std::unordered_set::merge(3)

✨ std::unordered_set::merge ✨

Hey bestie 💖 this is the page for std::unordered_set::merge(3) from the C++ Standard Libary manual (yes, that’s the spelling on the page — iconic 💅). It’s giving node-transfer realness, so let’s get into it.

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🌸 NAME 🌸

std::unordered_set::merge - std::unordered_set::merge

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🦋 SYNOPSIS 🦋

Four overloads, all (since C++17), all here to do the thing:

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

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💜 DESCRIPTION 💜

Okay so what it does: it attempts to extract ("splice") each element in source and insert it into *this using the hash function and key equality predicate of *this. If there is an element in *this with key equivalent to the key of an element from source, then that element is not extracted from source — it stays put. 🛑

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. Iterators to elements remaining in source remain valid.

⚠️ Undefined behavior alert: the behavior is undefined if get_allocator() != source.get_allocator(). Do not mess with that, bestie. 💀

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🎀 PARAMETERS 🎀

• source — compatible container to transfer the nodes from. 🧚

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💅 RETURN VALUE 💅

(none)

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🔥 COMPLEXITY 🔥

Average case O(N), worst case O(N * size() + N), where N is source.size(). Math girlie moment, no cap. 🧮

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🌈 EXAMPLE 🌈

Here’s the example exactly as the page gives it — including the // Run this code marker:

// Run this code

#include <iostream>
#include <unordered_set>

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

int main()
{
    std::unordered_set<char>
        p{'C', 'B', 'B', 'A'},
        q{'E', 'D', 'E', 'C'};

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

    p.merge(q);

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

💖 POSSIBLE OUTPUT 💖

p: [3] { A, B, C }
q: [3] { C, D, E }
p.merge(q);
p: [5] { E, D, A, B, C }
q: [1] { C }

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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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Source: http://cppreference.com · version/date: 2024.06.10 · page: std::unordered_set::merge(3) — bye bestie, go merge responsibly 💋✨