std::unordered_map::try_emplace

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)...))

If rehashing occurs due to the insertion, all iterators are invalidated. Otherwise iterators are not affected. References are not invalidated. Rehashing occurs only if the new number of elements is greater than max_load_factor()*bucket_count().

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 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::unordered_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)

Example

Run this code

#include <iostream>
#include <utility>
#include <string>
 
#include <unordered_map>
int main()
{
    using namespace std::literals;
    std::unordered_map<std::string, std::string> m;
 
    m.try_emplace("a", "a"s);
    m.try_emplace("b", "abcd");
    m.try_emplace("c", 10, 'c');
 
    for (const auto &p : m) {
        std::cout << p.first << " => " << p.second << '\n';
    }
}

Possible output:

a => a
b => abcd
c => cccccccccc

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

array (C++11)
vector
deque
forward_list (C++11)
list
set
multiset
map
multimap
unordered_set (C++11)
unordered_multiset (C++11)
unordered_map (C++11)
unordered_multimap (C++11)
stack
queue
priority_queue

Member functions
unordered_map::unordered_map
unordered_map::~unordered_map
unordered_map::operator=
unordered_map::get_allocator
Iterators
unordered_map::beginunordered_map::cbegin
unordered_map::endunordered_map::cend
Capacity
unordered_map::erase
unordered_map::size
unordered_map::max_size
Modifiers
unordered_map::clear
unordered_map::insert
unordered_map::insert_or_assign (C++17)
unordered_map::emplace
unordered_map::emplace_hint
unordered_map::try_emplace (C++17)
unordered_map::erase
unordered_map::swap
unordered_map::extract (C++17)
unordered_map::merge (C++17)
Lookup
unordered_map::count
unordered_map::find
unordered_map::equal_range
Bucket interface
unordered_map::begin2
unordered_map::end2
unordered_map::bucket_count
unordered_map::max_bucket_count
unordered_map::bucket_size
unordered_map::bucket
Hash policy
unordered_map::load_factor
unordered_map::max_load_factor
unordered_map::rehash
unordered_map::reserve
Observers
unordered_map::hash_function
unordered_map::key_eq