std::inplace_vector

 

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std::inplace_vector

Member types
Member functions
inplace_vector::inplace_vector inplace_vector::~inplace_vector    inplace_vector::operator= inplace_vector::assign inplace_vector::assign_range Size and capacity inplace_vector::empty inplace_vector::size inplace_vector::max_size inplace_vector::capacity inplace_vector::resize inplace_vector::reserve inplace_vector::shrink_to_fit Iterators inplace_vector::begininplace_vector::cbegin inplace_vector::endinplace_vector::cend inplace_vector::rbegininplace_vector::crbegin inplace_vector::rendinplace_vector::crend	Element access inplace_vector::at inplace_vector::operator[] inplace_vector::front inplace_vector::back inplace_vector::data Modifiers inplace_vector::clear inplace_vector::erase inplace_vector::swap inplace_vector::insert inplace_vector::insert_range inplace_vector::emplace inplace_vector::emplace_back inplace_vector::try_emplace_back inplace_vector::unchecked_emplace_back inplace_vector::push_back inplace_vector::try_push_back inplace_vector::unchecked_push_back inplace_vector::pop_back inplace_vector::append_range inplace_vector::try_append_range
Non-member functions
erase(std::inplace_vector)erase_if(std::inplace_vector) swap(std::inplace_vector)	operator==operator<=>

 

Defined in header <inplace_vector>
template<     class T,     std::size_t N > struct inplace_vector;		(since C++26)

inplace_vector is a dynamically-resizable array with contiguous inplace storage. The elements of type T are stored and properly aligned within the object itself. The capacity of internal storage is fixed at compile-time and is equal to N.

The elements are stored contiguously, which means that elements can be accessed not only through iterators or random-access operator[], but also using offsets to regular pointers to elements. A pointer to an element of an inplace_vector may be passed to any function that expects a pointer to an element of a C-array.

The inplace_vector models Container, ReversibleContainer, ContiguousContainer, and SequenceContainer, including most of the optional sequence container requirements, except that the push_front, emplace_front, pop_front, and prepend_range member functions are not provided.

For any N, std::inplace_vector<T, N>::iterator and std::inplace_vector<T, N>::const_iterator meet the ConstexprIterator requirements.

If N > 0 and std::is_trivial_v<T> is false, then member functions of inplace_vector are not usable in constant expressions.

The specialization std::inplace_vector<T, 0> is a TrivialType and is empty.

Any member function of std::inplace_vector<T, N> that would cause insertion beyond the capacity N throws std::bad_alloc.

The complexity of common operations on inplace_vectors is as follows:

• Random access to an element via operator[] or at() – constant: 𝓞(1).
• Insertion or removal of an element at the end – constant: 𝓞(1).
• Insertion or removal of elements at the end – linear in the number of elements inserted/removed: 𝓞(n).
• Insertion or removal of elements in the beginning or in the middle – linear in the number of elements inserted/removed plus the distance to the end of the vector: 𝓞(n).

Iterator invalidation

std::inplace_vector iterator invalidation guarantees differ from std::vector:

• moving an inplace_vector invalidates all iterators;
• swapping two inplace_vectors invalidates all iterators (during swap, the iterator will continue to point to the same array element, and may thus change its value).

The following member functions potentially invalidate iterators: operator=, assign, assign_range, clear, emplace, erase, insert, insert_range, pop_back, resize, and swap.

The following member functions potentially invalidate end iterator only: append_range, emplace_back, push_back, try_append_range, try_emplace_back, try_push_back, unchecked_emplace_back, and unchecked_push_back.

Template parameters

T	-	element type. Must be MoveConstructible and MoveAssignable.
N	-	capacity, i.e. the maximum number of elements in the inplace_vector (might be ​0​).

Member types

Type	Definition
value_type	T
size_type	std::size_t
difference_type	std::ptrdiff_t
reference	value_type&
const_reference	const value_type&
pointer	value_type*
const_pointer	const value_type*
iterator	implementation-defined LegacyRandomAccessIterator and random_access_iterator to value_type
const_iterator	implementation-defined LegacyRandomAccessIterator and random_access_iterator to const value_type
reverse_iterator	std::reverse_iterator<iterator>
const_reverse_iterator	std::reverse_iterator<const_iterator>

Member functions

(constructor)	constructs the inplace_vector (public member function)
(destructor)	destructs the inplace_vector (public member function)
operator=	assigns values to the container (public member function)
assign	assigns values to the container (public member function)
assign_range	assigns a range of values to the container (public member function)
Element access
at	access specified element with bounds checking (public member function)
operator[]	access specified element (public member function)
front	access the first element (public member function)
back	access the last element (public member function)
data	direct access to the underlying contiguous storage (public member function)
Iterators
begincbegin	returns an iterator to the beginning (public member function)
endcend	returns an iterator to the end (public member function)
rbegincrbegin	returns a reverse iterator to the beginning (public member function)
rendcrend	returns a reverse iterator to the end (public member function)
Size and capacity
empty	checks whether the container is empty (public member function)
size	returns the number of elements (public member function)
max_size [static]	returns the maximum possible number of elements (public static member function)
capacity [static]	returns the number of elements that can be held in currently allocated storage (public static member function)
resize	changes the number of elements stored (public member function)
reserve [static]	reserves storage (public static member function)
shrink_to_fit [static]	reduces memory usage by freeing unused memory (public static member function)
Modifiers
insert	inserts elements (public member function)
insert_range	inserts a range of elements (public member function)
emplace	constructs element in-place (public member function)
emplace_back	constructs an element in-place at the end (public member function)
try_emplace_back	tries to construct an element in-place at the end (public member function)
unchecked_emplace_back	unconditionally constructs an element in-place at the end (public member function)
push_back	adds an element to the end (public member function)
try_push_back	tries to add an element to the end (public member function)
unchecked_push_back	unconditionally adds an element to the end (public member function)
pop_back	removes the last element (public member function)
append_range	adds a range of elements to the end (public member function)
try_append_range	tries to add a range of elements to the end (public member function)
clear	clears the contents (public member function)
erase	erases elements (public member function)
swap	swaps the contents (public member function)

Non-member functions

std::swap(std::inplace_vector) (C++26)	specializes the std::swap algorithm (function template)
erase(std::inplace_vector)erase_if(std::inplace_vector) (C++26)	erases all elements satisfying specific criteria (function template)
operator==operator<=> (C++26)	lexicographically compares the values of two inplace_vectors (function template)

Notes

The number of elements in a inplace_vector may vary dynamically up to a fixed capacity because elements are stored within the object itself similarly to std::array. However, objects are initialized as they are inserted into inplace_vector unlike C arrays or std::array , which must construct all elements on instantiation.

inplace_vector is useful in environments where dynamic memory allocations are undesired.

Feature-test macro	Value	Std	Feature
__cpp_lib_inplace_vector	202406L	(C++26)	std::inplace_vector: dynamically-resizable vector with fixed capacity inplace storage

Example

Run this code

#include <algorithm>
#include <array>
#include <cassert>
#include <inplace_vector>
 
int main()
{
    std::inplace_vector<int, 4> v1{0, 1, 2};
    assert(v1.max_size() == 4);
    assert(v1.capacity() == 4);
    assert(v1.size() == 3);
    assert(std::ranges::equal(v1, std::array{0, 1, 2}));
    assert(v1[0] == 0);
    assert(v1.at(0) == 0);
    assert(v1.front() == 0);
    assert(*v1.begin() == 0);
    assert(v1.back() == 2);
    v1.push_back(3);
    assert(v1.back() == 3);
    assert(std::ranges::equal(v1, std::array{0, 1, 2, 3}));
    v1.resize(3);
    assert(std::ranges::equal(v1, std::array{0, 1, 2}));
    assert(v1.try_push_back(3) != nullptr);
    assert(v1.back() == 3);
    assert(v1.size() == 4);
    assert(v1.try_push_back(13) == nullptr); // no place
    assert(v1.back() == 3);
    assert(v1.size() == 4);
    v1.clear();
    assert(v1.size() == 0);
    assert(v1.empty());
}

See also

vector	dynamic contiguous array (class template)
array (C++11)	fixed-sized inplace contiguous array (class template)
deque	double-ended queue (class template)

External links

	inplace_vector — A reference implementation of P0843R14 (std::inplace_vector).
	static_vector — Boost.Container implements inplace vector as a standalone type with its own guarantees.
	fixed_vector — EASTL implements inplace vector via an extra template parameter.
	small_vector — Folly also implements inplace vector via an extra template parameter.
	stack_alloc — Howard Hinnant's Custom allocators that emulate std::inplace_vector on top of std::vector.