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```c++ template<class... Args> std::pair<iterator, bool> emplace(Args&&... args); ```

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Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/EmplaceConstructible[EmplaceConstructible^] into `X` from `args`. Returns:;; The bool component of the return type is true if an insert took place. + + If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. Throws:;; If an exception is thrown by an operation other than a call to `hasher` the function has no effect. Notes:;; Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. + + Pointers and references to elements are never invalidated. + + If `args...` is of the form `k,v`, it delays constructing the whole object until it is certain that an element should be inserted, using only the `k` argument to check. This optimization happens when the map's `key_type` is move constructible or when the `k` argument is a `key_type`.

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```c++ template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args); ```

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Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/EmplaceConstructible[EmplaceConstructible^] into `X` from `args`. Returns:;; If an insert took place, then the iterator points to the newly inserted element. Otherwise, it points to the element with equivalent key. Throws:;; If an exception is thrown by an operation other than a call to `hasher` the function has no effect. Notes:;; The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key. + + Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor. + + Pointers and references to elements are never invalidated. + + If `args...` is of the form `k,v`, it delays constructing the whole object until it is certain that an element should be inserted, using only the `k` argument to check. This optimization happens when the map's `key_type` is move constructible or when the `k` argument is a `key_type`.

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```c++ template<class... Args> std::pair<iterator, bool> try_emplace(const key_type& k, Args&&... args); template<class... Args> std::pair<iterator, bool> try_emplace(key_type&& k, Args&&... args); template<class K, class... Args> std::pair<iterator, bool> try_emplace(K&& k, Args&&... args) ```

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value_type(std::piecewise_construct, std::forward_as_tuple(std::forward<Key>(k)), std::forward_as_tuple(std::forward<Args>(args)...))

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value_type(std::piecewise_construct, std::forward_as_tuple(std::forward<K>(k)), std::forward_as_tuple(std::forward<Args>(args)...)) ```

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The `template<class K, class\... Args>` overload only participates in overload resolution if `Hash::is_transparent` and `Pred::is_transparent` are valid member typedefs and neither `iterator` nor `const_iterator` are implicitly convertible from `K`. The library assumes that `Hash` is callable with both `K` and `Key` and that `Pred` is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the `Key` type. --

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```c++ template<class... Args> iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args); template<class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args); template<class K, class... Args> iterator try_emplace(const_iterator hint, K&& k, Args&&... args); ```

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value_type(std::piecewise_construct, std::forward_as_tuple(std::forward<Key>(k)), std::forward_as_tuple(std::forward<Args>(args)...))

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value_type(std::piecewise_construct, std::forward_as_tuple(std::forward<K>(k)), std::forward_as_tuple(std::forward<Args>(args)...)) ```

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The `template<class K, class\... Args>` overload only participates in overload resolution if `Hash::is_transparent` and `Pred::is_transparent` are valid member typedefs and neither `iterator` nor `const_iterator` are implicitly convertible from `K`. The library assumes that `Hash` is callable with both `K` and `Key` and that `Pred` is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the `Key` type. --