std::unordered_map try_emplace() method

since C++17

// (1) Non const version only
template< class... Args >
pair<iterator, bool> try_emplace( const Key& k, Args&&... args );

// (2) Non const version only
template< class... Args >
pair<iterator, bool> try_emplace( Key&& k, Args&&... args );

// (3) Non const version only
template< class... Args >
iterator try_emplace( const_iterator hint, const Key& k, Args&&... args );

// (4) Non const version only
template< class... Args >
iterator try_emplace( const_iterator hint, Key&& k, Args&&... args );

Inserts a new element into the container with key k and value constructed with args, if there is no element with the key in the container.

• (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(std::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(std::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(std::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(std::forward<Args>(args)...))

danger

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().

Exceptions

If an exception is thrown by any operation, this function has no effect (strong exception guarantee).

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).

Feature testing macro: __cpp_lib_unordered_map_try_emplace.

Example

Main.cpp

#include <iostream>
#include <utility>
#include <string>
 
#include <unordered_map>
 
auto print_node = [](const auto &node) {
    std::cout << "[" << node.first << "] = " << node.second << '\n';
};
 
auto 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); }
}

Possible output

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