithm>

auto main() -> int
{
  auto const numbers = std::vector<int>{
    5, 2, 8, 1, 14, 3, 2, 10, 3, 2
  };
  auto const max_it = std::max_element(
    numbers.begin(),
    numbers.end()
  );
  auto const max_index = std::distance(numbers.begin(), max_it);
  std::cout << "Max: " << *max_it << " at index: " << max_index;
}

Result

Max: 14 at index: 4

Erase nth element

#include <iostream>
#include <vector>
#include <string>

auto main() -> int
{
  auto player_names = std::vector<std::string>{
    "Sad Crab", "Happy Wolf", "Angry Panda",
    "Wicked Witch", "Crazy Clown", "Funny Monkey",
  };

  auto const n = 2; // arbitrary, 0-based index
  // Erase the nth element ("Angry Panda")
  player_names.erase(player_names.begin() + n);

  for (auto const& name : player_names) {
    std::cout << name << '\n';
  }
}

Result

Sad Crab
Happy Wolf
Wicked Witch
Crazy Clown
Funny Monkey