# std::flat_map<Key,T,Compare,KeyContainer,MappedContainer>::try_emplace

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< cpp‎ | container‎ | flat map

template< class... Args > std::pair <iterator, bool> try_emplace( const key_type& k, Args&&... args ); | (1) | (since C++23)
template< class... Args > std::pair <iterator, bool> try_emplace( key_type&& k, Args&&... args ); | (2) | (since C++23)
template< class K, class... Args > std::pair <iterator, bool> try_emplace( K&& k, Args&&... args ); | (3) | (since C++23)
template< class... Args > iterator try_emplace( const_iterator hint, const key_type& k, Args&&... args ); | (4) | (since C++23)
template< class... Args > iterator try_emplace( const_iterator hint, key_type&& k, Args&&... args ); | (5) | (since C++23)
template< class K, class... Args > iterator try_emplace( const_iterator hint, K&& k, Args&&... args ); | (6) | (since C++23)

If a key equivalent to k already exists in the container, does nothing. Otherwise, inserts a new element into the underlying containers `c` with key k and value constructed with args.

1,2,4,5) Equivalent to:

    auto key_it = ranges::upper_bound(c.keys, k, compare);
    auto value_it = c.values.begin() + std::distance(c.keys.begin(), key_it);
    c.keys.insert(key_it, std::forward<decltype(k)>(k));
    c.values.emplace(value_it, std::forward<Args>(args)...);

3,6) Equivalent to:

    auto key_it = ranges::upper_bound(c.keys, k, compare);
    auto value_it = c.values.begin() + std::distance(c.keys.begin(), key_it);
    c.keys.emplace(key_it, std::forward<K>(k));
    c.values.emplace(value_it, std::forward<Args>(args)...);

The conversion from k into `key_type` must construct an object u, for which find(k) == find(u) is true. Otherwise, the behavior is undefined.
These overloads participate in overload resolution only if :

- The qualified-id `Compare::is_transparent` is valid and denotes a type.
- std::is_constructible_v <key_type, K> is true.
- std::is_assignable_v <mapped_type&, Args...> is true.
- For (3) only, std::is_convertible_v <K&&, const_iterator> and std::is_convertible_v <K&&, iterator> are both false.

| Information on iterator invalidation is copied from here

### 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

1-3) Same as for `emplace`.
4-6) Same as for `emplace_hint`.

### Complexity

1-3) Same as for `emplace`.
4-6) 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::flat_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).

Overloads (3,6) can be called without constructing an object of type `key_type`.

### Example

Run this code

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

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

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

    int main()
    {
        using namespace std::literals;
        std::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);
    }

Output:

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

### See also

emplace | constructs element in-place (public member function)
emplace_hint | constructs elements in-place using a hint (public member function)
insert | inserts elements (public member function)
insert_or_assign | inserts an element or assigns to the current element if the key already exists (public member function)

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