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

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.

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

#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