C++

参考

<unordered_map>

unordered_multimap

public member function
<unordered_map>

std::unordered_multimap::cbegin

container iterator (1)
const_iterator cbegin() const noexcept;
bucket iterator (2)
const_local_iterator cbegin ( size_type n ) const;
返回指向开头的 const_iterator
返回一个const_iteratorpointing to the first element in the unordered_multimap container (1) or in one of its buckets (2).

Aconst_iterator是一个指向常量内容的迭代器。这个迭代器可以像iteratorreturned by unordered_multimap::begin, but it cannot be used to modify the contents it points to.

参数

n
Bucket number. This shall be lower than bucket_count.
It is an optional parameter that changes the behavior of this member function: if set, the iterator retrieved points to the first element of a bucket, otherwise it points to the first element of the container.
成员类型size_type是一种无符号整型类型。

返回值

Aconst_iteratorto the first element in the container (1) or the bucket (2).

两者都const_iterator和const_local_iteratorare member types. In the unordered_multimap class template, these are forward iterator types.
const_local_iteratoris an interator of the same category asconst_iterator. Theirvalue_type, difference_type, 指针和引用member types are also the same. But the iterators themselves are not necessarily of the same type.

示例

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// unordered_multimap::cbegin/cend example
#include <iostream>
#include <unordered_map>

int main ()
{
  std::unordered_multimap<std::string,std::string> myumm = {
    {"apple","red"},
    {"apple","green"},
    {"orange","orange"},
    {"strawberry","red"}
  };

  std::cout << "myumm contains:";
  for ( auto it = myumm.cbegin(); it != myumm.cend(); ++it )
    std::cout << " " << it->first << ":" << it->second;  // cannot modify *it
  std::cout << std::endl;

  std::cout << "myumm's buckets contain:\n";
  for ( unsigned i = 0; i < myumm.bucket_count(); ++i) {
    std::cout << "bucket #" << i << " contains:";
    for ( auto local_it = myumm.cbegin(i); local_it!= myumm.cend(i); ++local_it )
      std::cout << " " << local_it->first << ":" << local_it->second;
    std::cout << std::endl;
  }

  return 0;
}

可能的输出
myumm contains: apple:red apple:green orange:orange strawberry:red
myumm's buckets contain:
bucket #0 contains:
bucket #1 contains:
bucket #2 contains: apple:red apple:green
bucket #3 contains: orange:orange
bucket #4 contains: strawberry:red


复杂度

常量。

迭代器有效性

没有变化。

另见

unordered_multimap::end
返回指向末尾的迭代器 （公开成员类型）
unordered_multimap::cbegin
返回指向起始位置的 const_iterator (公开成员函数)
unordered_multimap::find
获取指向元素的迭代器 (公共成员函数)