std::map::try_emplace

 

C++

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Technical Specifications

 

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template <class... Args> pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);	(1)	(since C++17)
template <class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);	(2)	(since C++17)
template <class... Args> iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);	(3)	(since C++17)
template <class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);	(4)	(since C++17)

1) If a key equivalent to k already exists in the container, does nothing. Otherwise, behaves like emplace except that the element is constructed as value_type(std::piecewise_construct, std::forward_as_tuple(k), std::forward_as_tuple(forward<Args>(args)...))

2) If a key equivalent to k already exists in the container, does nothing. Otherwise, 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(forward<Args>(args)...))

3) If a key equivalent to k already exists in the container, does nothing. Otherwise, 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(forward<Args>(args)...))

4) If a key equivalent to k already exists in the container, does nothing. Otherwise, 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(forward<Args>(args)...))

No iterators or references are 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

1,2) Same as for emplace

3,4) Same as for emplace_hint

Complexity

1,2) Same as for emplace

3,4) Same as for emplace_hint

Notes

Unlike insert or emplace, these functions do not steal from move-only 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)

See also

emplace (C++11)	constructs element in-place (public member function)
emplace_hint (C++11)	constructs elements in-place using a hint (public member function)
insert	inserts elements (public member function)