std::unorde...imap::merge(3)

💖 NAME 💖

okay so bestie, this is the manual page for std::unordered_multimap::merge — literally the page named std::unorde...imap::merge(3) from the C++ Standard Libary manual (yes, that’s how the header spells it, and yes the manual name is abbreviated exactly like that 💅). The NAME section is, iconically, just this: std::unordered_multimap::merge — std::unordered_multimap::merge. periodt.

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📜 Synopsis 📜

here are all four overloads, bestie, and every single one is (since C++17) ✨

          template< class H2, class P2 >
          void merge( std::unordered_map<Key, T, H2, P2, Allocator>& source
       (1) (since C++17)
          );
          template< class H2, class P2 >
          void merge( std::unordered_map<Key, T, H2, P2, Allocator>&& source
       (2) (since C++17)
          );
          template< class H2, class P2 >
          void merge( std::unordered_multimap<Key, T, H2, P2, Allocator>&
       (3) (since C++17)
          source );
          template< class H2, class P2 >
          void merge( std::unordered_multimap<Key, T, H2, P2, Allocator>&&
       (4) (since C++17)
          source );

Attempts to extract ("splice") each element in source and insert it into *this using the hash function and key equality predicate of *this. no cap, that’s the whole move 🫶

No elements are copied or moved, only the internal pointers of the container nodes are repointed. All pointers and references to the transferred elements remain valid, but now refer into *this, not into source. Iterators referring to the transferred elements and all iterators referring to *this are invalidated.

🚨 undefined behavior alert: the behavior is undefined if get_allocator() != source.get_allocator(). not a vibe. do not do it. 💀

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🌸 Parameters 🌸

• source — compatible container to transfer the nodes from.

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🎀 Return value 🎀

literally (none) — that’s it, bestie, nothing comes back 😌

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🔥 Complexity 🔥

Average case O(N), worst case O(N * size() + N), where N is source.size().

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🧚 Example 🧚

// Run this code

#include <iostream>
#include <string>
#include <unordered_map>
#include <utility>

// print out a std::pair
template<class Os, class U, class V>
Os& operator<<(Os& os, const std::pair<U,V>& p)
{
    return os << '{' << p.first << ", " << p.second << '}';
}

// print out an associative container
template<class Os, class K, class V>
Os& operator<<(Os& os, const std::unordered_multimap<K, V>& v)
{
    os << '[' << v.size() << "] {";
    bool o{};
    for (const auto& e : v)
        os << (o ? ", " : (o = 1, "")) << e;
    return os << "}\n";
}

int main()
{
    std::unordered_multimap<std::string, int>
        p{{"C", 3}, {"B", 2}, {"A", 1}, {"A", 0}},
        q{{"E", 6}, {"E", 7}, {"D", 5}, {"A", 4}};

    std::cout << "p: " << p << "q: " << q;

    p.merge(q);

    std::cout << "p.merge(q);\n" << "p: " << p << "q: " << q;
}

✨ Possible output ✨

p: [4] {{A, 1}, {A, 0}, {B, 2}, {C, 3}}
q: [4] {{A, 4}, {D, 5}, {E, 6}, {E, 7}}
p.merge(q);
p: [8] {{E, 6}, {E, 7}, {C, 3}, {A, 1}, {A, 0}, {A, 4}, {D, 5}, {B, 2}}
q: [0] {}

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👀 See also 👀

• extract — extracts nodes from the container. (C++17) (public member function)
• insert — inserts elements or nodes. (since C++17) (public member function)

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mwah 💋 source/version: http://cppreference.com; date: 2024.06.10; page: std::unorde...imap::merge(3). bye bestie, go merge responsibly 🫶✨