NAME¶

std::unordered_map::try_emplace - std::unordered_map::try_emplace

Synopsis¶

template< class... Args >
std::pair<iterator, bool> try_emplace( const Key& k, Args&&... (1) (since C++17)
args );
template< class... Args > (2) (since C++17)
std::pair<iterator, bool> try_emplace( Key&& k, Args&&... args );
template< class K, class... Args > (3) (since C++26)
std::pair<iterator, bool> try_emplace( K&& k, Args&&... args );
template< class... Args >
iterator try_emplace( const_iterator hint, const Key& k, Args&&... (4) (since C++17)
args );
template< class... Args >
iterator try_emplace( const_iterator hint, Key&& k, Args&&... args (5) (since C++17)
);
template< class K, class... Args >
iterator try_emplace( const_iterator hint, K&& k, Args&&... args (6) (since C++26)
);

If a key equivalent to k already exists in the container, does nothing. Otherwise,
inserts a new element into the container with key k and value constructed with args.
In such case:

1) 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) 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) Behaves like emplace except that the element is constructed as
value_type(std::piecewise_construct,

std::forward_as_tuple(std::forward<K>(k)),

std::forward_as_tuple(std::forward<Args>(args)...))
4) 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)...))
5) 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)...))
6) Behaves like emplace_hint except that the element is constructed as
value_type(std::piecewise_construct,

std::forward_as_tuple(std::forward<K>(k)),

std::forward_as_tuple(std::forward<Args>(args)...))
1-6) If value_type is not EmplaceConstructible into unordered_map from the
corresponding expression, the behavior is undefined.
3) This overload participates in overload resolution only if all following
conditions are satisfied:
* std::is_convertible_v<K&&, const_iterator> and std::is_convertible_v<K&&,
iterator> are both false.
* Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a
type.
If hash_function()(u.first) != hash_function()(k) || contains(u.first) is true, the
behavior is undefined, where u is the new element to be inserted.
6) This overload participates in overload resolution only if Hash::is_transparent
and KeyEqual::is_transparent are both valid and each denotes a type.
If hash_function()(u.first) != hash_function()(k) || contains(u.first) is true, the
behavior is undefined, where u is the new element to be inserted.

If after the operation the new number of elements is greater than old
max_load_factor() * bucket_count() a rehashing takes place.
If rehashing occurs (due to the insertion), all iterators are invalidated. Otherwise
(no rehashing), iterators are not invalidated.

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::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).

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

Feature-test macro Value Std Feature __cpp_lib_unordered_map_try_emplace 201411L (C++17) std::unordered_map::try_emplace,
std::unordered_map::insert_or_assign
Heterogeneous overloads for the __cpp_lib_associative_heterogeneous_insertion 202311L (C++26) remaining member functions in
ordered and unordered associative
containers. Overloads (3) and (6).

Example¶

// Run this code

#include <iostream>
#include <string>
#include <unordered_map>
#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::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 constructs element in-place
(public member function)
emplace_hint constructs elements in-place using a hint
(public member function)
inserts elements
insert or nodes
(since C++17)
(public member function)