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std::vector

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< cpp‎ | container
 
C++
 
Containers library
 
std::vector
Member types
Member functions
Non-member functions
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)
Deduction guides(C++17)
 
Defined in header <vector>
template<

    class T,
    class Allocator = std::allocator<T>

> class vector;
 (1)
namespace pmr {

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

}
 (2) (since C++17)
1) std::vector is a sequence container that encapsulates dynamic size arrays.
2) std::pmr::vector is an alias template that uses a polymorphic allocator.

Except for the std::vector<bool> partial specialization, the elements are stored contiguously, which means that elements can be accessed not only through iterators, but also using offsets to regular pointers to elements. 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.

The storage of the vector is handled automatically, being expanded 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()[1].

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.

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).

std::vector (for T other than bool) meets the requirements of Container, AllocatorAwareContainer(since C++11), SequenceContainer, ContiguousContainer(since C++17) and ReversibleContainer.

All member functions of std::vector are constexpr: it is possible to create and use std::vector objects in the evaluation of a constant expression.

However, std::vector objects generally cannot be constexpr, because any dynamically allocated storage must be released in the same evaluation of constant expression.

 (since C++20)
  1. ↑ In libstdc++, shrink_to_fit() is not available in C++98 mode.

Contents

Template parameters

T - The type of the elements.
T must meet the requirements of CopyAssignable and CopyConstructible. (until C++11)
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. (since C++11)
(until 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.

Feature-test macro Value Std Feature
__cpp_lib_incomplete_container_elements 201505L (C++17) Minimal incomplete type support
(since C++17)

[edit]

Allocator - An allocator that is used to acquire/release memory and to construct/destroy the elements in that memory. The type must meet the requirements of Allocator. The behavior is undefined(until C++20)The program is ill-formed(since C++20) if Allocator::value_type is not the same as T.[edit]

Specializations

The standard library provides a specialization of std::vector for the type bool, which may be optimized for space efficiency.

space-efficient dynamic bitset
(class template specialization) [edit]

Iterator invalidation

Operations Invalidated
All read only operations Never.
swap, std::swap end()
clear, operator=, assign Always.
reserve, shrink_to_fit If the vector changed capacity, all of them. If not, none.
erase Erased elements and all elements after them (including end()).
push_back, emplace_back If the vector changed capacity, all of them. If not, only end().
insert, emplace If the vector changed capacity, all of them.
If not, only those at or after the insertion point (including end()).
resize If the vector changed capacity, all of them. If not, only end() and any elements erased.
pop_back The element erased and end().

Member types

Member type Definition
value_type T[edit]
allocator_type Allocator[edit]
size_type Unsigned integer type (usually std::size_t)[edit]
difference_type Signed integer type (usually std::ptrdiff_t)[edit]
reference value_type&[edit]
const_reference const value_type&[edit]
pointer

Allocator::pointer

(until C++11)

std::allocator_traits<Allocator>::pointer

(since C++11)
[edit]
const_pointer

Allocator::const_pointer

(until C++11)

std::allocator_traits<Allocator>::const_pointer

(since C++11)
[edit]
iterator

LegacyRandomAccessIterator and LegacyContiguousIterator to value_type

(until C++20)

LegacyRandomAccessIterator, contiguous_iterator, and ConstexprIterator to value_type

(since C++20)
[edit]
const_iterator

LegacyRandomAccessIterator and LegacyContiguousIterator to const value_type

(until C++20)

LegacyRandomAccessIterator, contiguous_iterator, and ConstexprIterator to const value_type

(since C++20)