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slist<T, Alloc>

Category: containers Component type: type

Description

An slist is a singly linked list: a list where each element is linked to the next element, but not to the previous element. [1] That is, it is a Sequence that supports forward but not backward traversal, and (amortized) constant time insertion and removal of elements. Slists, like 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, slist<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. [2]

The main difference between slist and list is that list's iterators are bidirectional iterators, while slist's iterators are forward iterators. This means that slist is less versatile than list; frequently, however, bidirectional iterators are unnecessary. You should usually use slist unless you actually need the extra functionality of list, because singly linked lists are smaller and faster than double linked lists.

Important performance note: like every other Sequence, slist defines the member functions insert and erase. Using these member functions carelessly, however, can result in disastrously slow programs. The problem is that insert's first argument is an iterator pos, and that it inserts the new element(s) before pos. This means that insert must find the iterator just before pos; this is a constant-time operation for list, since list has bidirectional iterators, but for slist it must find that iterator by traversing the list from the beginning up to pos. In other words: insert and erase are slow operations anywhere but near the beginning of the slist.

Slist provides the member functions insert_after and erase_after, which are constant time operations: you should always use insert_after and erase_after whenever possible. If you find that insert_after and erase_after aren't adequate for your needs, and that you often need to use insert and erase in the middle of the list, then you should probably use list instead of slist.

Definition

Defined in the header slist, and in the backward-compatibility header slist.h. The slist class, and the slist header, are an SGI extension; they are not part of the C++ standard.

Example

int main() {
  slist<int> L;
  L.push_front(0);
  L.push_front(1);
  L.insert_after(L.begin(), 2);
  copy(L.begin(), L.end(),        // The output is 1 2 0
       ostream_iterator<int>(cout, " "));
  cout << endl;

  slist<int>::iterator back = L.previous(L.end());
  back = L.insert_after(back, 3); 
  back = L.insert_after(back, 4);
  back = L.insert_after(back, 5);
  copy(L.begin(), L.end(),        // The output is 1 2 0 3 4 5
       ostream_iterator<int>(cout, " "));
  cout << endl;
}

Template parameters

Parameter Description Default
T The slist's value type: the type of object that is stored in the list.  
Alloc The slist's allocator, used for all internal memory management. alloc

Model of

Front Insertion Sequence

Type requirements

None, except for those imposed by the requirements of Front Insertion Sequence.

Public base classes

None.

Members

Member Where defined Description
value_type Container The type of object, T, stored in the slist.
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 an slist.
const_iterator Container Const iterator used to iterate through an slist.
iterator begin() Container Returns an iterator pointing to the beginning of the slist.
iterator end() Container Returns an iterator pointing to the end of the slist.
const_iterator begin() const Container Returns a const_iterator pointing to the beginning of the slist.
const_iterator end() const Container Returns a const_iterator pointing to the end of the slist.
size_type size() const Container Returns the size of the slist. 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 slist. If you wish to test whether an slist 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 slist.
bool empty() const Container true if the slist's size is 0.
slist() Container Creates an empty slist.
slist(size_type n) Sequence Creates an slist with n elements, each of which is a copy of T().
slist(size_type n, const T& t) Sequence Creates an slist with n copies of t.
slist(const slist&) Container The copy constructor.
template <class InputIterator>
slist(InputIterator f, InputIterator l) 
[3]
Sequence Creates an slist with a copy of a range.
~slist() Container The destructor.
slist& operator=(const slist&) Container The assignment operator
void swap(slist&) Container Swaps the contents of two slists.
reference front() Front Insertion Sequence Returns the first element.
const_reference front() const Front Insertion Sequence Returns the first element.
void push_front(const T&) Front Insertion Sequence Inserts a new element at the beginning.
void pop_front() Front Insertion Sequence Removes the first element.
iterator previous(iterator pos) slist See below
const_iterator previous(const_iterator pos) slist See below
iterator insert(iterator pos, const T& x) Sequence Inserts x before pos.
template<class InputIterator>
void insert(iterator pos, InputIterator f, InputIterator l)
[3]
Sequence Inserts the range [first, last) before pos.
void insert(iterator pos,
            size_type n, const value_type& 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.
iterator insert_after(iterator pos) slist See below.
iterator insert_after(iterator pos, const value_type& x) slist See below.
template<class InputIterator>
void insert_after(iterator pos,
                  InputIterator f, InputIterator l)
slist See below.
void insert_after(iterator pos, 
                  size_type n, const value_type& x)
slist See below.
iterator erase_after(iterator pos) slist See below.
iterator erase_after(iterator before_first, iterator last) slist See below.
void splice(iterator position, slist& L) slist See below.
void splice(iterator position, slist& L, iterator i) slist See below.
void splice(iterator position, slist& L, iterator f, iterator l) slist See below.
void splice_after(iterator pos, iterator prev) slist See below.
void splice_after(iterator pos, 
                  iterator before_first, 
                  iterator before_last)
slist See below.
void remove(const T& value) slist See below.
void unique() slist See below.
void merge(slist& L) slist See below.
void sort() slist See below.
bool operator==(const slist&, 
                const slist&)
Forward Container Tests two slists for equality. This is a global function, not a member function.
bool operator<(const slist&, 
               const slist&)
Forward Container Lexicographical comparison. This is a global function, not a member function.

New members

These members are not defined in the Front Insertion Sequence requirements, but are specific to slist:
Function Description
iterator previous(iterator pos) pos must be a valid iterator in *this. The return value is an iterator prev such that ++prev == pos. Complexity: linear in the number of iterators in the range [begin(), pos).
const_iterator previous(const_iterator pos) pos must be a valid iterator in *this. The return value is an iterator prev such that ++prev == pos. Complexity: linear in the number of iterators in the range [begin(), pos).
iterator insert_after(iterator pos) pos must be a dereferenceable iterator in *this. (That is, pos may not be end().) Inserts a copy of T() immediately following pos. The return value is an iterator that points to the new element. Complexity: constant time.
iterator insert_after(iterator pos,
                      const value_type& x)
pos must be a dereferenceable iterator in *this. (That is, pos may not be end().) Inserts a copy of x immediately following pos. The return value is an iterator that points to the new element. Complexity: constant time.
template<class InputIterator>
void insert_after(iterator pos,
                  InputIterator f, InputIterator l)
Inserts elements from the range [f, l) immediately following pos. Complexity: linear in last - first.
void insert_after(iterator pos, 
                  size_type n, const value_type& x)
Inserts n copies of x immediately following pos. Complexity: linear in n.
iterator erase_after(iterator pos) Erases the element pointed to by the iterator following pos. Complexity: constant time.
iterator erase_after(iterator before_first, iterator last) Erases all elements in the range [before_first + 1, last). Complexity: linear in last - (before_first + 1).
void splice(iterator position, 
            slist<T, Alloc>& x);
position must be a valid iterator in *this, and x must be an slist 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. [4] Complexity: proportional to c1 (position - begin()) + c2(x.size()), where c1 and c2 are unknown constants.
 
void splice(iterator position, 
            slist<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. [4] If position == i or position == ++i, this function is a null operation. Complexity: proportional to c1 (position - begin()) + c2 (i - x.begin()), where c1 and c2 are unknown constants.
 
void splice(iterator position, 
            slist<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. [4] Complexity: proportional to c1 (position - begin()) + c2 (f - x.begin()) + c3 (l - f), where c1, c2, and c3 are unknown constants.
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.
void splice_after(iterator pos, iterator prev) pos must be a dereferenceable iterator in *this, and prev must be a dereferenceable iterator either in *this or in some other slist. (Note: "dereferenceable iterator" implies that neither pos nor prev may be an off-the-end iterator.) Moves the element following prev to *this, inserting it immediately after pos. Complexity: constant time.
void splice_after(iterator pos, 
                  iterator before_first, 
                  iterator before_last)
pos must be a dereferenceable iterator in *this, and before_first and before_last must be dereferenceable iterators either in *this or in some other slist. (Note: "dereferenceable iterator" implies that none of these iterators may be off-the-end iterators.) Moves the elements in the range [before_first + 1, before_last + 1) to *this, inserting them immediately after pos. Complexity: constant time.
template<class Predicate> 
void remove_if(Predicate p); 
[5]
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); 
[5]
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(slist<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(slist<T, Alloc>& x, 
           BinaryPredicate Comp); 
[5]
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 slists 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 slist. All iterators remain valid and continue to point to the same elements. [6] 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. [7] The number of comparisons is approximately N log N, where N is the slist's size.
template<class BinaryPredicate>
void sort(BinaryPredicate comp); 
[5]
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 slist *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. [7] The number of comparisons is approximately N log N, where N is the slist's size.

Notes

[1] The lists in such languages as Common Lisp, Scheme, and ML are singly linked lists. In some programming languages, almost all data structures are represented as singly linked lists.

[2] 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. An slist is exactly the opposite: iterators will not be invalidated, and will not be made to point to different elements, but, for slist iterators, the predecessor/successor relationship is not invariant.

[3] 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 slist::const_iterator.

[4] A similar property holds for all versions of insert() and erase(). Slist<T, Alloc>::insert() never invalidates any iterators, and slist<T, Alloc>::erase() only invalidates iterators pointing to the elements that are actually being erased.

[5] 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.

[6] The reverse algorithm works only for bidirectional iterators. Even if reverse were extended to work with forward iterators, however, it would still be useful to have the reverse member function: it has different iterator invalidation semantics. That is, the reverse member function preserves the value that each iterator points to. Note also that the algorithm reverse(L.begin(), L.end()) uses T's assignment operator, but the member function L.reverse() does not.

[7] The sort algorithm works only for random access iterators. In principle, however, it would be possible to write a sort algorithm that also accepted forward iterators. Even if there were such a version of sort, it would still be useful for slist 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, list, vector


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