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Difference between revisions of "cpp/container/flat map/try emplace"

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< cpp‎ | container‎ | flat map
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{{cpp/container/flat_map/title|try_emplace}}
 
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@@ The conversion from {{c|k}} into {{tt|key_type}} must construct an object {{c|u}}, for which {{c|1=find(k) == find(u)}} is {{c|true}}. Otherwise, the behavior is undefined.
 
@@ The conversion from {{c|k}} into {{tt|key_type}} must construct an object {{c|u}}, for which {{c|1=find(k) == find(u)}} is {{c|true}}. Otherwise, the behavior is undefined.
  
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* The qualified-id {{tt|Compare::is_transparent}} is valid and denotes a type.
 
* The qualified-id {{tt|Compare::is_transparent}} is valid and denotes a type.
 
* {{c|std::is_constructible_v<key_type, K>}} is {{c|true}}.
 
* {{c|std::is_constructible_v<key_type, K>}} is {{c|true}}.
 
* {{c|std::is_assignable_v<mapped_type&, Args...>}} is {{c|true}}.
 
* {{c|std::is_assignable_v<mapped_type&, Args...>}} is {{c|true}}.
 
* For {{v|3}} only, {{c|std::is_convertible_v<K&&, const_iterator>}} and {{c|std::is_convertible_v<K&&, iterator>}} are both {{c|false}}.
 
* For {{v|3}} only, {{c|std::is_convertible_v<K&&, const_iterator>}} and {{c|std::is_convertible_v<K&&, iterator>}} are both {{c|false}}.
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|plural=yes}}
 
{{cpp/container/note iterator invalidation|flat_map|try_emplace}}
 
{{cpp/container/note iterator invalidation|flat_map|try_emplace}}
  

Latest revision as of 22:29, 22 May 2024

 
C++
 
Containers library
 
std::flat_map
Member types
Member functions
Non-member functions
Helper classes
Tags
Deduction guides
 
template< class... Args >
std::pair<iterator, bool> try_emplace( const key_type& k, Args&&... args );
 (1) (since C++23)
template< class... Args >
std::pair<iterator, bool> try_emplace( key_type&& k, Args&&... args );
 (2) (since C++23)
template< class K, class... Args >
std::pair<iterator, bool> try_emplace( K&& k, Args&&... args );
 (3) (since C++23)
template< class... Args >
iterator try_emplace( const_iterator hint, const key_type& k, Args&&... args );
 (4) (since C++23)
template< class... Args >
iterator try_emplace( const_iterator hint, key_type&& k, Args&&... args );
 (5) (since C++23)
template< class K, class... Args >
iterator try_emplace( const_iterator hint, K&& k, Args&&... args );
 (6) (since C++23)

If a key equivalent to k already exists in the container, does nothing. Otherwise, inserts a new element into the underlying containers c with key k and value constructed with args.

1,2,4,5) Equivalent to: 
auto key_it = ranges::upper_bound(c.keys, k, compare);
auto value_it = c.values.begin() + std::distance(c.keys.begin(), key_it);
c.keys.insert(key_it, std::forward<decltype(k)>(k));
c.values.emplace(value_it, std::forward<Args>(args)...);
3,6) Equivalent to: 
auto key_it = ranges::upper_bound(c.keys, k, compare);
auto value_it = c.values.begin() + std::distance(c.keys.begin(), key_it);
c.keys.emplace(key_it, std::forward<K>(k));
c.values.emplace(value_it, std::forward<Args>(args)...);
The conversion from k into key_type must construct an object u, for which find(k) == find(u) is true. Otherwise, the behavior is undefined.
These overloads participate in overload resolution only if :
Information on iterator invalidation is copied from here

Contents

[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.
4-6) Same as for emplace_hint.

[edit] Complexity

1-3) Same as for emplace.
4-6) Same as for emplace_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::flat_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_type.

[edit] Example

Run this code
#include <flat_map>
#include <iostream>
#include <string>
#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::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
(public member function) [edit]
inserts an element or assigns to the current element if the key already exists
(public member function) [edit]
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