public member type
<unordered_map>
| container iterator (1) | iterator begin() noexcept;const_iterator begin() const noexcept; |
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| bucket iterator (2) | local_iterator begin ( size_type n );const_local_iterator begin ( size_type n ) const; |
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Return iterator to beginning
Returns an iterator pointing to the first element in the unordered_multimap container (1) or in one of its buckets (2).
Notice that an unordered_multimap object makes no guarantees on which specific element is considered its first element. But, in any case, the range that goes from itsbeginto itsendcovers all the elements in the container (or the bucket), until invalidated.
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
An iterator to the first element in the container (1) or the bucket (2).
All return types (iterator, const_iterator, local_iterator和const_local_iterator) are member types. In the unordered_multimap class template, these are forward iterator types.
Local iterators are of the same category as non-local iterators. 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::begin/end example
#include <iostream>
#include <string>
#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.begin(); it != myumm.end(); ++it )
std::cout << " " << it->first << ":" << it->second;
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.begin(i); local_it!= myumm.end(i); ++local_it )
std::cout << " " << local_it->first << ":" << local_it->second;
std::cout << std::endl;
}
return 0;
}
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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
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