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Latest revision as of 08:11, 28 November 2021

[edit template]
 
C++
 
Containers library
 
std::map
Member functions
Non-member functions
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)
Deduction guides (C++17)
 
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) Behaves like emplace except that the element is constructed as
value_type(std::piecewise_construct,

           std::forward_as_tuple(k),

           std::forward_as_tuple(std::forward<Args>(args)...))
2) 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(std::forward<Args>(args)...))
3) Behaves like emplace except that the element is constructed as
value_type(std::piecewise_construct,

           std::forward_as_tuple(std::forward<K>(k)),

           std::forward_as_tuple(std::forward<Args>(args)...))
4) 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(std::forward<Args>(args)...))
5) 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(std::forward<Args>(args)...))
6) Behaves like emplace_hint except that the element is constructed as
value_type(std::piecewise_construct,

           std::forward_as_tuple(std::forward<K>(k)),

           std::forward_as_tuple(std::forward<Args>(args)...))
1-6) If value_type is not EmplaceConstructible into map from the corresponding expression, the behavior is undefined.
3) This overload participates in overload resolution only if all following conditions are satisfied:
If equal_range(u.first) == equal_range(k) is false, the behavior is undefined, where u is the new element to be inserted.
6) 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.

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:
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 (3,6) 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

Run this code
#include <iostream>
#include <string>
#include <map>
#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

(C++11)
 constructs element in-place
(public member function) [edit]
(C++11)
 constructs elements in-place using a hint
(public member function) [edit]
inserts elements or nodes(since C++17)
(public member function) [edit]
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