std::unordered_map::unordered_map

(1) empty container constructor (default constructor)

Constructs an empty unordered_map object, containing no elements and with a size of zero.
It can construct the container with specific hasher, key_equal and allocator objects, along with a minimum number of hash buckets.

(2) 範圍建構函式 ((2) range constructor)

Constructs an unordered_map object containing copies of each of the elements in the range[first,last).

(3) copy constructor (and copy with allocator)

The object is initialized to have the same contents and properties as the ump unordered_map object.

(4) move constructor (and moving with allocator)

The object acquires the contents of the rvalue ump.

(5) initializer list

Initializes the container with the contents of the list.

引數

n

Minimum number of initial buckets.
This is not the number of elements in the container, but the minimum number of slots desired for the internal hash table on construction.
If this argument is not specified, the constructor determines this automatically (in a way that depends on the particular library implementation).
成員型別size_type是一種無符號整型型別。

hf

Hasher function object. A hasher is a function that returns an integral value based on the container object key passed to it as argument.
成員型別hasheris defined in unordered_map as an alias of its third template parameter (Hash).

eql

Comparison function object, that returnstrueif the two container object keys passed as arguments are to be considered equal.
成員型別key_equalis defined in unordered_map as an alias of its fourth template parameter (Pred).

alloc

要使用的分配器物件，而不是構造一個新的分配器物件。
對於使用其預設 分配器類模板版本的類例項化，此引數不相關。
成員型別allocator_typeis defined in unordered_map as an alias of its fifth template parameter (Alloc).

first, last

指向範圍內初始位置和最終位置的輸入迭代器。 使用的範圍是[first,last)，它包括first和last之間的所有元素，包括first指向的元素，但不包括last指向的元素。
The function template type can be any type of input iterator.

ump

Another unordered_map object of the same type (with the same class template arguments), whose contents are either copied or moved.

il

il
這些物件是從初始化列表宣告符自動構造的。
成員型別value_typeis the type of the elements in the container, defined in unordered_map aspair<const key_type,mapped_type>，其中成員型別key_type是第一個模板引數（鍵型別）的別名，而mapped_type是第二個模板引數（對映型別，T).

示例

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// constructing unordered_maps
#include <iostream>
#include <string>
#include <unordered_map>

typedef std::unordered_map<std::string,std::string> stringmap;

stringmap merge (stringmap a,stringmap b) {
  stringmap temp(a); temp.insert(b.begin(),b.end()); return temp;
}

int main ()
{
  stringmap first;                              // empty
  stringmap second ( {{"apple","red"},{"lemon","yellow"}} );       // init list
  stringmap third ( {{"orange","orange"},{"strawberry","red"}} );  // init list
  stringmap fourth (second);                    // copy
  stringmap fifth (merge(third,fourth));        // move
  stringmap sixth (fifth.begin(),fifth.end());  // range

  std::cout << "sixth contains:";
  for (auto& x: sixth) std::cout << " " << x.first << ":" << x.second;
  std::cout << std::endl;

  return 0;
}

sixth contains: apple:red lemon:yellow orange:orange strawberry:red

複雜度

另見

unordered_map::operator=

賦新值 (公開成員函式)

unordered_map::insert

插入元素 (公開成員函式)

unordered_map::reserve

請求容量更改 (公共成員函式)