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std::vector(3) C++ Standard Libary std::vector(3) NAME std::vector - std::vector Synopsis Defined in header <vector> template< class T, (1) class Allocator = std::allocator<T> > class vector; namespace pmr { template< class T > using vector = std::vector<T, (2) (since C++17) std::pmr::polymorphic_allocator<T>>; } 1) std::vector is a sequence container that encapsulates dynamic size arrays. 2) std::pmr::vector is an alias template that uses a polymorphic al- locator. 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 mem- ory 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 el- ements is known beforehand. The complexity (efficiency) of common operations on vectors is as follows: * Random access - constant (1). * Insertion or removal of elements at the end - amortized constant (1). * Insertion or removal of elements - linear in the distance to the end of the vector (n). std::vector (for T other than bool) meets the requirements of Con- tainer , 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. (since C++20) However, std::vector objects generally cannot be constexpr, because any dynamically allocated storage must be released in the same evaluation of constant expression. 1. In libstdc++, shrink_to_fit() is not available in C++98 mode. Template parameters The type of the elements. T must meet the requirements of CopyAssignable and (until CopyConstructible. C++11) The requirements that are imposed on the elements depend on the (since actual operations performed on the container. Generally, it is C++11) required that element type is a complete type and meets the (until requirements of Erasable, but many member functions im- pose C++17) stricter requirements. The requirements that are imposed on the elements depend on the actual operations performed on the container. Generally, it is T - 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 incom- plete element type if the allocator satisfies the allocator completeness (since requirements. C++17) Feature-test macro Value Std Feature Min- imal __cpp_lib_incomplete_container_elements 201505L (C++17) incomplete type sup- port 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. 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. 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) 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(). If the vector changed capacity, all of them. insert, emplace If not, only those at or after the inser- tion 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 allocator_type Allocator size_type Unsigned integer type (usually std::size_t) difference_type Signed integer type (usually std::ptrdiff_t) reference value_type& const_reference const value_type& Allocator::pointer (until C++11) pointer std::allocator_traits<Allocator>::pointer (since C++11) Allocator::const_pointer (until C++11) const_pointer std::allocator_traits<Alloca- tor>::const_pointer (since C++11) LegacyRandomAccessIterator and (until C++20) LegacyContiguousIterator to value_type LegacyRandomAccessIterator, iterator contiguous_iterator, and ConstexprIterator to (since C++20) value_type LegacyRandomAccessIterator and (until C++20) LegacyContiguousIterator to const value_type LegacyRandomAccessIterator, const_iterator contiguous_iterator, and ConstexprIterator to (since C++20) const value_type reverse_iterator std::reverse_iterator<iterator> const_reverse_iterator std::reverse_iterator<const_iterator> Member functions constructor constructs the vector (public member function) destructor destructs the 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 (C++23) (public member function) get_allocator returns the associated allocator (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 begin returns an iterator to the beginning cbegin (public member function) (C++11) end returns an iterator to the end cend (public member function) (C++11) rbegin returns a reverse iterator to the beginning crbegin (public member function) (C++11) rend returns a reverse iterator to the end crend (public member function) (C++11) Capacity empty checks whether the container is empty (public member function) size returns the number of elements (public member function) max_size returns the maximum possible number of elements (public member function) reserve reserves storage (public member function) returns the number of elements that can be held in currently allocated capacity storage (public member function) shrink_to_fit reduces memory usage by freeing unused memory (DR*) (public member function) Modifiers clear clears the contents (public member function) insert inserts elements (public member function) insert_range inserts a range of elements (C++23) (public member function) emplace constructs element in-place (C++11) (public member function) erase erases elements (public member function) push_back adds an element to the end (public member function) emplace_back constructs an element in-place at the end (C++11) (public member function) append_range adds a range of elements to the end (C++23) (public member function) pop_back removes the last element (public member function) resize changes the number of elements stored (public member function) swap swaps the contents (public member function) Non-member functions operator== operator!= operator< operator<= operator> operator>= lexicographically compares the values of two vectors operator<=> (function template) (removed in C++20) (removed in C++20) (removed in C++20) (removed in C++20) (removed in C++20) (C++20) std::swap(std::vector) specializes the std::swap algorithm (function template) erase(std::vector) erases all elements satisfying specific cri- teria erase_if(std::vector) (function template) (C++20) 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 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 Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct behavior LWG 69 C++98 contiguity of the storage for elements of re- quired vector was not required T was not required to be CopyConstructible T is also required to LWG 230 C++98 (an element of type T might not be able to be CopyConstructible be constructed) LWG 464 C++98 access to the underlying storage of an data function provided empty vector resulted in UB http://cppreference.com 2024.06.10 std::vector(3)
NAME | Synopsis | Template parameters | Specializations | Member types | Member functions | Element access | Iterators | Capacity | Modifiers | Non-member functions | Notes | Example | Output:
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