public member function
<unordered_map>
template <class... Args> iterator emplace ( Args&&... args );
Construct and insert element
Inserts a new element in the unordered_multimap. This new element is constructed in place using args as the arguments for the element's constructor.
This effectively increases the container size by one.
A similar member function exists, insert, which either copies or moves existing objects into the container.
参数
- args
Arguments used to construct a new object of the mapped type for the inserted element.
Arguments forwarded to construct the new element (of type
pair<const key_type, mapped_type>).
This can be one of
- Two arguments: one for the
key, the other for the
mapped value.
- A single argument of a
pair type with a value for the
key as first member, and a value for the
mapped value as second.
-
piecewise_construct as first argument, and two additional arguments with
tuples to be forwarded as arguments for the
key value and for the
mapped value respectivelly.
See
pair::pair for more info.
返回值
指向新插入元素的迭代器。
成员类型iterator是一个 forward iterator 类型。
新元素的存储使用allocator_traits<allocator_type>::construct()分配，这在失败时可能会抛出异常（对于默认的 allocator，bad_alloc如果分配请求不成功，则抛出）。
示例
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// unordered_multimap::emplace
#include <iostream>
#include <string>
#include <unordered_map>
int main ()
{
std::unordered_multimap<std::string,std::string> myumm;
myumm.emplace ("NCC-1701", "C. Pike");
myumm.emplace ("NCC-1701", "J.T. Kirk");
myumm.emplace ("NCC-1701-D", "J.L. Picard");
myumm.emplace ("NCC-74656", "K. Janeway");
std::cout << "myumm contains:" << std::endl;
for (auto& x: myumm)
std::cout << x.first << ": " << x.second << std::endl;
std::cout << std::endl;
return 0;
}
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可能的输出
mymap contains:
NCC-1701: C. Pike
NCC-1701: J.T. Kirk
NCC-1701-D: J.L. Picard
NCC-74656: K. Janeway
|
复杂度
平均情况：常量。
最坏情况：容器大小的线性。
可能会触发 rehash（未包含）。
迭代器有效性
在大多数情况下，插入后容器中的所有迭代器都保持有效。唯一的例外是当容器的增长强制重新哈希时。在这种情况下，容器中的所有迭代器都将失效。
A rehash is forced if the new container size after the insertion operation would increase above its capacity threshold (calculated as the container's bucket_count multiplied by its max_load_factor).
在所有情况下，`unordered_multimap` 容器中元素的引用都保持有效，即使在 rehash 之后也是如此。