std:: vector
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.
- ↑ In libstdc++, shrink_to_fit() is not available in C++98 mode.
Contents
- 1 Template parameters
- 2 Specializations
- 3 Iterator invalidation
- 4 Member types
- 5 Member functions
- 5.1 Element access
- 5.2 Iterators
- 5.3 Capacity
- 5.4 Modifiers
[edit] Template parameters
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 |
[edit] Specializations
The standard library provides a specialization of std::vector for the type bool , which may be optimized for space efficiency.
[edit] Iterator invalidation
| Operations | Invalidated |
|---|
| All read only operations | Never. |
| swap , std::swap | end() |
| clear , 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
| Allocator::pointer | (until C++11) |
| std:: allocator_traits < Allocator >:: pointer | (since C++11) |
| Allocator::const_pointer | (until C++11) |
| std:: allocator_traits < Allocator >:: const_pointer | (since C++11) |
