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.
Parameters
- 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.
Return value
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 is true.
Otherwise, the pair object returned has as first component an iterator pointing to the element in the container with the same key, and false as its second component.
Member type iterator is a forward iterator type.
The storage for the new element is allocated using allocator_traits<allocator_type>::construct(), which may throw exceptions on failure (for the default allocator, bad_alloc is thrown if the allocation request does not succeed).
Example
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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;
}
| |
Possible output:
mymap contains:
NCC-1701: J.T. Kirk
NCC-1701-D: J.L. Picard
NCC-74656: K. Janeway
|
Complexity
Average case: constant.
Worst case: linear in container size.
May trigger a rehash (not included).
Iterator validity
On most cases, all iterators in the container remain valid after the insertion. The only exception being when the growth of the container forces a rehash. In this case, all iterators in the container are invalidated.
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).
References to elements in the unordered_map container remain valid in all cases, even after a rehash.