1) 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:
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.
shrink_to_fit() is not available in C++98 mode.T must meet the requirements of CopyAssignable and CopyConstructible. (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 of Erasable, but many member functions impose stricter requirements. (since 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 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)
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.
vector
vector
#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:
Defect reportsThe 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 ofvector was not required required LWG 230 C++98 T was not required to be CopyConstructible
T might not be able to be constructed) T is also required to
vector resulted in UB data function provided See also resizable, fixed capacity, inplace contiguous array
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