Difference between revisions of "cpp/container/vector"

Latest revision as of 21:20, 2 August 2024

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.

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.

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)

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

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]

[edit] Specializations

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

vector<bool>

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

[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().

[edit] 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)

[edit]

reverse_iterator std::reverse_iterator<iterator>[edit]

const_reverse_iterator std::reverse_iterator<const_iterator>[edit]

[edit] Member functions

(constructor)	constructs the `vector` (public member function) [edit]
(destructor)	destructs the `vector` (public member function) [edit]
operator=	assigns values to the container (public member function) [edit]
assign	assigns values to the container (public member function) [edit]
assign_range (C++23)	assigns a range of values to the container (public member function) [edit]
get_allocator	returns the associated allocator (public member function) [edit]
Element access
at	access specified element with bounds checking (public member function) [edit]
operator[]	access specified element (public member function) [edit]
front	access the first element (public member function) [edit]
back	access the last element (public member function) [edit]
data	direct access to the underlying contiguous storage (public member function) [edit]
Iterators
begincbegin (C++11)	returns an iterator to the beginning (public member function) [edit]
endcend (C++11)	returns an iterator to the end (public member function) [edit]
rbegincrbegin (C++11)	returns a reverse iterator to the beginning (public member function) [edit]
rendcrend (C++11)	returns a reverse iterator to the end (public member function) [edit]
Capacity
empty	checks whether the container is empty (public member function) [edit]
size	returns the number of elements (public member function) [edit]
max_size	returns the maximum possible number of elements (public member function) [edit]
reserve	reserves storage (public member function) [edit]
capacity	returns the number of elements that can be held in currently allocated storage (public member function) [edit]
shrink_to_fit (DR*)	reduces memory usage by freeing unused memory (public member function) [edit]
Modifiers
clear	clears the contents (public member function) [edit]
insert	inserts elements (public member function) [edit]
insert_range (C++23)	inserts a range of elements (public member function) [edit]
emplace (C++11)	constructs element in-place (public member function) [edit]
erase	erases elements (public member function) [edit]
push_back	adds an element to the end (public member function) [edit]
emplace_back (C++11)	constructs an element in-place at the end (public member function) [edit]
append_range (C++23)	adds a range of elements to the end (public member function) [edit]
pop_back	removes the last element (public member function) [edit]
resize	changes the number of elements stored (public member function) [edit]
swap	swaps the contents (public member function) [edit]

[edit] Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)	lexicographically compares the values of two `vector`s (function template) [edit]
std::swap(std::vector)	specializes the std::swap algorithm (function template) [edit]
erase(std::vector)erase_if(std::vector) (C++20)	erases all elements satisfying specific criteria (function template) [edit]

Deduction guides

(since C++17)

[edit] Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_containers_ranges`	202202L	(C++23)	Ranges construction and insertion for containers

[edit] Example

Run this code

#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

[edit] 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 to be CopyConstructible
LWG 464	C++98	access to the underlying storage of an empty `vector` resulted in UB	`data` function provided

[edit] See also

inplace_vector (C++26)	dynamically-resizable, fixed capacity, inplace contiguous array (class template) [edit]
array (C++11)	fixed-sized inplace contiguous array (class template) [edit]
deque	double-ended queue (class template) [edit]

[1] In libstdc++, shrink_to_fit() is not available in C++98 mode.

[1]

@@ Line 20: / Line 20: @@
 @2@ {{tt|std::pmr::vector}} is an alias template that uses a [[cpp/memory/polymorphic_allocator|polymorphic allocator]].
-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 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 {{lc|capacity()}} function. Extra memory can be returned to the system via a call to {{lc|shrink_to_fit()}}<ref>In libstdc++, {{tt|shrink_to_fit()}} is [https://gcc.gnu.org/onlinedocs/libstdc++/manual/strings.html#strings.string.shrink not available] in C++98 mode.</ref>.
@@ Line 60: / Line 60: @@
 |-
 |All read only operations
-|Never
+|Never.
 |-
 |{{lc|swap}}, {{lc|std::swap}}
@@ Line 66: / Line 66: @@
 |-
 |{{lc|clear}}, {{lc|1=operator=}}, {{lc|assign}}
-|Always
+|Always.
 |-
 |{{lc|reserve}}, {{lc|shrink_to_fit}}
@@ Line 72: / Line 72: @@
 |-
 |{{lc|erase}}
-|Erased elements and all elements after them (including {{lc|end()}})
+|Erased elements and all elements after them (including {{lc|end()}}).
 |-
 |{{lc|push_back}}, {{lc|emplace_back}}
@@ Line 155: / Line 155: @@
 {{dsc end}}
-{{rrev|noborder=true|since=c++17|{{=}}{{=}}{{=}}{{rl|deduction guides|Deduction guides}}{{=}}{{=}}{{=}}}}
+{{rrev|since=c++17|
+==={{rl|deduction guides|Deduction guides}}===
+}}
 ===Notes===
@@ Line 195: / Line 197: @@
 {{dr list item|wg=lwg|dr=464|std=C++98|before=access to the underlying storage of an empty {{tt|vector}} resulted in UB|after={{tt|data}} function provided}}
 {{dr list end}}
+===See also===
+{{dsc begin}}
+{{dsc inc|cpp/container/dsc inplace_vector}}
+{{dsc inc|cpp/container/dsc array}}
+{{dsc inc|cpp/container/dsc deque}}
+{{dsc end}}
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Difference between revisions of "cpp/container/vector"

Latest revision as of 21:20, 2 August 2024

Contents

[edit] Template parameters

[edit] Specializations

[edit] Iterator invalidation

[edit] Member types

[edit] Member functions

Element access

Iterators

Capacity

Modifiers

[edit] Non-member functions

Deduction guides

[edit] Notes

[edit] Example

[edit] Defect reports

[edit] See also

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