public member function
<unordered_map>
template <class... Args>pair<iterator, bool> emplace ( Args&&... args );
Construct and insert element
Inserts a new element in the unordered_map if its key is unique. This new element is constructed in place using args as the arguments for the element's constructor.
The insertion only takes place if no element in the container has a key equivalent to the one being emplaced (keys in an unordered_map are unique).
If inserted, 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
If the insertion takes place (because no other element existed with the same key), the function returns a pair object, whose first component is an iterator to the inserted element, and whose second component istrue.
Otherwise, the pair object returned has as first component an iterator pointing to the element in the container with the same key, andfalseas its second component.
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_map::emplace
#include <iostream>
#include <string>
#include <unordered_map>
int main ()
{
std::unordered_map<std::string,std::string> mymap;
mymap.emplace ("NCC-1701", "J.T. Kirk");
mymap.emplace ("NCC-1701-D", "J.L. Picard");
mymap.emplace ("NCC-74656", "K. Janeway");
std::cout << "mymap contains:" << std::endl;
for (auto& x: mymap)
std::cout << x.first << ": " << x.second << std::endl;
std::cout << std::endl;
return 0;
}
|
可能的输出
mymap contains:
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。
May trigger a rehash (not included).
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).
无论如何，指向 unordered_map 容器中元素的引用都保持有效，即使在 rehash 之后也是如此。