std::unordered_map<Key,T,Hash,KeyEqual,Allocator>:: try_emplace

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:

If after the operation the new number of elements is greater than old max_load_factor() * bucket_count() a rehashing takes place.
If rehashing occurs (due to the insertion), all iterators are invalidated. Otherwise (no rehashing), iterators are not invalidated.

Parameters

Return value

Complexity

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 ).

Overloads (3,6) can be called without constructing an object of type Key .

Example

#include <iostream>
#include <string>
#include <unordered_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::unordered_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);
}

inserted: [a] = a
inserted: [b] = abcd
inserted: [c] = cccccccccc
ignored:  [c] = cccccccccc
[a] = a
[b] = abcd
[c] = cccccccccc

See also