std::unordered_map::unordered_map

	(1)
explicit unordered_map( size_type bucket_count = /implementation-defined/, const Hash& hash = Hash(), const KeyEqual& equal = KeyEqual(), const Allocator& alloc = Allocator() );	(since C++11) (until C++14)
unordered_map() : unordered_map( size_type(/implementation-defined/) ) {} explicit unordered_map( size_type bucket_count, const Hash& hash = Hash(), const KeyEqual& equal = KeyEqual(), const Allocator& alloc = Allocator() );	(since C++14)
unordered_map( size_type bucket_count, const Allocator& alloc ) : unordered_map(bucket_count, Hash(), KeyEqual(), alloc) {} unordered_map( size_type bucket_count, const Hash& hash, const Allocator& alloc ) : unordered_map(bucket_count, hash, KeyEqual(), alloc) {}	(1)	(since C++14)
explicit unordered_map( const Allocator& alloc );	(1)	(since C++11)
template< class InputIt > unordered_map( InputIt first, InputIt last, size_type bucket_count = /implementation-defined/, const Hash& hash = Hash(), const KeyEqual& equal = KeyEqual(), const Allocator& alloc = Allocator() );	(2)	(since C++11)
template< class InputIt > unordered_map( InputIt first, InputIt last, size_type bucket_count, const Allocator& alloc ) : unordered_map(first, last, bucket_count, Hash(), KeyEqual(), alloc) {}	(2)	(since C++14)
template< class InputIt > unordered_map( InputIt first, InputIt last, size_type bucket_count, const Hash& hash, const Allocator& alloc ) : unordered_map(first, last, bucket_count, hash, KeyEqual(), alloc) {}	(2)	(since C++14)
unordered_map( const unordered_map& other );	(3)	(since C++11)
unordered_map( const unordered_map& other, const Allocator& alloc );	(3)	(since C++11)
unordered_map( unordered_map&& other );	(4)	(since C++11)
unordered_map( unordered_map&& other, const Allocator& alloc );	(4)	(since C++11)
unordered_map( std::initializer_list<value_type> init, size_type bucket_count = /implementation-defined/, const Hash& hash = Hash(), const KeyEqual& equal = KeyEqual(), const Allocator& alloc = Allocator() );	(5)	(since C++11)
unordered_map( std::initializer_list<value_type> init, size_type bucket_count, const Allocator& alloc ) : unordered_map(init, bucket_count, Hash(), KeyEqual(), alloc) {}	(5)	(since C++14)
unordered_map( std::initializer_list<value_type> init, size_type bucket_count, const Hash& hash, const Allocator& alloc ) : unordered_map(init, bucket_count, hash, KeyEqual(), alloc) {}	(5)	(since C++14)

从各种数据源构造新容器。可选地使用所提供的用户bucket_count作为要创建的最少数量的桶，hash作为散列函数，equal作为比较键和alloc作为分配器。

1%29构造空容器。集max_load_factor()到1.0。对于默认构造函数，桶的数量是实现定义的。

2%29构造包含范围内容的容器。[first, last).成套max_load_factor()如果范围中的多个元素具有比较等价的键，则未指定插入了%28挂起的元素LWG2844%29

3%29复制构造函数。的内容的副本构造容器。other，复制加载因子、谓词和哈希函数。如果alloc不提供，则通过调用std::allocator_traits<allocator_type>::select_on_container_copy_construction(other.get_allocator())...

4%29移动构造函数。的内容构造容器。other使用移动语义。如果alloc如果不提供分配器，则通过移动构造从属于other...

5%29使用初始化程序列表的内容构造容器。init，和无序一样[医]映射%28 init.start%28%29，init.end%28%29%29。

参数

alloc	-	allocator to use for all memory allocations of this container
bucket_count	-	minimal number of buckets to use on initialization. If it is not specified, implementation-defined default value is used
hash	-	hash function to use
equal	-	comparison function to use for all key comparisons of this container
first, last	-	the range to copy the elements from
other	-	another container to be used as source to initialize the elements of the container with
init	-	initializer list to initialize the elements of the container with

类型要求

-输入必须符合输入器的要求。

复杂性

1%29常数

2%29平均情况下线性最坏情况二次之间的距离first和last

3%29线性other

4%29常数。如果alloc被赋予和alloc != other.get_allocator()，然后是线性的。

5%29平均线性最坏情况二次型init

注记

在容器移动构造%28重载%284%29%29之后，引用、指针和迭代器%28---other保持有效，但引用当前在*this.现行标准通过第23.2.1节中的总括声明作出这一保证。集装箱。所需经费/12，目前正在考虑通过以下方式提供更直接的担保：lwg 2321...

例

二次

#include <unordered_map>
#include <vector>
#include <bitset>
#include <string>
#include <utility>
 
struct Key {
    std::string first;
    std::string second;
};
 
struct KeyHash {
 std::size_t operator()(const Key& k) const
 {
     return std::hash<std::string>()(k.first) ^
            (std::hash<std::string>()(k.second) << 1);
 }
};
 
struct KeyEqual {
 bool operator()(const Key& lhs, const Key& rhs) const
 {
    return lhs.first == rhs.first && lhs.second == rhs.second;
 }
};
 
struct Foo {
    Foo(int val_) : val(val_) {}
    int val;
    bool operator==(const Foo &rhs) const { return val == rhs.val; }
};
 
namespace std {
    template<> struct hash<Foo> {
        std::size_t operator()(const Foo &f) const {
            return std::hash<int>{}(f.val);
        }  
    };
}
 
int main()
{
    // default constructor: empty map
    std::unordered_map<std::string, std::string> m1;
 
    // list constructor
    std::unordered_map<int, std::string> m2 =
    {
        {1, "foo"},
        {3, "bar"},
        {2, "baz"},
    };
 
    // copy constructor
    std::unordered_map<int, std::string> m3 = m2;
 
    // move constructor
    std::unordered_map<int, std::string> m4 = std::move(m2);
 
    // range constructor
    std::vector<std::pair<std::bitset<8>, int>> v = { {0x12, 1}, {0x01,-1} };
    std::unordered_map<std::bitset<8>, double> m5(v.begin(), v.end());
 
    //Option 1 for a constructor with a custom Key type
    // Define the KeyHash and KeyEqual structs and use them in the template
    std::unordered_map<Key, std::string, KeyHash, KeyEqual> m6 = {
            { {"John", "Doe"}, "example"},
            { {"Mary", "Sue"}, "another"}
    };
 
    //Option 2 for a constructor with a custom Key type
    // Define a const == operator for the class/struct and specialize std::hash
    // structure in the std namespace
    std::unordered_map<Foo, std::string> m7 = {
        { Foo(1), "One"}, { 2, "Two"}, { 3, "Three"}
    };
 
    //Option 3: Use lambdas
    // Note that the initial bucket count has to be passed to the constructor
    struct Goo {int val; };
    auto hash = [](const Goo &g){ return std::hash<int>{}(g.val); };
    auto comp = [](const Goo &l, const Goo &r){ return l.val == r.val; };
    std::unordered_map<Goo, double, decltype(hash), decltype(comp)> m8(10, hash, comp);
}

二次

另见

operator=	assigns values to the container (public member function)

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最后更新于：2017-12-18