std::vector
<vector>
class T,
class Allocator = std::allocator <T>
template< class T >
using vector = std::vector<T, std::pmr::polymorphic_allocator <T>>;
std::vector is a sequence container that encapsulates dynamic size arrays.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.
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.
- ↑ In libstdc++,
shrink_to_fit()is not available in C++98 mode.
Contents
Template parameters
| T | - | The type of the elements.
| ||||||||||||||
| 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.
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
pointer
Allocator::pointer
std::allocator_traits <Allocator>::pointer
(since C++11)const_pointer
Allocator::const_pointer
std::allocator_traits <Allocator>::const_pointer
(since C++11)iterator
LegacyRandomAccessIterator and LegacyContiguousIterator to value_type
LegacyRandomAccessIterator, contiguous_iterator, and ConstexprIterator to value_type
const_iterator
LegacyRandomAccessIterator and LegacyContiguousIterator to const value_type
(until C++20)LegacyRandomAccessIterator, contiguous_iterator, and ConstexprIterator to const value_type
reverse_iterator
const_reverse_iterator
Member functions
Element access
Iterators
Capacity
(public member function) [edit]
Modifiers
Non-member functions
vectors (function template) [edit]
Deduction guides
(since C++17)Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_containers_ranges |
202202L |
(C++23) | Ranges construction and insertion for containers |
__cpp_lib_ranges_reserve_hint |
202502L |
(C++26) | ranges::approximately_sized_range, ranges::reserve_hint, and changes to std::vector
|
Example
#include <iostream> #include <vector> int main() { // Create a vector containing integers std::vector<int> v = {8, 4, 5, 9}; // Add two more integers to vector v.push_back(6); v.push_back(9); // Overwrite element at position 2 v[2] = -1; // Print out the vector for (int n : v) std::cout << n << ' '; std::cout << '\n'; }
Output:
8 4 -1 9 6 9
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 69 | C++98 | contiguity of the storage for elements of vector was not required
|
required |
| LWG 230 | C++98 | T was not required to be CopyConstructible (an element of type T might not be able to be constructed)
|
T is also required tobe CopyConstructible |
| LWG 464 | C++98 | access to the underlying storage of an empty vector resulted in UB
|
data function provided
|