# std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::try_emplace

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< cpp‎ | container‎ | unordered map

template< class... Args > std::pair<iterator, bool> try_emplace( const Key& k, Args&&... args ); | (1) | (since C++17)
template< class... Args > std::pair<iterator, bool> try_emplace( Key&& k, Args&&... args ); | (2) | (since C++17)
template< class... Args > iterator try_emplace( const_iterator hint, const Key& k, Args&&... args ); | (3) | (since C++17)
template< class... Args > iterator try_emplace( const_iterator hint, Key&& k, Args&&... args ); | (4) | (since C++17)

Inserts a new element into the container with key `k` and value constructed with `args`, if there is no element with the key in the container.

1) If a key equivalent to `k` already exists in the container, does nothing. Otherwise, behaves like `emplace` except that the element is constructed as
value_type(std::piecewise_construct,

std::forward_as_tuple(k),

std::forward_as_tuple(std::forward<Args>(args)...))
2) If a key equivalent to `k` already exists in the container, does nothing. Otherwise, behaves like `emplace` except that the element is constructed as
value_type(std::piecewise_construct,

std::forward_as_tuple(std::move(k)),

std::forward_as_tuple(std::forward<Args>(args)...))
3) If a key equivalent to `k` already exists in the container, does nothing. Otherwise, behaves like `emplace_hint` except that the element is constructed as
value_type(std::piecewise_construct,

std::forward_as_tuple(k),

std::forward_as_tuple(std::forward<Args>(args)...))
4) If a key equivalent to `k` already exists in the container, does nothing. Otherwise, behaves like `emplace_hint` except that the element is constructed as
value_type(std::piecewise_construct,

std::forward_as_tuple(std::move(k)),

std::forward_as_tuple(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 greater than max_load_factor() * bucket_count().

### Parameters

k | - | the key used both to look up and to insert if not found
hint | - | iterator to the position before which the new element will be inserted
args | - | arguments to forward to the constructor of the element

### Return value

1,2) Same as for `emplace`
3,4) Same as for `emplace_hint`

### Complexity

1,2) Same as for `emplace`
3,4) Same as for `emplace_hint`

### Notes

Unlike `insert` or `emplace`, these functions do not move from rvalue arguments if the insertion does not happen, which makes it easy to manipulate maps whose values are move-only types, such as std::unordered_map<std::string, std::unique_ptr<foo>>. In addition, `try_emplace` treats the key and the arguments to the `mapped_type` separately, unlike `emplace`, which requires the arguments to construct a `value_type` (that is, a std::pair).

Feature-test macro | Value | Std | Comment
--- | --- | --- | ---
`__cpp_lib_unordered_map_try_emplace` | 201411L | (C++17) | `std::unordered_map::try_emplace`, std::unordered_map::insert_or_assign

### Example

Run this code

    #include <iostream>
    #include <utility>
    #include <string>

    #include <unordered_map>

    auto print_node = [](const auto &node) {
        std::cout << "[" << node.first << "] = " << node.second << '\n';
    };

    auto print_result = [](auto const &pair) {
        std::cout << (pair.second ? "inserted: " : "ignored:  ");
        print_node(*pair.first);
    };

    int main()
    {
        using namespace std::literals;
        std::unordered_map<std::string, std::string> m;

        print_result( m.try_emplace("a", "a"s) );
        print_result( m.try_emplace("b", "abcd") );
        print_result( m.try_emplace("c", 10, 'c') );
        print_result( m.try_emplace("c", "Won't be inserted") );

        for (const auto &p : m) { print_node(p); }
    }

Possible output:

    inserted: [a] = a
    inserted: [b] = abcd
    inserted: [c] = cccccccccc
    ignored:  [c] = cccccccccc
    [a] = a
    [b] = abcd
    [c] = cccccccccc

### See also

emplace (C++11) | constructs element in-place (public member function)
emplace_hint (C++11) | constructs elements in-place using a hint (public member function)
insert (C++11) | inserts elements or nodes (since C++17) (public member function)

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