std::map<Key,T,Compare,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 map from the corresponding expression, the behavior is undefined.
- 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.
- The qualified-id Compare::is_transparent is valid and denotes a type.
If equal_range(u.first) == equal_range(k) is false, the behavior is undefined, where u is the new element to be inserted.
- This overload participates in overload resolution only if the qualified-id Compare::is_transparent is valid and denotes a type.
If equal_range(u.first) == equal_range(k) is false, the behavior is undefined, where u is the new element to be inserted.
No iterators or references are 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:
Logarithmic in the size of the container.
4-6) Same as for emplace_hint:
Logarithmic in the size of the container in general, but amortized constant if the new element is inserted just before hint.
[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::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_map_try_emplace | 201411L | (C++17) | std::map::try_emplace, std::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::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
[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] |