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list<T, Alloc>

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, " "));
// The values that are printed are 1 2 0

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. alloc

Model of

Reversible Container, Front Insertion Sequence, Back Insertion Sequence.

Type requirements

None, except for those imposed by the requirements of Reversible Container, Front Insertion Sequence, and Back Insertion Sequence.

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 Reversible Container Iterator used to iterate backwards through a list.
const_reverse_iterator Reversible Container 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() Reversible Container Returns a reverse_iterator pointing to the beginning of the reversed list.
reverse_iterator rend() Reversible Container Returns a reverse_iterator pointing to the end of the reversed list.
const_reverse_iterator rbegin() const Reversible Container Returns a const_reverse_iterator pointing to the beginning of the reversed list.
const_reverse_iterator rend() const Reversible Container 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)
[2]
Sequence Creates a list with a copy of a range.
~list() Container The destructor.
list& operator=(const list&) Container The assignment operator
reference front() Front Insertion Sequence Returns the first element.
const_reference front() const Front Insertion Sequence Returns the first element.
reference back() Sequence Returns the last element.
const_reference back() const Back Insertion Sequence Returns the last element.
void push_front(const T&) Front Insertion Sequence Inserts a new element at the beginning.
void push_back(const T&) Back Insertion Sequence Inserts a new element at the end.
void pop_front() Front Insertion Sequence Removes the first element.
void pop_back() Back Insertion Sequence 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)
[2]
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&)
Forward Container Tests two lists for equality. This is a global function, not a member function.
bool operator<(const list&, 
               const list&)
Forward Container Lexicographical comparison. This is a global function, not a member function.

New members

These members are not defined in the Reversible Container, Front Insertion Sequence, and Back Insertion Sequence 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); 
[4]
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); 
[4]
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); 
[4]
Comp must be a comparison function that induces a strict weak ordering (as defined in the LessThan Comparable 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); 
[4]
Comp must be a comparison function that induces a strict weak ordering (as defined in the LessThan Comparable 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 input iterator. 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 random access iterators. In principle, however, it would be possible to write a sort algorithm that also accepted bidirectional iterators. 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

Bidirectional Iterator, Reversible Container, Sequence, slist vector.


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