容器库(8)-std::multiset
std::multiset是以key为元素的有序关联容器，和std::set不同的是，std::multiset中的key可以重复。搜索、移除和插入的时间复杂度是对数复杂度。
template<
class Key,
class Compare = std::less,
class Allocator = std::allocator
> class multiset; 本文章的代码库：
https://gitee.com/gamestorm577/CppStd
成员函数
构造、析构和赋值
构造函数
可以用迭代器、另一个multiset或者元素列表来构造一个multiset。代码示例：
std::vector vec{1, 1, 2, 2, 3};
std::multiset m1(vec.begin(), vec.end());
std::multiset m2(m1);
std::multiset m3{1, 2, 2, 2, 2, 3, 3, 4};
std::cout << "m1 size = " << m1.size() << std::endl;
std::cout << "m2 size = " << m2.size() << std::endl;
std::cout << "m3 size = " << m3.size() << std::endl; 输出结果：
m1 size = 5
m2 size = 5
m3 size = 8析构函数
删除multiset时，会调用各元素的析构函数。代码示例：
struct MyStruct
{
MyStruct(int i)
: Index(i)
{
}
~MyStruct()
{
std::cout << "destruct, Index = " << Index << std::endl;
}
int Index = 0;
};
struct MyStructCmp
{
bool operator()(const MyStruct& lhs, const MyStruct& rhs) const
{
return lhs.Index < rhs.Index;
}
};
std::multiset s{1, 2, 2, 3};
std::cout << "end" << std::endl; 输出结果：
destruct, Index = 3
destruct, Index = 2
destruct, Index = 2
destruct, Index = 1
end
destruct, Index = 1
destruct, Index = 3
destruct, Index = 2
destruct, Index = 2赋值函数
可以用另一个set或者元素列表给set赋值。代码示例：
std::multiset tmp{1, 1, 2, 2};
std::multiset m1;
std::multiset m2;
m1 = tmp;
m2 = {1, 1, 2, 2, 3};
std::cout << "m1 size = " << m1.size() << std::endl;
std::cout << "m2 size = " << m2.size() << std::endl; 输出结果：
m1 size = 4
m2 size = 5迭代器
接口begin、cbegin指向multiset起始的迭代器，end、cend指向末尾的迭代器。rbegin、crbegin指向起始的逆向迭代器，rend、crend指向末尾的逆向迭代器。无论什么迭代器都不能修改元素的值。代码示例：
std::multiset m{1, 2, 2, 3};
for (auto iter = m.begin(); iter != m.end(); ++iter)
{
std::cout << "num is: " << *iter << std::endl;
} 输出结果：
num is: 1
num is: 2
num is: 2
num is: 3容量
empty
检查multiset是否为空。代码示例：
std::multiset m1{1, 2, 2, 3};
std::multiset m2;
std::cout << std::boolalpha;
std::cout << "m1 empty: " << m1.empty() << std::endl;
std::cout << "m2 empty: " << m2.empty() << std::endl; 输出结果：
m1 empty: false
m2 empty: truesize
获取multiset中元素的个数。代码示例：
std::set m1{1, 2, 2, 3};
std::set m2;
std::cout << "m1 size = " << m1.size() << std::endl;
std::cout << "m2 size = " << m2.size() << std::endl; 输出结果：
m1 size = 3
m2 size = 0max_size
返回可以容纳的最大元素个数。代码示例：
std::set m1;
std::set m2;
std::cout << "m1 max size = " << m1.max_size() << std::endl;
std::cout << "m2 max size = " << m2.max_size() << std::endl; 输出结果：
m1 max size = 576460752303423487
m2 max size = 461168601842738790修改器
clear
清除所有的元素。代码示例：
std::set m{1, 2, 2, 3};
std::cout << "m size = " << m.size() << std::endl;
m.clear();
std::cout << "m size = " << m.size() << std::endl; 输出结果：
m size = 3
m size = 0insert
插入元素，参数可以是元素、迭代器或者元素节点。代码示例：
std::multiset m{1, 2, 2};
m.insert(2);
std::cout << "m size = " << m.size() << std::endl;
std::vector tmp{2, 2, 3, 3, 3};
m.insert(tmp.begin(), tmp.end());
std::cout << "m size = " << m.size() << std::endl; 输出结果：
m size = 4
m size = 9emplace
构造一个元素到multiset中。代码示例：
struct MyStruct
{
MyStruct(int num1, int num2)
: Num1(num1)
, Num2(num2)
{
}
int Num1;
int Num2;
};
struct MyStructCmp
{
bool operator()(const MyStruct& lhs, const MyStruct& rhs) const
{
return lhs.Num1 < rhs.Num2;
}
};
std::multiset m;
m.emplace(1, 10);
std::cout << "m size = " << m.size() << std::endl;
m.emplace(1, 10);
std::cout << "m size = " << m.size() << std::endl; 输出结果：
m size = 1
m size = 2emplace_hint
向multiset中尽可能靠近hint之前的位置插入新元素：
template
iterator emplace_hint(const_iterator hint, Args&&... args); 不同的hint会导致插入元素的效率不同。代码示例：
auto timer = [](std::function func, std::string tag) -> void
{
auto start = std::chrono::system_clock::now();
std::size_t size = func();
auto end = std::chrono::system_clock::now();
std::chrono::duration time = end - start;
std::cout << tag << ", size: " << size << ", use time: " << time.count()
<< std::endl;
};
const int count = 2000000;
auto multiset_emplace = [=]() -> std::size_t
{
std::multiset m;
for (int i = 0; i < count; ++i)
{
m.emplace(i);
}
return m.size();
};
auto multiset_emplace_hint1 = [=]() -> std::size_t
{
std::multiset m;
auto iter = m.begin();
for (int i = 0; i < count; ++i)
{
m.emplace_hint(iter, i);
iter = m.end();
}
return m.size();
};
auto multiset_emplace_hint2 = [=]() -> std::size_t
{
std::multiset m;
auto iter = m.begin();
for (int i = 0; i < count; ++i)
{
m.emplace_hint(iter, i);
iter = m.begin();
}
return m.size();
};
auto multiset_emplace_hint3 = [=]() -> std::size_t
{
std::multiset m;
auto iter = m.begin();
for (int i = 0; i < count; ++i)
{
iter = m.emplace_hint(iter, i);
}
return m.size();
};
timer(multiset_emplace, "multiset_emplace");
timer(multiset_emplace_hint1, "multiset_emplace_hint1");
timer(multiset_emplace_hint2, "multiset_emplace_hint2");
timer(multiset_emplace_hint3, "multiset_emplace_hint3"); 输出结果：
multiset_emplace, size: 2000000, use time: 779.65
multiset_emplace_hint1, size: 2000000, use time: 639.442
multiset_emplace_hint2, size: 2000000, use time: 750.817
multiset_emplace_hint3, size: 2000000, use time: 860.849erase
移除指定位置或者指定值的元素。代码示例：
std::multiset m{1, 1, 1, 2, 2, 2, 3, 17, 17, 17};
std::cout << "m size = " << m.size() << std::endl;
m.erase(m.begin());
std::cout << "m size = " << m.size() << std::endl;
m.erase(m.begin(), std::next(m.begin(), 2));
std::cout << "m size = " << m.size() << std::endl;
m.erase(17);
std::cout << "m size = " << m.size() << std::endl; 输出结果：
m size = 10
m size = 9
m size = 7
m size = 4swap
和另一个multiset交换元素内容。代码示例：
std::multiset m1{1, 1, 1, 2};
std::multiset m2{1, 2};
m1.swap(m2);
std::cout << "m1 size = " << m1.size() << std::endl;
std::cout << "m2 size = " << m2.size() << std::endl; 输出结果：
m1 size = 2
m2 size = 4extract
提取map中的某个位置或首个等于某个值的元素节点，提取后multiset不再拥有该元素。代码示例：
std::multiset m{1, 1, 1, 2};
std::cout << "m size = " << m.size() << std::endl;
m.extract(1);
std::cout << "m size = " << m.size() << std::endl;
auto iter = m.extract(2);
std::cout << "m size = " << m.size() << std::endl;
m.insert(std::move(iter));
std::cout << "m size = " << m.size() << std::endl; 输出结果：
m size = 4
m size = 3
m size = 2
m size = 3merge
合并另一个multiset。代码示例：
std::multiset m1{1, 1, 1, 2};
std::multiset m2{1, 2, 2, 2};
m1.merge(m2);
std::cout << "m1 size = " << m1.size() << std::endl; 输出结果：
m1 size = 8查找
count
返回匹配特定元素值的数量。代码示例：
std::multiset m{1, 1, 1, 2, 2};
std::cout << "m key 1 count: " << m.count(1) << std::endl;
std::cout << "m key 2 count: " << m.count(2) << std::endl; 输出结果：
m key 1 count: 3
m key 2 count: 2find
返回匹配key值的迭代器。代码示例：
std::multiset m{1, 1, 1, 2, 2};
auto iter = m.find(2);
std::cout << "key = " << *iter << std::endl; 输出结果：
key = 2contains
检查是否包含某个key值，返回布尔值。代码示例：
std::multiset m{1, 1, 1, 2, 2};
std::cout << std::boolalpha;
std::cout << "m contains 1: " << m.contains(1) << std::endl;
std::cout << "m contains 3: " << m.contains(3) << std::endl; 输出结果：
m contains 1: true
m contains 3: falselower_bound
返回首个大于等于给定键值的迭代器。代码示例：
std::multiset m{2, 3, 5, 7, 11, 13, 17};
auto iter = m.lower_bound(5);
std::cout << "key = " << *iter << std::endl; 输出结果：
key = 5upper_bound
返回首个大于给定键值的迭代器。代码示例：
std::multiset m{2, 3, 5, 7, 11, 13, 17};
auto iter = m.upper_bound(5);
std::cout << "key = " << *iter << std::endl; 输出结果：
key = 7equal_range
返回一对迭代器，分别是lower_bound和upper_range的结果。代码示例：
std::multiset m{2, 3, 5, 7, 11, 13, 17};
auto [iter1, iter2] = m.equal_range(5);
std::cout << "iter1 = " << *iter1 << std::endl;
std::cout << "iter2 = " << *iter2 << std::endl; 输出结果：
iter1 = 5
iter2 = 7观察器
key_comp
返回用于比较key值的函数。代码示例：
std::multiset m;
auto comp = m.key_comp();
std::cout << std::boolalpha;
std::cout << comp(1, 2) << std::endl; 输出结果：
truevalue_comp
返回用于比较key值的函数。代码示例：
std::multiset m;
auto comp = m.value_comp();
std::cout << std::boolalpha;
std::cout << comp(1, 2) << std::endl; 输出结果：
true非成员函数
比较运算符
operator==,!=,<,<=,>,>=用于比较两个multiset。代码示例：
std::multiset m1 = {1, 1, 2};
std::multiset m2 = {1, 1};
std::cout << std::boolalpha;
std::cout << "m1 == m2: " << (m1 == m2) << std::endl;
std::cout << "m1 != m2: " << (m1 != m2) << std::endl;
std::cout << "m1 < m2: " << (m1 < m2) << std::endl;
std::cout << "m1 <= m2: " << (m1 <= m2) << std::endl;
std::cout << "m1 > m2: " << (m1 > m2) << std::endl;
std::cout << "m1 >= m2: " << (m1 >= m2) << std::endl; 输出结果：
m1 == m2: false
m1 != m2: true
m1 < m2: false
m1 <= m2: false
m1 > m2: true
m1 >= m2: trueswap
交换两个multiset的元素内容。代码示例：
std::multiset m1 = {1, 1, 2};
std::multiset m2 = {1, 1};
std::swap(m1, m2);
std::cout << "m1 size is: " << m1.size() << std::endl;
std::cout << "m2 size is: " << m2.size() << std::endl; 输出结果：
m1 size is: 2
m2 size is: 3erase_if
删除满足要求的元素。代码示例：
std::multiset s = {1, 1, 2, 3, 4};
std::cout << "s size = " << s.size() << std::endl;
std::erase_if(s,
[](int a)
{
return a <= 2;
});
std::cout << "s size = " << s.size() << std::endl; 输出结果：
s size = 5
s size = 2