std::flat_m...rt_or_assign(3)

💅 NAME 💅

std::flat_map::insert_or_assign — literally std::flat_map::insert_or_assign, no cap. Bestie, this page header is giving std::flat_m...rt_or_assign(3) from the C++ Standard Libary manual, and we're about to get into it. 🫶

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✨ Synopsis ✨

          template< class M >
          std::pair<iterator, bool> insert_or_assign( const key_type& k, M&&
       (1) (since C++23)
          obj );
          template< class M >
          std::pair<iterator, bool> insert_or_assign( key_type&& k, M&& obj
       (2) (since C++23)
          );
          template< class K, class M >
       (3) (since C++23)
          std::pair<iterator, bool> insert_or_assign( K&& k, M&& obj );
          template< class M >
          iterator insert_or_assign( const_iterator hint, const key_type& k,
       (4) (since C++23)
          M&& obj );
          template< class M >
          iterator insert_or_assign( const_iterator hint, key_type&& k, M&&
       (5) (since C++23)
          obj );
          template< class K, class M >
       (6) (since C++23)
          iterator insert_or_assign( const_iterator hint, K&& k, M&& obj );

1,2) okay so if a key equivalent to k already exists in the container, it assigns std::forward<M>(obj) to the mapped_type corresponding to the key k. If the key does not exist, it inserts the new value as if by:

• (1,2) try_emplace(std::forward<decltype(k)>(k), std::forward<M>(obj)),
• (4,5) try_emplace(hint, std::forward<decltype(k)>(k), std::forward<M>(obj)).

The program is ill-formed if either std::is_assignable_v<mapped_type&, M> or std::is_constructible_v<mapped_type, M> is false. periodt.

3,6) if a key equivalent to k already exists in the container, it assigns std::forward<M>(obj) to the mapped_type corresponding to the key k. Otherwise, it's equivalent to:

• (3) try_emplace(std::forward<K>(k), std::forward<M>(obj)),
• (6) try_emplace(hint, std::forward<K>(k), std::forward<M>(obj)).

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 — and not in a cute way, bestie. 😭

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&, M> is true.
• std::is_constructible_v<mapped_type, M> is true.

Information on iterator invalidation is copied from here. 👀

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🫶 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
• obj — the value to insert or assign

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🌈 Return value 🌈

• 1-3) The bool component is true if the insertion took place and false if the assignment took place. The iterator component is pointing at the element that was inserted or updated.
• 4-6) Iterator pointing at the element that was inserted or updated.

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🔥 Complexity 🔥

• 1-3) Same as for emplace.
• 4-6) Same as for emplace_hint.

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🎀 Notes 🎀

insert_or_assign returns more information than operator[] and does not require default-constructibility of the mapped type. slay for not making you default-construct stuff for no reason. 💅

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🧚 Example 🧚

Run this code, bestie — it's giving main character energy:

// Run this code

#include <flat_map>
#include <iostream>
#include <string>

void print_node(const auto& node)
{
    std::cout << '[' << node.first << "] = " << node.second << '\n';
}

void print_result(auto const& pair)
{
    std::cout << (pair.second ? "inserted: " : "assigned: ");
    print_node(*pair.first);
}

int main()
{
    std::flat_map<std::string, std::string> map;

    print_result(map.insert_or_assign("a", "apple"));
    print_result(map.insert_or_assign("b", "banana"));
    print_result(map.insert_or_assign("c", "cherry"));
    print_result(map.insert_or_assign("c", "clementine"));

    for (const auto& node : map)
        print_node(node);
}

Output:

inserted: [a] = apple
inserted: [b] = banana
inserted: [c] = cherry
assigned: [c] = clementine
[a] = apple
[b] = banana
[c] = clementine

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👑 See also 👑

• operator[] — access or insert specified element (public member function)
• at — access specified element with bounds checking (public member function)
• insert — inserts elements (public member function)
• emplace — constructs element in-place (public member function)
• try_emplace — inserts in-place if the key does not exist, does nothing if the key exists (public member function)

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source/version/date sign-off, because we keep every detail: http://cppreference.com, 2024.06.10, page std::flat_m...rt_or_assign(3) — mwah 💋✨