相较于vector的连续线性空间，list就显得复杂许多，它的好处是每次插入或删除一个元素，就配置或释放一个元素空间。因此，list对于空间的运用有绝对的精准，一点也不浪费。而且，对于任何位置的元素插入或元素移除，list永远是常数时间。
list 内部为双向链表，内部元素互相以link串接起来，每个元素都知道其前一个元素以及下一个元素的位置。
template <class T>
struct __list_node {
typedef void* void_pointer;
void_pointer next;
void_pointer prev;
T data;
};list节点并不一定在连续空间上，所以不能像vector一样用原生指针做迭代器，其迭代器必须实现前移、后移、取值等操作。
list插入操作和接合操作都不会造成原有的list迭代器失效，这在vector是不成立的。因为vector的插入操作可能造成内存重新配置，导致原有的迭代器全部失效。甚至list的元素删除操作（erase），也只有“指向被删除元素”的那个迭代器失效，其他迭代器不受任何影响。
来看源码：
template<class T, class Ref, class Ptr>
struct __list_iterator {
typedef __list_iterator<T, T&, T*> iterator;
typedef __list_iterator<T, const T&, const T*> const_iterator;
typedef __list_iterator<T, Ref, Ptr> self;
typedef bidirectional_iterator_tag iterator_category;
typedef T value_type;
typedef Ptr pointer;
typedef Ref reference;
typedef __list_node<T>* link_type;
typedef size_t size_type;
typedef ptrdiff_t difference_type;
link_type node; //内部保留一个原生指针，指向list结点
__list_iterator(link_type x) : node(x) {}
__list_iterator() {}
__list_iterator(const iterator& x) : node(x.node) {}
bool operator==(const self& x) const { return node == x.node; }
bool operator!=(const self& x) const { return node != x.node; }
reference operator*() const { return (*node).data; }
self& operator++() {
node = (link_type)((*node).next);
return *this;
}
self operator++(int) {
self tmp = *this;
++*this;
return tmp;
}
self& operator--() {
node = (link_type)((*node).prev);
return *this;
}
self operator--(int) {
self tmp = *this;
--*this;
return tmp;
}
};
template <class T, class Alloc = alloc>
class list {
protected:
typedef void* void_pointer;
typedef __list_node<T> list_node;
typedef simple_alloc<list_node, Alloc> list_node_allocator;
public:
typedef __list_iterator<T, T&, T*> iterator;
typedef __list_iterator<T, const T&, const T*> const_iterator;
protected:
link_type node;
protected:
link_type get_node() { return list_node_allocator::allocate(); }
void put_node(link_type p) { list_node_allocator::deallocate(p); }
link_type create_node(const T& x) {
link_type p = get_node();
__STL_TRY {
construct(&p->data, x);
}
__STL_UNWIND(put_node(p));
return p;
}
void destroy_node(link_type p) {
destroy(&p->data);
put_node(p);
}
protected:
void empty_initialize() {
node = get_node();
node->next = node;
node->prev = node;
}
void fill_initialize(size_type n, const T& value) {
empty_initialize();
__STL_TRY {
insert(begin(), n, value);
}
__STL_UNWIND(clear(); put_node(node));
}
protected: //将[first,last)内元素迁移到position处
void transfer(iterator position, iterator first, iterator last) {
if (position != last) {
(*(link_type((*last.node).prev))).next = position.node;
(*(link_type((*first.node).prev))).next = last.node;
(*(link_type((*position.node).prev))).next = first.node;
link_type tmp = link_type((*position.node).prev);
(*position.node).prev = (*last.node).prev;
(*last.node).prev = (*first.node).prev;
(*first.node).prev = tmp;
}
}
public:
void splice(iterator position, list& x) {
if (!x.empty())
transfer(position, x.begin(), x.end());
}
public:
list() { empty_initialize(); }
iterator begin() { return (link_type)((*node).next); }
const_iterator begin() const { return (link_type)((*node).next); }
iterator end() { return node; }
const_iterator end() const { return node; }
bool empty() const { return node->next == node; }
size_type size() const {
size_type result = 0;
distance(begin(), end(), result);
return result;
}
size_type max_size() const { return size_type(-1); }
reference front() { return *begin(); }
const_reference front() const { return *begin(); }
reference back() { return *(--end()); }
const_reference back() const { return *(--end()); }
void swap(list<T, Alloc>& x) { __STD::swap(node, x.node); }
iterator insert(iterator position, const T& x) {
link_type tmp = create_node(x);
tmp->next = position.node;
tmp->prev = position.node->prev;
(link_type(position.node->prev))->next = tmp;
position.node->prev = tmp;
return tmp;
}
void push_front(const T& x) { insert(begin(), x); }
void push_back(const T& x) { insert(end(), x); }
iterator erase(iterator position) {
link_type next_node = link_type(position.node->next);
link_type prev_node = link_type(position.node->prev);
prev_node->next = next_node;
next_node->prev = prev_node;
destroy_node(position.node);
return iterator(next_node);
}
void pop_front() { erase(begin()); }
void pop_back() {
iterator tmp = end();
erase(--tmp);
}
};
template <class T, class Alloc>
void list<T, Alloc>::merge(list<T, Alloc>& x) { //归并排序，归并操作
iterator first1 = begin();
iterator last1 = end();
iterator first2 = x.begin();
iterator last2 = x.end();
while (first1 != last1 && first2 != last2)
if (*first2 < *first1) {
iterator next = first2;
transfer(first1, first2, ++next);
first2 = next;
}
else
++first1;
if (first2 != last2) transfer(last1, first2, last2);
}
//1)Creates a series of buckets (64 total).
//2)Removes the first element of the list to sort and merges it with the first (i=0th) bucket.
//3)If, before the merge, the ith bucket is not empty, merge the ith bucket with the i+1th bucket.
//4)Repeat step 3 until we merge with an empty bucket.
//5)Repeat step 2 and 3 until the list to sort is empty.
//6)Merge all the remaining non-empty buckets together starting from smallest to largest.
template <class T, class Alloc>
void list<T, Alloc>::sort() {
if (node->next == node || link_type(node->next)->next == node) return;
list<T, Alloc> carry;
list<T, Alloc> counter[64];
int fill = 0;
while (!empty()) {
carry.splice(carry.begin(), *this, begin()); //将begin处的元素从list取下，insert到carry中
int i = 0;
while(i < fill && !counter[i].empty()) {
counter[i].merge(carry);
carry.swap(counter[i++]);
}
carry.swap(counter[i]);
if (i == fill) ++fill;
}
for (int i = 1; i < fill; ++i) counter[i].merge(counter[i-1]);
swap(counter[fill-1]);
}
template <class T, class Alloc> template <class Predicate>
void list<T, Alloc>::remove_if(Predicate pred) {
iterator first = begin();
iterator last = end();
while (first != last) {
iterator next = first;
++next;
if (pred(*first)) erase(first);
first = next;
}
}版权声明：本文为博主原创文章，未经博主允许不得转载。
时间： 2024-10-08 16:17:38