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# std::unordered_map::emplace

From cppreference.com
< cpp‎ | container‎ | unordered map

[edit template]

template< class... Args > std::pair<iterator,bool> emplace( Args&&... args ); |  | (since C++11)

Inserts a new element into the container by constructing it in-place with the given `args`.

Careful use of `emplace` allows the new element to be constructed while avoiding unnecessary copy or move operations. The constructor of the new element (i.e. std::pair<const Key, T>) is called with exactly the same arguments as supplied to `emplace`, forwarded via std::forward<Args>(args)....

If rehashing occurs due to the insertion, all iterators are invalidated. Otherwise iterators are not affected. References are not invalidated. Rehashing occurs only if the new number of elements is equal to or greater than max_load_factor()*bucket_count().

### [edit] Parameters

args | - | arguments to forward to the constructor of the element

### [edit] Return value

Returns a pair consisting of a bool denoting whether the insertion took place and an iterator to the inserted element, or the already-existing element if no insertion happened.

### [edit] Exceptions

If an exception is thrown by any operation, this function has no effect.

### [edit] Complexity

Amortized constant on average, worst case linear in the size of the container.

### [edit] Example

Run this code

    #include <iostream>
    #include <utility>

    #include <unordered_map>
    int main()
    {
        std::unordered_map<std::string, std::string> m;

        // uses pair's move constructor (overload 8)
        m.emplace(std::make_pair(std::string("a"), std::string("a")));

        // uses pair's converting move constructor (overload 5)
        m.emplace(std::make_pair("b", "abcd"));

        // uses pair's template constructor (overload 3)
        m.emplace("d", "ddd");

        // uses pair's piecewise constructor (overload 6)
        m.emplace(std::piecewise_construct,
                  std::forward_as_tuple("c"),
                  std::forward_as_tuple(10, 'c'));

        for (const auto &p : m) {
            std::cout << p.first << " => " << p.second << '\n';
        }
    }

Possible output:

    a => a
    b => abcd
    c => cccccccccc
    d => ddd

### [edit] See also

emplace_hint | constructs elements in-place using a hint (public member function) [edit]
insert | inserts elements (public member function) [edit]

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