std::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)...))

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 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)

Example

Run this code

#include <iostream>
#include <utility>
#include <string>
 
#include <map>
int main()
{
    using namespace std::literals;
    std::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';
    }
}

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
map::map
map::~map
map::operator=
map::get_allocator
Element access
map::at
map::operator[]
Iterators
map::beginmap::cbegin (C++11)
map::endmap::cend (C++11)
map::rbeginmap::crbegin (C++11)
map::rendmap::crend (C++11)
Capacity
map::empty
map::size
map::max_size
Modifiers
map::clear
map::insert
map::insert_or_assign (C++17)
map::emplace (C++11)
map::emplace_hint (C++11)
map::try_emplace (C++17)
map::erase
map::swap
map::extract (C++17)
map::merge (C++17)
Lookup
map::count
map::find
map::equal_range
map::lower_bound
map::upper_bound
Observers
map::key_comp
map::value_comp