std::flat_map<Key,T,Compare,KeyContainer,MappedContainer>::try_emplace

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Containers library

Sequence

array (C++11)

vector

vector<bool>

inplace_vector (C++26)

deque

forward_list (C++11)

list

Associative

set

multiset

map

multimap

Unordered associative

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

Adaptors

stack

queue

priority_queue

flat_set (C++23)

flat_multiset (C++23)

flat_map (C++23)

flat_multimap (C++23)

Views

span (C++20)

mdspan (C++23)

Tables

Iterator invalidation

Member function table

Non-member function table

 

std::flat_map

Member types

Member functions

flat_map::flat_map flat_map::operator= Element access flat_map::at flat_map::operator[] Iterators flat_map::beginflat_map::cbegin flat_map::endflat_map::cend flat_map::rbeginflat_map::crbegin flat_map::rendflat_map::crend Lookup flat_map::count flat_map::find flat_map::contains flat_map::equal_range    flat_map::lower_bound flat_map::upper_bound Capacity flat_map::size flat_map::max_size flat_map::empty Modifiers flat_map::clear flat_map::erase flat_map::swap flat_map::emplace flat_map::extract flat_map::replace flat_map::insert flat_map::insert_range flat_map::insert_or_assign flat_map::emplace_hint flat_map::try_emplace Observers flat_map::key_comp flat_map::keys flat_map::value_comp flat_map::values

Non-member functions

operator==operator<=>         swap(std::flat_map) erase_if(std::flat_map)

Helper classes

uses_allocator<std::flat_map>

Tags

sorted_unique       sorted_unique_t

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 :

• The qualified-id Compare::is_transparent is valid and denotes a type.
• std::is_constructible_v<key_type, K> is true.
• std::is_assignable_v<mapped_type&, Args...> is true.
• For (3) only, std::is_convertible_v<K&&, const_iterator> and std::is_convertible_v<K&&, iterator> are both false.

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

1-3) Same as for emplace.

4-6) Same as for emplace_hint.

Complexity

1-3) Same as for emplace.

4-6) 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::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.

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

See also

emplace	constructs element in-place (public member function)
emplace_hint	constructs elements in-place using a hint (public member function)
insert	inserts elements (public member function)
insert_or_assign	inserts an element or assigns to the current element if the key already exists (public member function)