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| Category: containers | | Component type: type |
Description
A list is a doubly linked list. That is, it is a Sequence that supports both forward and backward traversal, and (amortized) constant time insertion and removal of elements at the beginning or the end, or in the middle. Lists have the important property that insertion and splicing do not invalidate iterators to list elements, and that even removal invalidates only the iterators that point to the elements that are removed. The ordering of iterators may be changed (that is, list<T>iterator might have a different predecessor or successor after a list operation than it did before), but the iterators themselves will not be invalidated or made to point to different elements unless that invalidation or mutation is explicit. [1]
Note that singly linked lists, which only support forward traversal, are also sometimes useful. If you do not need backward traversal, then Slist may be more efficient than list.
Definition
Defined in the standard header list, and in the nonstandard backward-compatibility header list.h.
Example
list<int> L;
L.push_back(0);
L.push_front(1);
L.insert(++L.begin(), 2);
copy(L.begin(), L.end(), ostream_iterator<int>(cout, " "));
Template parameters
| Parameter | Description | Default |
T | The list's value type: the type of object that is stored in the list. | |
Alloc | The list's allocator, used for all internal memory management. | Allocators |
Model of
ReversibleContainer, FrontInsertionSequence, BackInsertionSequence.
Type requirements
None, except for those imposed by the requirements of ReversibleContainer, FrontInsertionSequence, and BackInsertionSequence.
Public base classes
None.
Members
| Member | Where defined | Description |
value_type | Container | The type of object, T, stored in the list. |
pointer | Container | Pointer to T. |
reference | Container | Reference to T |
const_reference | Container | Const reference to T |
size_type | Container | An unsigned integral type. |
difference_type | Container | A signed integral type. |
iterator | Container | Iterator used to iterate through a list. |
const_iterator | Container | Const iterator used to iterate through a list. |
reverse_iterator | ReversibleContainer | Iterator used to iterate backwards through a list. |
const_reverse_iterator | ReversibleContainer | Const iterator used to iterate backwards through a list. |
iterator begin() | Container | Returns an iterator pointing to the beginning of the list. |
iterator end() | Container | Returns an iterator pointing to the end of the list. |
const_iterator begin() const | Container | Returns a const_iterator pointing to the beginning of the list. |
const_iterator end() const | Container | Returns a const_iterator pointing to the end of the list. |
reverse_iterator rbegin() | ReversibleContainer | Returns a reverse_iterator pointing to the beginning of the reversed list. |
reverse_iterator rend() | ReversibleContainer | Returns a reverse_iterator pointing to the end of the reversed list. |
const_reverse_iterator rbegin() const | ReversibleContainer | Returns a const_reverse_iterator pointing to the beginning of the reversed list. |
const_reverse_iterator rend() const | ReversibleContainer | Returns a const_reverse_iterator pointing to the end of the reversed list. |
size_type size() const | Container | Returns the size of the list. Note: you should not assume that this function is constant time. It is permitted to be O(N), where N is the number of elements in the list. If you wish to test whether a list is empty, you should write L.empty() rather than L.size() == 0. |
size_type max_size() const | Container | Returns the largest possible size of the list. |
bool empty() const | Container | true if the list's size is 0. |
list() | Container | Creates an empty list. |
list(size_type n) | Sequence | Creates a list with n elements, each of which is a copy of T(). |
list(size_type n, const T& t) | Sequence | Creates a list with n copies of t. |
list(const list&) | Container | The copy constructor. |
template <class InputIterator>
list(InputIterator f, InputIterator l)
| Sequence | Creates a list with a copy of a range. |
~list() | Container | The destructor. |
list& operator=(const list&) | Container | The assignment operator |
reference front() | FrontInsertionSequence | Returns the first element. |
const_reference front() const | FrontInsertionSequence | Returns the first element. |
reference back() | Sequence | Returns the last element. |
const_reference back() const | BackInsertionSequence | Returns the last element. |
void push_front(const T&) | FrontInsertionSequence | Inserts a new element at the beginning. |
void push_back(const T&) | BackInsertionSequence | Inserts a new element at the end. |
void pop_front() | FrontInsertionSequence | Removes the first element. |
void pop_back() | BackInsertionSequence | Removes the last element. |
void swap(list&) | Container | Swaps the contents of two lists. |
iterator insert(iterator pos, const T& x) | Sequence | Inserts x before pos. |
template <class InputIterator>
void insert(iterator pos,
InputIterator f,
InputIterator l)
| Sequence | Inserts the range [f, l) before pos. |
void insert(iterator pos,
size_type n, const T& x)
| Sequence | Inserts n copies of x before pos. |
iterator erase(iterator pos) | Sequence | Erases the element at position pos. |
iterator erase(iterator first, iterator last) | Sequence | Erases the range [first, last) |
void clear() | Sequence | Erases all of the elements. |
void resize(n, t = T()) | Sequence | Inserts or erases elements at the end such that the size becomes n. |
void splice(iterator pos, list& L) | list | See below. |
void splice(iterator pos,
list& L,
iterator i)
| list | See below. |
void splice(iterator pos,
list& L,
iterator f, iterator l)
| list | See below. |
void remove(const T& value) | list | See below. |
void unique() | list | See below. |
void merge(list& L) | list | See below. |
void sort() | list | See below. |
bool operator==(const list&,
const list&)
| ForwardContainer | Tests two lists for equality. This is a global function, not a member function. |
bool operator<(const list&,
const list&)
| ForwardContainer | Lexicographical comparison. This is a global function, not a member function. |
New members
These members are not defined in the ReversibleContainer, FrontInsertionSequence, and BackInsertionSequence requirements, but are specific to list.
| Function | Description |
void splice(iterator position,
list<T, Alloc>& x);
| position must be a valid iterator in *this, and x must be a list that is distinct from *this. (That is, it is required that &x != this.) All of the elements of x are inserted before position and removed from x. All iterators remain valid, including iterators that point to elements of x. [3] This function is constant time. |
void splice(iterator position,
list<T, Alloc>& x,
iterator i);
| position must be a valid iterator in *this, and i must be a dereferenceable iterator in x. Splice moves the element pointed to by i from x to *this, inserting it before position. All iterators remain valid, including iterators that point to elements of x. [3] If position == i or position == ++i, this function is a null operation. This function is constant time. |
void splice(iterator position,
list<T, Alloc>& x,
iterator f, iterator l);
| position must be a valid iterator in *this, and [first, last) must be a valid range in x. position may not be an iterator in the range [first, last). Splice moves the elements in [first, last) from x to *this, inserting them before position. All iterators remain valid, including iterators that point to elements of x. [3] This function is constant time. |
void remove(const T& val); | Removes all elements that compare equal to val. The relative order of elements that are not removed is unchanged, and iterators to elements that are not removed remain valid. This function is linear time: it performs exactly size() comparisons for equality. |
template<class Predicate>
void remove_if(Predicate p);
| Removes all elements *i such that p(*i) is true. The relative order of elements that are not removed is unchanged, and iterators to elements that are not removed remain valid. This function is linear time: it performs exactly size() applications of p. |
void unique(); | Removes all but the first element in every consecutive group of equal elements. The relative order of elements that are not removed is unchanged, and iterators to elements that are not removed remain valid. This function is linear time: it performs exactly size() - 1 comparisons for equality. |
template<class BinaryPredicate>
void unique(BinaryPredicate p);
| Removes all but the first element in every consecutive group of equivalent elements, where two elements *i and *j are considered equivalent if p(*i, *j) is true. The relative order of elements that are not removed is unchanged, and iterators to elements that are not removed remain valid. This function is linear time: it performs exactly size() - 1 comparisons for equality. |
void merge(list<T, Alloc>& x); | Both *this and x must be sorted according to operator<, and they must be distinct. (That is, it is required that &x != this.) This function removes all of x's elements and inserts them in order into *this. The merge is stable; that is, if an element from *this is equivalent to one from x, then the element from *this will precede the one from x. All iterators to elements in *this and x remain valid. This function is linear time: it performs at most size() + x.size() - 1 comparisons. |
template<class BinaryPredicate>
void merge(list<T, Alloc>& x,
BinaryPredicate Comp);
| Comp must be a comparison function that induces a strict weak ordering (as defined in the LessThanComparable requirements) on objects of type T, and both *this and x must be sorted according to that ordering. The lists x and *this must be distinct. (That is, it is required that &x != this.) This function removes all of x's elements and inserts them in order into *this. The merge is stable; that is, if an element from *this is equivalent to one from x, then the element from *this will precede the one from x. All iterators to elements in *this and x remain valid. This function is linear time: it performs at most size() + x.size() - 1 applications of Comp. |
void reverse(); | Reverses the order of elements in the list. All iterators remain valid and continue to point to the same elements. [5] This function is linear time. |
void sort(); | Sorts *this according to operator<. The sort is stable, that is, the relative order of equivalent elements is preserved. All iterators remain valid and continue to point to the same elements. [6] The number of comparisons is approximately N log N, where N is the list's size. |
template<class BinaryPredicate>
void sort(BinaryPredicate comp);
| Comp must be a comparison function that induces a strict weak ordering (as defined in the LessThanComparable requirements on objects of type T. This function sorts the list *this according to Comp. The sort is stable, that is, the relative order of equivalent elements is preserved. All iterators remain valid and continue to point to the same elements. [6] The number of comparisons is approximately N log N, where N is the list's size. |
Notes
[1] A comparison with Vector is instructive. Suppose that i is a valid Vector<T>iterator. If an element is inserted or removed in a position that precedes i, then this operation will either result in i pointing to a different element than it did before, or else it will invalidate i entirely. (A Vector<T>iterator will be invalidated, for example, if an insertion requires a reallocation.) However, suppose that i and j are both iterators into a Vector, and there exists some integer n such that i == j + n. In that case, even if elements are inserted into the vector and i and j point to different elements, the relation between the two iterators will still hold. A list is exactly the opposite: iterators will not be invalidated, and will not be made to point to different elements, but, for list iterators, the predecessor/successor relationship is not invariant.
[2] This member function relies on member template functions, which at present (early 1998) are not supported by all compilers. If your compiler supports member templates, you can call this function with any type of InputIterator. If your compiler does not yet support member templates, though, then the arguments must either be of type const value_type* or of type list::const_iterator.
[3] A similar property holds for all versions of insert() and erase(). List<T, Alloc>insert() never invalidates any iterators, and list<T, Alloc>erase() only invalidates iterators pointing to the elements that are actually being erased.
[4] This member function relies on member template functions, which at present (early 1998) are not supported by all compilers. You can only use this member function if your compiler supports member templates.
[5] If L is a list, note that L.reverse() and reverse(L.begin(), L.end()) are both correct ways of reversing the list. They differ in that L.reverse() will preserve the value that each iterator into L points to but will not preserve the iterators' predecessor/successor relationships, while reverse(L.begin(), L.end()) will not preserve the value that each iterator points to but will preserve the iterators' predecessor/successor relationships. Note also that the algorithm reverse(L.begin(), L.end()) will use T's assignment operator, while the member function L.reverse() will not.
[6] The sort algorithm works only for RandomAccessIterator. In principle, however, it would be possible to write a sort algorithm that also accepted BidirectionalIterator. Even if there were such a version of sort, it would still be useful for list to have a sort member function. That is, sort is provided as a member function not only for the sake of efficiency, but also because of the property that it preserves the values that list iterators point to.
See also
BidirectionalIterator, ReversibleContainer, Sequence, Slist, Vector.
