# std::map<Key,T,Compare,Allocator>::try_emplace

From cppreference.com
< cpp‎ | container‎ | map

template< class... Args > std::pair <iterator, bool> try_emplace( const Key& k, Args&&... args ); | (1) | (since C++17)
template< class... Args > std::pair <iterator, bool> try_emplace( Key&& k, Args&&... args ); | (2) | (since C++17)
template< class K, class... Args > std::pair <iterator, bool> try_emplace( K&& k, Args&&... args ); | (3) | (since C++26)
template< class... Args > iterator try_emplace( const_iterator hint, const Key& k, Args&&... args ); | (4) | (since C++17)
template< class... Args > iterator try_emplace( const_iterator hint, Key&& k, Args&&... args ); | (5) | (since C++17)
template< class K, class... Args > iterator try_emplace( const_iterator hint, K&& k, Args&&... args ); | (6) | (since C++26)

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:

1) 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) 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) Behaves like `emplace` except that the element is constructed as
value_type( std::piecewise_construct,

std::forward_as_tuple ( std::forward <K>(k)),

std::forward_as_tuple ( std::forward <Args>(args)...))
4) 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)...))
5) 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)...))
6) Behaves like `emplace_hint` except that the element is constructed as
value_type( std::piecewise_construct,

std::forward_as_tuple ( std::forward <K>(k)),

std::forward_as_tuple ( std::forward <Args>(args)...))
1-6) If `value_type` is not EmplaceConstructible into `map` from the corresponding expression, the behavior is undefined.
3) This overload participates in overload resolution only if all following conditions are satisfied:

- std::is_convertible_v <K&&, const_iterator> and std::is_convertible_v <K&&, iterator> are both false.
- The qualified-id Compare::is_transparent is valid and denotes a type.

If equal_range(u.first) == equal_range(k) is false, the behavior is undefined, where u is the new element to be inserted.
6) This overload participates in overload resolution only if the qualified-id Compare::is_transparent is valid and denotes a type.
If equal_range(u.first) == equal_range(k) is false, the behavior is undefined, where u is the new element to be inserted.

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

1-3) Same as for `emplace`:
A pair consisting of an iterator to the inserted element (or to the element that prevented the insertion) and a bool value set to true if and only if the insertion took place.
4-6) Same as for `emplace_hint`:
An iterator to the inserted element, or to the element that prevented the insertion.

### Complexity

1-3) Same as for `emplace`:
Logarithmic in the size of the container.
4-6) Same as for `emplace_hint`:
Logarithmic in the size of the container in general, but amortized constant if the new element is inserted just before 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).

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

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_map_try_emplace` | `201411L` | (C++17) | `std::map::try_emplace`, std::map::insert_or_assign
`__cpp_lib_associative_heterogeneous_insertion` | `202311L` | (C++26) | Heterogeneous overloads for the remaining member functions in ordered and unordered associative containers. Overloads (3) and (6).

### Example

Run this code

    #include <iostream>
    #include <string>
    #include <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::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 (C++11) | constructs element in-place (public member function)
emplace_hint (C++11) | constructs elements in-place using a hint (public member function)
insert | inserts elements or nodes(since C++17) (public member function)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/container/map/try_emplace&oldid=135736"