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      std::vector

      From cppreference.com
      <cpp‎ |container
       
       
       
      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 apolymorphic allocator.

      Except for thestd::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 usingcapacity() function. Extra memory can be returned to the system via a call toshrink_to_fit()[1].

      Reallocations are usually costly operations in terms of performance. Thereserve() 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 - constant𝓞(1).
      • Insertion or removal of elements at the end - amortized constant𝓞(1).
      • Insertion or removal of elements - linear in the distance to the end of the vector𝓞(n).

      std::vector (forT other thanbool) meets the requirements ofContainer,AllocatorAwareContainer(since C++11),SequenceContainer,ContiguousContainer(since C++17) andReversibleContainer.

      All member functions ofstd::vector areconstexpr: it is possible to create and usestd::vector objects in the evaluation of a constant expression.

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

      		(since C++20)
      1. ↑In libstdc++,shrink_to_fit() isnot available in C++98 mode.

      Contents

      Template parameters

      T - The type of the elements.
      T must meet the requirements ofCopyAssignable andCopyConstructible.(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 ofErasable, 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 ofErasable, 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 theallocator completeness requirements.

      Feature-test macroValueStdFeature
      __cpp_lib_incomplete_container_elements201505L(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 ofAllocator.The behavior is undefined(until C++20)The program is ill-formed(since C++20) ifAllocator::value_type is not the same asT.[edit]

      Specializations

      The standard library provides a specialization ofstd::vector for the typebool, which may be optimized for space efficiency.

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

      Iterator invalidation

      OperationsInvalidated
      All read only operationsNever.
      swap,std::swapend()
      clear,operator=,assignAlways.
      reserve,shrink_to_fitIf the vector changed capacity, all of them. If not, none.
      eraseErased elements and all elements after them (includingend()).
      push_back,emplace_backIf the vector changed capacity, all of them. If not, onlyend().
      insert,emplaceIf the vector changed capacity, all of them.
      If not, only those at or after the insertion point (includingend()).
      resizeIf the vector changed capacity, all of them. If not, onlyend() and any elements erased.
      pop_backThe element erased andend().

      Member types

      Member type Definition
      value_typeT[edit]
      allocator_typeAllocator[edit]
      size_type Unsigned integer type (usuallystd::size_t)[edit]
      difference_type Signed integer type (usuallystd::ptrdiff_t)[edit]
      referencevalue_type&[edit]
      const_referenceconst 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 andLegacyContiguousIterator tovalue_type

      		(until C++20)

      LegacyRandomAccessIterator,contiguous_iterator, andConstexprIterator tovalue_type

      		(since C++20)
      [edit]
      const_iterator

      LegacyRandomAccessIterator andLegacyContiguousIterator toconst value_type

      		(until C++20)

      LegacyRandomAccessIterator,contiguous_iterator, andConstexprIterator toconst value_type

      		(since C++20)
      [edit]
      reverse_iteratorstd::reverse_iterator<iterator>[edit]
      const_reverse_iteratorstd::reverse_iterator<const_iterator>[edit]

      Member functions

      constructs thevector
      (public member function)[edit]
      destructs thevector
      (public member function)[edit]
      assigns values to the container
      (public member function)[edit]
      assigns values to the container
      (public member function)[edit]
      assigns a range of values to the container
      (public member function)[edit]
      returns the associated allocator
      (public member function)[edit]
      Element access
      access specified element with bounds checking
      (public member function)[edit]
      access specified element
      (public member function)[edit]
      access the first element
      (public member function)[edit]
      access the last element
      (public member function)[edit]
      direct access to the underlying contiguous storage
      (public member function)[edit]
      Iterators
      returns an iterator to the beginning
      (public member function)[edit]
      (C++11)
      		 returns an iterator to the end
      (public member function)[edit]
      returns a reverse iterator to the beginning
      (public member function)[edit]
      (C++11)
      		 returns a reverse iterator to the end
      (public member function)[edit]
      Capacity
      checks whether the container is empty
      (public member function)[edit]
      returns the number of elements
      (public member function)[edit]
      returns the maximum possible number of elements
      (public member function)[edit]
      reserves storage
      (public member function)[edit]
      returns the number of elements that can be held in currently allocated storage
      (public member function)[edit]
      reduces memory usage by freeing unused memory
      (public member function)[edit]
      Modifiers
      clears the contents
      (public member function)[edit]
      inserts elements
      (public member function)[edit]
      inserts a range of elements
      (public member function)[edit]
      (C++11)
      		 constructs element in-place
      (public member function)[edit]
      erases elements
      (public member function)[edit]
      adds an element to the end
      (public member function)[edit]
      constructs an element in-place at the end
      (public member function)[edit]
      adds a range of elements to the end
      (public member function)[edit]
      removes the last element
      (public member function)[edit]
      changes the number of elements stored
      (public member function)[edit]
      swaps the contents
      (public member function)[edit]

      Non-member functions

      (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 twovectors
      (function template)[edit]
      specializes thestd::swap algorithm
      (function template)[edit]
      erases all elements satisfying specific criteria
      (function template)[edit]

      Deduction guides

      		(since C++17)

      Notes

      Feature-test macroValueStdFeature
      __cpp_lib_containers_ranges202202L(C++23)Ranges construction and insertion for containers
      __cpp_lib_ranges_reserve_hint202502L(C++26)ranges::approximately_sized_range,ranges::reserve_hint, and changes tostd::vector

      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 vectorfor(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.

      DRApplied toBehavior as publishedCorrect behavior
      LWG 69C++98contiguity of the storage for elements ofvector was not requiredrequired
      LWG 230C++98T was not required to beCopyConstructible
      (an element of typeT might not be able to be constructed)      		T is also required to
      beCopyConstructible
      LWG 464C++98access to the underlying storage of an emptyvector resulted in UBdata function provided

      See also

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