NAME¶

std::flat_map::try_emplace - std::flat_map::try_emplace

Synopsis¶

template< class... Args >
std::pair<iterator, bool> try_emplace( const key_type& k, (1) (since C++23)
Args&&... args );
template< class... Args >
std::pair<iterator, bool> try_emplace( key_type&& k, Args&&... (2) (since C++23)
args );
template< class K, class... Args > (3) (since C++23)
std::pair<iterator, bool> try_emplace( K&& k, Args&&... args );
template< class... Args >
iterator try_emplace( const_iterator hint, const key_type& k, (4) (since C++23)
Args&&... args );
template< class... Args >
iterator try_emplace( const_iterator hint, key_type&& k, Args&&... (5) (since C++23)
args );
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)
inserts an element or assigns to the current element if the key
insert_or_assign already exists
(public member function)