std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::try_emplace - cppreference.com (original) (raw)

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

1-6) If value_type is not EmplaceConstructible into unordered_map from the corresponding expression, the behavior is undefined.

  1. This overload participates in overload resolution only if all following conditions are satisfied:

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.

  1. 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.

[edit] 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

[edit] Return value

1-3) Same as for emplace:
A pair consisting of an iterator to the inserted element (or to the element that prevented the insertion) and a bool value set to true if and only if the insertion took place.

4-6) Same as for emplace_hint:
An iterator to the inserted element, or to the element that prevented the insertion.

[edit] Complexity

1-3) Same as for emplace:
Amortized constant on average, worst case linear in the size of the container.

4-6) Same as for emplace_hint:
Amortized constant on average, worst case linear in the size of the container.

[edit] 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) and (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
__cpp_lib_associative_heterogeneous_insertion 202311L (C++26) Heterogeneous overloads for the remaining member functions in ordered and unordered associative containers. Overloads (3) and (6).

[edit] Example

#include #include #include #include   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

[edit] See also

| | constructs element in-place (public member function) [edit] | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | constructs elements in-place using a hint (public member function) [edit] | | | inserts elements or nodes(since C++17) (public member function) [edit] |